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Fix offset used to read the second part of scalar pairs from a const - #159148

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scottmcm:fix-159116
Jul 12, 2026
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Fix offset used to read the second part of scalar pairs from a const#159148
rust-bors[bot] merged 3 commits into
rust-lang:mainfrom
scottmcm:fix-159116

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@scottmcmscottmcm commented Jul 11, 2026•
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@rustbotrustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 11, 2026
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scottmcm
scottmcm commented Jul 11, 2026
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Comment threadcompiler/rustc_codegen_ssa/src/mir/operand.rs
} => {
let (a_size, b_size) = (a.size(bx), b.size(bx));
assert!(b_offset.bytes() > 0);
let alloc_b_offset = offset + local_b_offset;

@scottmcmscottmcmJul 11, 2026•
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Hooray for over 20 year old techniques.

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@workingjubileeworkingjubileeJul 11, 2026

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whoops.

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Ahh yeah, that was easy to miss unfortunately. Needed to look closer I guess, if I wanted to make sure I got it. But yeah, important to just not make things look the same if they aren't.

@scottmcmscottmcmJul 11, 2026

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Yeah. Super easy when scanning to do the "well obviously this b_offset is exactly the same as the literally dozens of other b_offset variables I'm touching".

Especially when I'm not trying to change anything so am more inclined to trust tests passing that it's right -- but apparently we didn't have any coverage for this block with offset > 0.

scottmcm
scottmcm commented Jul 11, 2026
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Comment threadcompiler/rustc_codegen_ssa/src/mir/operand.rs
let (a_size, b_size) = (a.size(bx), b.size(bx));
assert!(b_offset.bytes() > 0);
let alloc_b_offset = offset + local_b_offset;
assert!(alloc_b_offset.bytes() > 0);

@scottmcmscottmcmJul 11, 2026•
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It's not obvious to me what the intent of this assert! is, but this is what it was checking before #158666

rust/compiler/rustc_codegen_ssa/src/mir/operand.rs

Lines 230 to 231 in b0cbd79

let b_offset = (offset + a_size).align_to(b.default_align(bx).abi);
assert!(b_offset.bytes() > 0);

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RalfJung
RalfJung reviewed Jul 11, 2026
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Comment threadcompiler/rustc_codegen_ssa/src/mir/operand.rs
Comment on lines 235 to 240
let a_val = read_scalar(
offset,
a_size,
a,
bx.scalar_pair_element_backend_type(layout, 0, true),
);

@RalfJungRalfJungJul 11, 2026•
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We do have much nicer APIs for reading stuff from a const Allocation... but they need an InterpCx. Those are not very expensive to set up though. Maybe codegen should have one and then use the nicer APIs?

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@scottmcmscottmcmJul 11, 2026•
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Can you start a zulip thread and ping me there, Ralf? I started #t-compiler/const-eval > InterpCx in codegen @ 💬

It absolutely sounds interesting, but I'd not want to do it in this PR since I'd rather get back to correct with a simple PR, then do the more-churn-for-better-maintainability separately.

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Oh yeah definitely not for this PR, this was more of a "in case you are interested to clean this up further".

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@workingjubilee

workingjubilee commented Jul 11, 2026

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@bors r+

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📌 Commit fb07057 has been approved by workingjubilee

It is now in the queue for this repository.

@rust-borsrust-borsBot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 11, 2026
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@bors p=1 we really need to not miscompile this.

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rust-borsBot pushed a commit that referenced this pull request Jul 11, 2026
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Auto merge of#159148- scottmcm:fix-159116, r=workingjubilee
8799719
Fix offset used to read the second part of scalar pairs from a `const`
Fixes#159116
r? @workingjubilee
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💔 Test for 8799719 failed: CI. Failed job:

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Spurious.

