crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936

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adamperlin merged 2 commits into
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adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
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adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix

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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.

CopilotAI review requested due to automatic review settings May 8, 2026 00:20
@github-actionsgithub-actionsBot added the area-crossgen2-coreclr only use for closed issues label May 8, 2026
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Pull request overview

This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.

Changes:

  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.

Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs Outdated
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Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs Outdated
@adamperlinadamperlin changed the title crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?

@adamperlin

adamperlin commented May 8, 2026

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Looks reasonable to me. Are you able to validate crossgen2 output with this change?

Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.

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, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
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adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
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crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
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adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
Outdated

AndyAyersMS
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Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
@adamperlin
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
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May 8, 2026

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/ba-g infrastructure issues
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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
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adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
Merged
crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
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adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
CopilotAI
review requested due to automatic review settings
May 8, 2026 00:20
@github-actionsgithub-actionsBot
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only use for closed issues
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@adamperlinadamperlin
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
Outdated

AndyAyersMS
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View reviewed changes
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
@adamperlin
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May 8, 2026

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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
@adamperlin
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May 8, 2026

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/ba-g infrastructure issues
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merged commit f7280f5
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109 of 111 checks passed

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, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
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adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
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crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
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adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
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review requested due to automatic review settings
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
Outdated

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Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
@adamperlin
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May 8, 2026

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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
@adamperlin
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May 8, 2026

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/ba-g infrastructure issues
@adamperlin
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merged commit f7280f5
into
dotnet:mainMay 11, 2026

109 of 111 checks passed

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mentioned this pull request
May 12, 2026
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This was referenced Jun 2, 2026
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@adamperlin@AndyAyersMS@pavelsavara

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

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crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
Merged
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
Merged
crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
CopilotAI
review requested due to automatic review settings
May 8, 2026 00:20
@github-actionsgithub-actionsBot
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@adamperlinadamperlin
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
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Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
@adamperlin
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May 8, 2026

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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
@adamperlin
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May 8, 2026

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/ba-g infrastructure issues
@adamperlin
adamperlin
merged commit f7280f5
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dotnet:mainMay 11, 2026

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@AndyAyersMSAndyAyersMSAndyAyersMS approved these changes
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@adamperlin@AndyAyersMS@pavelsavara

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
Merged
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
Merged
crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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May 8, 2026•

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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
CopilotAI
review requested due to automatic review settings
May 8, 2026 00:20
@github-actionsgithub-actionsBot
added
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area-crossgen2-coreclr
only use for closed issues
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May 8, 2026
@adamperlinadamperlin
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
Outdated

AndyAyersMS
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Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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changed the title
crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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mentioned this pull request
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
@adamperlin
adamperlin
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May 8, 2026

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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
@adamperlin
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May 8, 2026

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/ba-g infrastructure issues
@adamperlin
adamperlin
merged commit f7280f5
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dotnet:mainMay 11, 2026

109 of 111 checks passed

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mentioned this pull request
May 12, 2026
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@AndyAyersMSAndyAyersMSAndyAyersMS approved these changes
@MichalStrehovskyMichalStrehovskyAwaiting requested review from MichalStrehovskyMichalStrehovsky is a code owner
@davidwrightondavidwrightonAwaiting requested review from davidwrighton

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@adamperlin@AndyAyersMS@pavelsavara

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
Merged
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
Merged
crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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@adamperlin
@adamperlinadamperlin
commented
May 8, 2026•

edited

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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
CopilotAI
review requested due to automatic review settings
May 8, 2026 00:20
@github-actionsgithub-actionsBot
added
the 
area-crossgen2-coreclr
only use for closed issues
label
May 8, 2026
@adamperlinadamperlin
added this to the 11.0.0 milestone
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
Outdated

AndyAyersMS
AndyAyersMS
reviewed
May 8, 2026
View reviewed changes
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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@adamperlinadamperlin
changed the title
crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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mentioned this pull request
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
@adamperlin
adamperlin
commented
May 8, 2026

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ContributorAuthor
Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
@adamperlin
adamperlin
commented
May 8, 2026

