tree f263a5994ba8d6b3428d8bed7aa8ad9ee804a6e5
parent ceefd2339402e0cb651596dd16f50c0c46c80c44
author Yingwei Zheng <dtcxzyw2333@gmail.com> 1704026688 +0800
committer Copybara-Service <copybara-worker@google.com> 1704026729 -0800

[FuncAttrs] Deduce `noundef` attributes for return values (#76553)

This patch deduces `noundef` attributes for return values.
IIUC, a function returns `noundef` values iff all of its return values
are guaranteed not to be `undef` or `poison`.
Definition of `noundef` from LangRef:
```
noundef
This attribute applies to parameters and return values. If the value representation contains any
undefined or poison bits, the behavior is undefined. Note that this does not refer to padding
introduced by the type’s storage representation.
```
Alive2: https://alive2.llvm.org/ce/z/g8Eis6

Compile-time impact: http://llvm-compile-time-tracker.com/compare.php?from=30dcc33c4ea3ab50397a7adbe85fe977d4a400bd&to=c5e8738d4bfbf1e97e3f455fded90b791f223d74&stat=instructions:u
|stage1-O3|stage1-ReleaseThinLTO|stage1-ReleaseLTO-g|stage1-O0-g|stage2-O3|stage2-O0-g|stage2-clang|
|--|--|--|--|--|--|--|
|+0.01%|+0.01%|-0.01%|+0.01%|+0.03%|-0.04%|+0.01%|

The motivation of this patch is to reduce the number of `freeze` insts
and enable more optimizations.

GitOrigin-RevId: 1228becf7df28c68579f2b9b390b74aa41149a0a