Add Windows ARM64 support by sosukesuzuki · Pull Request #138 · oven-sh/WebKit

sosukesuzuki · 2026-01-16T04:04:37Z

Summary

This PR adds Windows ARM64 (WoA) support to JavaScriptCore.

Source Code Changes

CPU Detection

Add _M_ARM64 MSVC macro to ARM64 CPU detection in PlatformCPU.h and BPlatform.h

Calling Conventions

Limit SYSV_ABI attribute to x86_64 only on Windows (ARM64 uses a different ABI)
- PlatformCallingConventions.h
- CodePtr.h
- FunctionPtr.h
- FunctionTraits.h (2 locations)
- AbstractMacroAssembler.h

ARM64 Assembler

Expand LinkRecord::CopyTypes to uint64_t content[5] for Windows (different struct packing)
Add FlushInstructionCache() API call to cacheFlush() for Windows

Frame Pointer Handling

Add Windows ARM64 DECLARE_CALL_FRAME macro with x29 clobber in CallFrame.h
Disable USE_BUILTIN_FRAME_ADDRESS for Windows ARM64 in PlatformUse.h

Register Mappings

Add Windows ARM64 register mappings to MachineContext.h:
- stackPointerImpl: regs.Sp
- framePointerImpl: regs.Fp
- instructionPointerImpl: regs.Pc
- argumentPointer<1>: regs.X1
- wasmInstancePointer: regs.X19
- llintInstructionPointer: regs.X4

Assembler Directives

Exclude .previous directive for Windows in MacroAssemblerARM64.cpp (COFF doesn't support it)
Add Windows ARM64 currentStackPointer implementation in StackPointer.cpp

JIT/Offlineasm

Disable inline ASM-based unaryDoubleOpWrapper for Windows ARM64 in ThunkGenerators.cpp
Add Windows ARM64 COFF-style addressing to globaladdr in arm64.rb

Build System

Add architecture detection for clang_rt.builtins library in OptionsMSVC.cmake (aarch64 vs x86_64)

CI Changes

Add Windows ARM64 matrix entries using windows-11-arm runner
Add ARM64-specific LLVM installation step (manual download since Scoop may not have ARM64 LLVM)
Update Build WebKit step with -Platform parameter
Add Windows ARM64 artifact download/upload in release job

Testing

This PR needs to be tested on Windows ARM64 hardware via CI.

robobun · 2026-01-16T09:48:20Z

/build-preview

github-actions · 2026-01-16T09:48:31Z

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This commit adds Windows ARM64 (WoA) support to JavaScriptCore. Source code changes: - Add _M_ARM64 MSVC macro to CPU detection (PlatformCPU.h, BPlatform.h) - Limit SYSV_ABI to x86_64 only on Windows (6 files) - Expand LinkRecord::CopyTypes for Windows and add FlushInstructionCache to cacheFlush (ARM64Assembler.h) - Add Windows ARM64 DECLARE_CALL_FRAME macro with x29 clobber (CallFrame.h) - Disable USE_BUILTIN_FRAME_ADDRESS for Windows ARM64 (PlatformUse.h) - Add Windows ARM64 register mappings to MachineContext.h (Sp, Fp, Pc, X1, X19, X4) - Exclude .previous directive for Windows (MacroAssemblerARM64.cpp) - Add Windows ARM64 currentStackPointer implementation (StackPointer.cpp) - Disable inline ASM unaryDoubleOpWrapper for Windows ARM64 (ThunkGenerators.cpp) - Add Windows ARM64 COFF-style addressing to globaladdr (arm64.rb) - Add architecture detection for clang_rt.builtins (OptionsMSVC.cmake) CI changes: - Add Windows ARM64 matrix entries (windows-11-arm runner) - Add ARM64-specific LLVM installation step - Update Build WebKit step with Platform parameter - Add Windows ARM64 artifact download/upload in release job

robobun · 2026-01-16T09:50:02Z

/build-preview

github-actions · 2026-01-16T09:50:19Z

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github-actions · 2026-01-16T09:50:29Z

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github-actions · 2026-01-16T09:51:46Z

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github-actions · 2026-01-16T10:27:29Z