@bors retry

@rust-borsrust-borsBot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 12, 2026
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rust-borsBot pushed a commit that referenced this pull request Jul 12, 2026
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Auto merge of#159148- scottmcm:fix-159116, r=workingjubilee
769702c
Fix offset used to read the second part of scalar pairs from a `const`
Fixes#159116
r? @workingjubilee
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@rust-borsrust-borsBot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jul 12, 2026
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💔 Test for 769702c failed: CI. Failed job:

@scottmcm
Use smaller scalars in the test and force optimizations on
d693dfe
Otherwise 32-bit has a different ABI and the `CHECK` fails.
And the noopt job fails without forcing MIR optimization here.
@rust-borsrust-borsBot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 12, 2026
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@bors try jobs=x86_64-gnu-nopt

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rust-borsBot pushed a commit that referenced this pull request Jul 12, 2026
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Auto merge of#159148- scottmcm:fix-159116, r=<try>
ab24c36
Fix offset used to read the second part of scalar pairs from a `const`
try-job: x86_64-gnu-nopt
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Ok, the codegen tests passed so the -O addition was enough.

@bors r=workingjubilee

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📌 Commit d693dfe has been approved by workingjubilee

It is now in the queue for this repository.

@rust-borsrust-borsBot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 12, 2026
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workingjubilee commented Jul 12, 2026

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scribbles a note

codegen PR (review?) checklist:

  • new test? set compile flags explicitly for optimization

(probably should actually add that to the rustc devguide... )

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☀️ Try build successful (CI)
Build commit: ab24c36 (ab24c36adcef556fbefb53379f052eb3c1317e23)
Base parent: be8e824 (be8e82435eb04fbe75ed5286b52735366e160bed)

@JonathanBrouwerJonathanBrouwer mentioned this pull request Jul 12, 2026
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rust-borsBot pushed a commit that referenced this pull request Jul 12, 2026
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Auto merge of#159170- JonathanBrouwer:rollup-UxyQ0Ig, r=JonathanBro…
170e61f
…uwer
Rollup of 3 pull requests
Successful merges:
- #159148 (Fix offset used to read the second part of scalar pairs from a `const`)
- #159167 (c-variadic: Update cfg for 128-bit integers to fix build error on AArch64 ILP32)
- #159153 (Rename `errors.rs` file to `diagnostics.rs` (12/N))
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⌛ Testing commit d693dfe with merge 5d7b931...

Workflow: https://github.com/rust-lang/rust/actions/runs/29187429671

rust-borsBot pushed a commit that referenced this pull request Jul 12, 2026
@bors
Auto merge of#159148- scottmcm:fix-159116, r=workingjubilee
5d7b931
Fix offset used to read the second part of scalar pairs from a `const`
Fixes#159116
r? @workingjubilee
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@bors yield
Yielding to enclosing rollup

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Auto build was cancelled. Cancelled workflows:

The next pull request likely to be tested is #159170.

rust-borsBot pushed a commit that referenced this pull request Jul 12, 2026
@bors
Auto merge of#159170- JonathanBrouwer:rollup-UxyQ0Ig, r=JonathanBro…
babf6ea
…uwer
Rollup of 3 pull requests
Successful merges:
- #159148 (Fix offset used to read the second part of scalar pairs from a `const`)
- #159167 (c-variadic: Update cfg for 128-bit integers to fix build error on AArch64 ILP32)
- #159153 (Rename `errors.rs` file to `diagnostics.rs` (12/N))
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rust-borsBot merged commit 3ab3767 into rust-lang:mainJul 12, 2026
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rust-timer added a commit that referenced this pull request Jul 12, 2026
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Unrolled build for#159148
a242f50
Rollup merge of #159148 - scottmcm:fix-159116, r=workingjubilee
Fix offset used to read the second part of scalar pairs from a `const`
Fixes#159116
r? @workingjubilee
@rustbotrustbot added this to the 1.99.0 milestone Jul 12, 2026
@scottmcm
scottmcm deleted the fix-159116 branch July 13, 2026 15:06
@jieyouxujieyouxu mentioned this pull request Jul 16, 2026
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