[all-commits] [llvm/llvm-project] a77ded: [InstSimplify][InstCombine][ConstantFold] Move vec...

[Yingwei Zheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: a77dedcacb4c5eb221395b69877981dd6ad98989
      https://github.com/llvm/llvm-project/commit/a77dedcacb4c5eb221395b69877981dd6ad98989
  Author: Yingwei Zheng <dtcxzyw2333 at gmail.com>
  Date:   2024-11-01 (Fri, 01 Nov 2024)

  Changed paths:
    M llvm/lib/Analysis/InstructionSimplify.cpp
    M llvm/lib/IR/ConstantFold.cpp
    M llvm/lib/Transforms/InstCombine/InstCombineInternal.h
    M llvm/lib/Transforms/InstCombine/InstCombineMulDivRem.cpp
    M llvm/test/Transforms/InstCombine/div.ll
    M llvm/test/Transforms/InstCombine/rem.ll
    M llvm/test/Transforms/InstCombine/vector-udiv.ll
    M llvm/test/Transforms/InstSimplify/div.ll
    M llvm/test/Transforms/InstSimplify/rem.ll

  Log Message:
  -----------
  [InstSimplify][InstCombine][ConstantFold] Move vector div/rem by zero fold to InstCombine (#114280)

Previously we fold `div/rem X, C` into `poison` if any element of the
constant divisor `C` is zero or undef. However, it is incorrect when
threading udiv over an vector select:
https://alive2.llvm.org/ce/z/3Ninx5
```
define <2 x i32> @vec_select_udiv_poison(<2 x i1> %x) {
  %sel = select <2 x i1> %x, <2 x i32> <i32 -1, i32 -1>, <2 x i32> <i32 0, i32 1>
  %div = udiv <2 x i32> <i32 42, i32 -7>, %sel
  ret <2 x i32> %div
}
```
In this case, `threadBinOpOverSelect` folds `udiv <i32 42, i32 -7>, <i32
-1, i32 -1>` and `udiv <i32 42, i32 -7>, <i32 0, i32 1>` into
`zeroinitializer` and `poison`, respectively. One solution is to
introduce a new flag indicating that we are threading over a vector
select. But it requires to modify both `InstSimplify` and
`ConstantFold`.

However, this optimization doesn't provide benefits to real-world
programs:

https://dtcxzyw.github.io/llvm-opt-benchmark/coverage/data/zyw/opt-ci/actions-runner/_work/llvm-opt-benchmark/llvm-opt-benchmark/llvm/llvm-project/llvm/lib/IR/ConstantFold.cpp.html#L908

https://dtcxzyw.github.io/llvm-opt-benchmark/coverage/data/zyw/opt-ci/actions-runner/_work/llvm-opt-benchmark/llvm-opt-benchmark/llvm/llvm-project/llvm/lib/Analysis/InstructionSimplify.cpp.html#L1107

This patch moves the fold into InstCombine to avoid breaking numerous
existing tests.

Fixes #114191 and #113866 (only poison-safety issue).



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