https://bugs.llvm.org/show_bug.cgi?id=46319
Bug ID: 46319
Summary: LLVM does not see that two writes are independent.
Product: clang
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
Assignee: unassignedclangb...@nondot.org
Reporter: denis.yaroshevs...@gmail.com
CC: llvm-bugs@lists.llvm.org, neeil...@live.com,
richard-l...@metafoo.co.uk
See this part of a simd optimized routine that doubles every element in an
array.
https://godbolt.org/z/SLp3Ec
Even though clang can in both cases successfully see that all 4 reads are
independent:
```
vmovdqa ymm0, ymmword ptr [rdi]
vmovdqa ymm1, ymmword ptr [rdi + 32]
vmovdqa ymm2, ymmword ptr [rdi + 64]
vmovdqa ymm3, ymmword ptr [rdi + 96]
```
It introduces dependencies between independent computations by default:
```
vpaddd ymm0, ymm0, ymm0
vmovdqa ymmword ptr [rdi], ymm0
vpaddd ymm0, ymm1, ymm1
vmovdqa ymmword ptr [rdi + 32], ymm0
...
```
While I suspect it should be just `loads, adds, stores`
This is also what clang generates when auto-vectorizing this:
https://godbolt.org/z/ik55Yv
NOTE: on my measurements hacking around this didn't have an effect, probably
the processor could figure it out on it's own. However, this is not ideal and I
can imagine you can encounter a situation when this will backfire.
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