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Use the official LLVM releases URL instead of aspect-build/llvm-project which doesn't have Windows ARM64 (woa64) builds. The correct URL format is: https://github.com/llvm/llvm-project/releases/download/llvmorg-<version>/LLVM-<version>-woa64.exe Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

github-actions · 2026-01-16T11:02:36Z

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The ICU project files specify Windows SDK 10.0.16299.0 which isn't available on the Windows ARM64 GitHub Actions runner. Add /p:WindowsTargetPlatformVersion=10.0 to MSBuild calls to let it pick the latest available SDK version. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

github-actions · 2026-01-16T11:35:51Z

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The VS Developer Shell adds VS's bundled x64 LLVM tools to PATH before our explicitly installed ARM64 LLVM. This caused cmake to use the x64 clang-cl which generates x64 code, but the ARM64 libraries were being linked, causing architecture mismatch errors. For ARM64 builds, explicitly specify the full path to the ARM64 LLVM toolchain we installed (C:/LLVM/bin/clang-cl.exe and lld-link.exe). Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

github-actions · 2026-01-16T12:09:56Z

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LLVM 19 has a bug in the AArch64 backend that causes SEH unwind info errors when compiling JSC on Windows ARM64: "fatal error: error in backend: Failed to evaluate function length in SEH unwind info" LLVM 21.1.8 has better Windows ARM64 support and likely fixes this issue. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

github-actions · 2026-01-16T12:44:21Z

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Use /clang: prefix instead of -Xclang when passing Clang-specific flags to clang-cl. The /clang: prefix is the proper clang-cl syntax for passing flags directly to the Clang frontend.

github-actions · 2026-01-16T12:55:06Z

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github-actions · 2026-01-16T13:27:27Z

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Add -mllvm -align-loops=1 flag for Windows ARM64 builds to work around the LLVM bug llvm/llvm-project#47432 which causes 'Failed to evaluate function length in SEH unwind info' errors when inline assembly contains alignment directives. This is a known issue that affects all current LLVM versions on Windows ARM64. The workaround disables loop alignment which prevents the compiler from generating the problematic alignment directives in inline assembly.

github-actions · 2026-01-16T14:03:42Z

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The -mllvm -align-loops option was removed in LLVM 21. Use the official Clang frontend -falign-loops=1 flag instead.

github-actions · 2026-01-16T14:39:09Z

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LLVM has a bug (llvm/llvm-project#47432) on Windows ARM64 that causes a fatal error 'Failed to evaluate function length in SEH unwind info' when inline assembly contains alignment directives like .balign. The probe trampoline in MacroAssemblerARM64.cpp uses .balign 16 for function alignment, which triggers this bug. Since the probe functionality is primarily used for debugging/profiling and is not critical for normal operation, we disable it on Windows ARM64 by wrapping the inline assembly and probe() function in a conditional compilation guard. A stub implementation is provided that triggers RELEASE_ASSERT if probe is ever called on Windows ARM64, ensuring any code paths that rely on it will be caught during development.

github-actions · 2026-01-16T15:18:57Z

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LLVM has a bug (llvm/llvm-project#47432) on Windows ARM64 that causes a fatal error 'Failed to evaluate function length in SEH unwind info' when inline assembly contains alignment directives like .balign/.balignl. The LowLevelInterpreter uses alignment directives for performance optimization (better instruction cache usage), but they are not required for correctness. On Windows ARM64, we disable these directives to work around the LLVM bug.

github-actions · 2026-01-16T15:55:11Z

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LLVM has a bug (llvm/llvm-project#47432) that causes a fatal error "Failed to evaluate function length in SEH unwind info" when inline assembly contains alignment directives like .p2align on Windows ARM64. The gcSafeZeroMemory function has ARM64-specific inline assembly with .p2align 4, which triggers this bug. Fall back to the generic scalar implementation on Windows ARM64.

github-actions · 2026-01-16T16:46:48Z

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Release: autobuild-preview-pr-138-a5af1deb

coderabbitai · 2026-01-16T19:50:54Z

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The pull request is closed.

Walkthrough

Adds Windows ARM64 support across the WebKit build infrastructure, including platform-specific CI workflows, assembly handling, calling convention adjustments, and toolchain configuration while maintaining x86_64 compatibility.

