Emit cold inline asm marker for cold_path intrinsic in LLVM backend#155272
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Redslayer112 wants to merge 1 commit intorust-lang:mainfrom
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Emit cold inline asm marker for cold_path intrinsic in LLVM backend#155272Redslayer112 wants to merge 1 commit intorust-lang:mainfrom
Redslayer112 wants to merge 1 commit intorust-lang:mainfrom
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The cold_path intrinsic was a no-op in the SSA layer, which meant LLVM never saw any cold hint in blocks reached through generic FnOnce::call_once boundaries. The !prof metadata emitted by find_cold_blocks was dropped by LLVM's InlinerPass when folding redundant PHI+branch patterns. This patch removes the SSA-level no-op for cold_path and adds an LLVM-specific handler that emits "asm sideeffect" with a "cold" call-site attribute. The sideeffect flag prevents any LLVM pass from removing the marker, while the cold attribute lets BranchProbabilityInfo mark the containing block as unlikely. The empty asm string produces zero machine instructions. Fixes rust-lang#155263
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jieyouxu
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| sym::cold_path => { | ||
| // This is a no-op. The intrinsic is just a hint to the optimizer. | ||
| return Ok(()); | ||
| } |
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I am closing this as per rust-lang/compiler-team#893.
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The cold_path intrinsic was a no-op in the SSA layer, which meant LLVM never saw any cold hint in blocks reached through generic FnOnce::call_once boundaries. The !prof metadata emitted by find_cold_blocks was dropped by LLVM's InlinerPass when folding redundant PHI+branch patterns.
This patch removes the SSA-level no-op for cold_path and adds an LLVM-specific handler that emits "asm sideeffect" with a "cold" call-site attribute. The sideeffect flag prevents any LLVM pass from removing the marker, while the cold attribute lets BranchProbabilityInfo mark the containing block as unlikely. The empty asm string produces zero machine instructions.
Fixes #155263
Testing
tests/codegen-llvm/hint/cold_path_propagation.rsregression testcold_path.rstest continues to passset_cold_blockhandler)