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Allow vscr to be marked as a clobber in PowerPC inline asm #148663

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Allow vscr to be marked as a clobber in PowerPC inline asm #148663

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@pmur
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@pmur pmur commented Nov 7, 2025

I don't think gcc or llvm do anything with vscr, but they do allow it to be marked as a clobber. This prevents needing to enumerate it as a preserved flag if an update does make use of it.

ELFv2 defines this as a "limited-access" register. This means the VSCR[SAT] bit is clobbered by a function call.

r? @Amanieu

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Allow vscr to be marked as a clobber in PowerPC inline asm
2b1a424 
ELFv2 defines this as a "limited-access" register. This means the
VSCR[SAT] bit is clobbered by a function call.
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rustbot commented Nov 7, 2025

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

@rustbot rustbot added A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. 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 Nov 7, 2025
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rust-log-analyzer commented Nov 7, 2025

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
spellchecking files
building external tool typos from package typos-cli@1.38.1
finished building tool typos
npm WARN deprecated puppeteer@22.15.0: < 24.10.2 is no longer supported
npm ERR! code 127
npm ERR! git dep preparation failed
npm ERR! command /node/bin/node /node/lib/node_modules/npm/bin/npm-cli.js install --force --cache=/home/user/.npm --prefer-offline=false --prefer-online=false --offline=false --no-progress --no-save --no-audit --include=dev --include=peer --include=optional --no-package-lock-only --no-dry-run
npm ERR! npm WARN using --force Recommended protections disabled.
npm ERR! npm WARN deprecated inflight@1.0.6: This module is not supported, and leaks memory. Do not use it. Check out lru-cache if you want a good and tested way to coalesce async requests by a key value, which is much more comprehensive and powerful.
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated readdir-scoped-modules@1.1.0: This functionality has been moved to @npmcli/fs
npm ERR! npm WARN deprecated debuglog@1.0.1: Package no longer supported. Contact Support at https://www.npmjs.com/support for more info.
npm ERR! npm WARN deprecated read-package-json@2.1.2: This package is no longer supported. Please use @npmcli/package-json instead.
npm ERR! npm WARN deprecated read-installed@4.0.3: This package is no longer supported.
npm ERR! npm ERR! code 127
npm ERR! npm ERR! path /home/user/.npm/_cacache/tmp/git-cloneXXXXXXDVR1XF/node_modules/rollup
npm ERR! npm ERR! command failed
npm ERR! npm ERR! command sh -c patch-package
npm ERR! npm ERR! sh: 1: patch-package: not found
npm ERR! 
npm ERR! npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T18_10_00_257Z-debug-0.log

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T18_09_52_993Z-debug-0.log
npm install did not exit successfully
tidy [extra_checks]: IO error: npm install returned exit code exit status: 127
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:58
  local time: Fri Nov  7 18:10:18 UTC 2025
  network time: Fri, 07 Nov 2025 18:10:18 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

@Amanieu
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Amanieu commented Nov 8, 2025

I don't think this is the correct approach, and I think this should be handled via preserves_flags instead. The main reason for this is that the different bits in both fpscr and vscr have different behavior with regards to the ABI.

Some bits are treated as global state:

  • FPSCR exception enable bits
  • FPSCR rounding mode bits
  • VSCR NJ bit

Some bits are treated as volatile state:

  • FPSCR exception status bits
  • VSCR SAT bit

At the very least we would need separate clobbers for the different types of flags, but I feel that here it's better to stay consistent with other architectures and put all of these under preserves_flags.

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A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. 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.

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