[all-commits] [llvm/llvm-project] c66476: [AssumeBundles] offset should be added to correctl...

[Juneyoung Lee via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: c6647693300be4b74575143db7429f284f3afeb1
      https://github.com/llvm/llvm-project/commit/c6647693300be4b74575143db7429f284f3afeb1
  Author: Juneyoung Lee <aqjune at gmail.com>
  Date:   2021-04-02 (Fri, 02 Apr 2021)

  Changed paths:
    M llvm/lib/Transforms/Scalar/AlignmentFromAssumptions.cpp
    M llvm/test/Transforms/AlignmentFromAssumptions/simple.ll
    M llvm/test/Transforms/AlignmentFromAssumptions/simple32.ll

  Log Message:
  -----------
  [AssumeBundles] offset should be added to correctly calculate align

This is a patch to fix the bug in alignment calculation (see https://reviews.llvm.org/D90529#2619492).

Consider this code:

```
call void @llvm.assume(i1 true) ["align"(i32* %a, i32 32, i32 28)]
%arrayidx = getelementptr inbounds i32, i32* %a, i64 -1
; aligment of %arrayidx?
```

The llvm.assume guarantees that `%a - 28` is 32-bytes aligned, meaning that `%a` is 32k + 28 for some k.
Therefore `a - 4` cannot be 32-bytes aligned but the existing code was calculating the pointer as 32-bytes aligned.

The reason why this happened is as follows.
`DiffSCEV` stores `%arrayidx - %a` which is -4.
`OffSCEV` stores the offset value of “align”, which is 28.
`DiffSCEV` + `OffSCEV` = 24 should be used for `a - 4`'s offset from 32k, but `DiffSCEV` - `OffSCEV` = 32 was being used instead.

Reviewed By: Tyker

Differential Revision: https://reviews.llvm.org/D98759