Changes

Cohort / File(s)	Summary
CI/CD Workflow Configuration `.github/workflows/build-reusable.yml`, `windows-release.ps1`	Introduces ARM64 build matrix entries and platform-specific LLVM/Ninja installation steps; updates MSBuild invocations with explicit Windows SDK version and ARM64-aware toolchain paths; adds ARM64 artifacts to release handling.
WTF Platform Abstractions `Source/WTF/wtf/PlatformCPU.h`, `Source/WTF/wtf/PlatformCallingConventions.h`, `Source/WTF/wtf/PlatformUse.h`	Expands ARM64 CPU detection to recognize MSVC's `_M_ARM64` macro; restricts SYSV_ABI calling convention to Windows x86_64 only; excludes Windows ARM64 from BUILTIN_FRAME_ADDRESS support.
Code Pointer and Function Traits `Source/WTF/wtf/CodePtr.h`, `Source/WTF/wtf/FunctionPtr.h`, `Source/WTF/wtf/FunctionTraits.h`	Narrows SYSV_ABI function pointer constructor overloads to Windows x86_64 only, removing them from Windows ARM64 builds.
Stack Pointer and Memory Allocator `Source/WTF/wtf/StackPointer.cpp`, `Source/bmalloc/bmalloc/BPlatform.h`	Adds ARM64 Windows inline-assembly implementation for stack pointer retrieval; extends ARM64 CPU detection macro for MSVC builds.
ARM64 Assembler and JIT Core `Source/JavaScriptCore/assembler/ARM64Assembler.h`, `Source/JavaScriptCore/assembler/AbstractMacroAssembler.h`, `Source/JavaScriptCore/assembler/MacroAssemblerARM64.cpp`	Extends CopyTypes struct for Windows ARM64; adjusts size assertion logic; adds cache flush support for Windows; disables probe trampoline generation on Windows ARM64 with stub implementation; restricts x86_64-specific constructor to that architecture.
Instruction Generation and Lowering `Source/JavaScriptCore/jit/ThunkGenerators.cpp`, `Source/JavaScriptCore/llint/LowLevelInterpreter.cpp`, `Source/JavaScriptCore/offlineasm/arm64.rb`	Excludes Windows ARM64 from ARM64 inline assembly wrapper path; disables alignment directives on Windows ARM64; adds COFF-style addressing for ARM64 globaladdr lowering.
Call Frame and Machine Context `Source/JavaScriptCore/interpreter/CallFrame.h`, `Source/JavaScriptCore/runtime/MachineContext.h`	Adds ARM64-specific DECLARE_CALL_FRAME implementation using `_AddressOfReturnAddress()` on Windows; extends ARM64 register mappings for stack, frame, instruction, and argument pointers.
Memory Operations `Source/JavaScriptCore/heap/GCMemoryOperations.h`	Prevents ARM64 inline assembly with alignment directives on Windows due to LLVM bug; falls back to scalar code path.
CMake Configuration `Source/cmake/OptionsMSVC.cmake`	Introduces architecture-aware Clang builtins library lookup; selects appropriate architecture variant based on CMAKE_SYSTEM_PROCESSOR.

Possibly related PRs

Replace Cygwin-based ICU build with MSBuild on Windows #136: Updates Windows build pipeline configuration and windows-release.ps1 with Platform parameter handling and toolchain modifications.
Fix Windows CI: Use pre-installed 7zip on GHA runners #127: Modifies Windows LLVM/Ninja installation procedures in CI workflows affecting build dependencies.

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robobun force-pushed the claude/windows-arm64-support branch from 8d20fd8 to a48085e Compare January 16, 2026 09:49

Fix -fno-c++-static-destructors flag syntax for clang-cl

6a30e7b

Use /clang: prefix instead of -Xclang when passing Clang-specific flags to clang-cl. The /clang: prefix is the proper clang-cl syntax for passing flags directly to the Clang frontend.

Use -falign-loops=1 frontend flag instead of -mllvm backend flag

a2e398e

The -mllvm -align-loops option was removed in LLVM 21. Use the official Clang frontend -falign-loops=1 flag instead.

Claude Bot added 2 commits January 16, 2026 14:46

Remove -falign-loops workaround since SEH bug is fixed in source code

3ba6d90

Jarred-Sumner marked this pull request as ready for review January 16, 2026 19:41

Jarred-Sumner merged commit c4efd9c into main Jan 16, 2026
30 checks passed

Jarred-Sumner deleted the claude/windows-arm64-support branch January 16, 2026 19:42

coderabbitai Bot mentioned this pull request Jan 18, 2026

Fix Windows ARM64 alignment directive issue with -fno-unwind-tables #140

Merged

3 tasks

Jarred-Sumner mentioned this pull request Jan 19, 2026

Support Windows ARM64 oven-sh/bun#9824

Closed

This was referenced Jan 21, 2026

fix: use unsigned bitfields for portable bytecode cache struct layout #141

Merged

fix libpas assertion on windows #147

Merged

Conversation

sosukesuzuki commented Jan 16, 2026

Summary

Source Code Changes

CPU Detection

Calling Conventions

ARM64 Assembler

Frame Pointer Handling

Register Mappings

Assembler Directives

JIT/Offlineasm

Build System

CI Changes

Testing

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