[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).

1 files changed, 0 insertions, 5 deletions

diff --git a/llvm/lib/IR/ConstantFold.cpp b/llvm/lib/IR/ConstantFold.cpp
index 07dfbc4..c2780fa 100644
--- a/llvm/lib/IR/ConstantFold.cpp
+++ b/llvm/lib/IR/ConstantFold.cpp
@@ -902,11 +902,6 @@ Constant *llvm::ConstantFoldBinaryInstruction(unsigned Opcode, Constant *C1,
         Constant *ExtractIdx = ConstantInt::get(Ty, i);
         Constant *LHS = ConstantExpr::getExtractElement(C1, ExtractIdx);
         Constant *RHS = ConstantExpr::getExtractElement(C2, ExtractIdx);
-
-        // If any element of a divisor vector is zero, the whole op is poison.
-        if (Instruction::isIntDivRem(Opcode) && RHS->isNullValue())
-          return PoisonValue::get(VTy);
-
         Constant *Res = ConstantExpr::isDesirableBinOp(Opcode)
                             ? ConstantExpr::get(Opcode, LHS, RHS)
                             : ConstantFoldBinaryInstruction(Opcode, LHS, RHS);