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/ba-g infrastructure issues
@adamperlin
adamperlin
merged commit f7280f5
into
dotnet:mainMay 11, 2026

109 of 111 checks passed

@dotnet-maestrodotnet-maestroBot
mentioned this pull request
May 12, 2026
 Merged


This was referenced Jun 2, 2026
 Closed

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@github-actionsgithub-actionsBot
locked and limited conversation to collaborators
Jun 11, 2026
@pavelsavarapavelsavara
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Copilot code reviewCopilot
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Copilot left review comments
@AndyAyersMSAndyAyersMSAndyAyersMS approved these changes
@MichalStrehovskyMichalStrehovskyAwaiting requested review from MichalStrehovskyMichalStrehovsky is a code owner
@davidwrightondavidwrightonAwaiting requested review from davidwrighton

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4 participants

@adamperlin@AndyAyersMS@pavelsavara

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

Skip to content
crossgen2: Add Length Prefix to Crossgen-Generated Wasm - #127936
Merged
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
May 11, 2026
Merged
crossgen2: Add Length Prefix to Crossgen-Generated Wasm#127936
adamperlin merged 2 commits into
dotnet:mainfrom 
adamperlin:adamperlin/crossgen-wasm-emitter-length-prefix
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May 8, 2026•

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#127773 adds length prefixes directly to emitted Wasm code in the JIT. This change is a follow up which adds length prefixes directly to generated wasm import thunks and stubs in crossgen. It also removes the logic in the Object Writer around handling length prefixes, since these will now be encoded in the ObjectData itself.
CopilotAI
review requested due to automatic review settings
May 8, 2026 00:20
@github-actionsgithub-actionsBot
added
the 
area-crossgen2-coreclr
only use for closed issues
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May 8, 2026
@adamperlinadamperlin
added this to the 11.0.0 milestone
May 8, 2026
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Pull request overview
This PR updates the Wasm object writer pipeline so that Wasm function bodies carry their own ULEB128 length prefix in ObjectData, rather than relying on SectionWriter/ObjectWriter to inject prefixes and adjust relocation offsets. It also updates Crossgen2’s Wasm import thunk emission to include the prefix directly in the emitted thunk bytes.
Changes:
  • Remove section-writer length-prefix support and the corresponding relocation-offset adjustment in ObjectWriter.
  • Update WasmFunctionBody relocation offset accounting to include the newly inlined body-size prefix.
  • Emit the body-size prefix directly into Crossgen2-generated Wasm import thunks.
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.
Show a summary per file
FileDescription
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.csRemoves Wasm-specific section-writer length-prefix configuration now that prefixes are encoded into ObjectData.
src/coreclr/tools/Common/Compiler/ObjectWriter/WasmInstructions.csAdjusts relocation offsets to account for the function-body length prefix being present in the encoded bytes.
src/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.csRemoves length-prefix encoding support from SectionWriter.EmitData and related plumbing.
src/coreclr/tools/Common/Compiler/ObjectWriter/ObjectWriter.csRemoves the (Wasm-only) relocation offset adjustment that previously compensated for writer-inserted length prefixes.
src/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.csPrepends the ULEB128 body-size prefix when encoding Wasm import thunks.
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/SectionWriter.cs
Comment threadsrc/coreclr/tools/Common/Compiler/DependencyAnalysis/Target_Wasm/WasmEmitter.cs
Outdated

AndyAyersMS
AndyAyersMS
reviewed
May 8, 2026
View reviewed changes
Comment threadsrc/coreclr/tools/Common/Compiler/ObjectWriter/WasmObjectWriter.cs
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@adamperlinadamperlin
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crossgen2: Add Length Prefix to Wasm Import Thunks crossgen2: Add Length Prefix to Crossgen-Generated WasmMay 8, 2026
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
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Looks reasonable to me. Are you able to validate crossgen2 output with this change?
Yes, for a smaller test case that I have that does include crossgen-generated Wasm in the compilation! I haven't been able to get through an SPC compilation but I think that is due to other issues.
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/ba-g infrastructure issues
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