[InstCombine] Consolidate another fold into select value equivalence

[nikic]

We had a separate fold that handled just the trivial case where we're replacing exactly the argument of the select. Handle this in select value equivalence by relaxing the infinite loop protection to allow a replacement of a non-constant with a constant.

This also fixes #113301, as the separate fold did not handle undef values correctly.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Nikita Popov (nikic)

Changes

We had a separate fold that handled just the trivial case where we're replacing exactly the argument of the select. Handle this in select value equivalence by relaxing the infinite loop protection to allow a replacement of a non-constant with a constant.

This also fixes #113301, as the separate fold did not handle undef values correctly.

---

Full diff: https://github.com/llvm/llvm-project/pull/117746.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+1-12)
• (modified) llvm/test/Transforms/InstCombine/select-value-equivalence.ll (+1-2)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index e5525133e5dbb5..245fecb775e8c0 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1348,7 +1348,7 @@ Instruction *InstCombinerImpl::foldSelectValueEquivalence(SelectInst &Sel,
     // If we will be able to evaluate f(Y) to a constant, we can allow undef,
     // otherwise Y cannot be undef as we might pick different values for undef
     // in the cmp and in f(Y).
-    if (TrueVal == OldOp)
+    if (TrueVal == OldOp && (isa<Constant>(OldOp) || !isa<Constant>(NewOp)))
       return nullptr;
 
     if (Value *V = simplifyWithOpReplaced(TrueVal, OldOp, NewOp, SQ,
@@ -1925,17 +1925,6 @@ Instruction *InstCombinerImpl::foldSelectInstWithICmp(SelectInst &SI,
   ICmpInst::Predicate Pred = ICI->getPredicate();
   Value *CmpLHS = ICI->getOperand(0);
   Value *CmpRHS = ICI->getOperand(1);
-  if (CmpRHS != CmpLHS && isa<Constant>(CmpRHS) && !isa<Constant>(CmpLHS)) {
-    if (CmpLHS == TrueVal && Pred == ICmpInst::ICMP_EQ) {
-      // Transform (X == C) ? X : Y -> (X == C) ? C : Y
-      replaceOperand(SI, 1, CmpRHS);
-      Changed = true;
-    } else if (CmpLHS == FalseVal && Pred == ICmpInst::ICMP_NE) {
-      // Transform (X != C) ? Y : X -> (X != C) ? Y : C
-      replaceOperand(SI, 2, CmpRHS);
-      Changed = true;
-    }
-  }
 
   if (Instruction *NewSel = foldSelectICmpEq(SI, ICI, *this))
     return NewSel;
diff --git a/llvm/test/Transforms/InstCombine/select-value-equivalence.ll b/llvm/test/Transforms/InstCombine/select-value-equivalence.ll
index d55766ddf40405..812b4d8f6be793 100644
--- a/llvm/test/Transforms/InstCombine/select-value-equivalence.ll
+++ b/llvm/test/Transforms/InstCombine/select-value-equivalence.ll
@@ -322,12 +322,11 @@ define <2 x i8> @select_vec_op_const_no_undef(<2 x i8> %x) {
   ret <2 x i8> %xr
 }
 
-; FIXME: This is a miscompile.
 define <2 x i8> @select_vec_op_const_undef(<2 x i8> %x) {
 ; CHECK-LABEL: define <2 x i8> @select_vec_op_const_undef(
 ; CHECK-SAME: <2 x i8> [[X:%.*]]) {
 ; CHECK-NEXT:    [[XZ:%.*]] = icmp eq <2 x i8> [[X]], <i8 1, i8 undef>
-; CHECK-NEXT:    [[XR:%.*]] = select <2 x i1> [[XZ]], <2 x i8> <i8 1, i8 undef>, <2 x i8> <i8 4, i8 3>
+; CHECK-NEXT:    [[XR:%.*]] = select <2 x i1> [[XZ]], <2 x i8> [[X]], <2 x i8> <i8 4, i8 3>
 ; CHECK-NEXT:    ret <2 x i8> [[XR]]
 ;
   %xz = icmp eq <2 x i8> %x, <i8 1, i8 undef>

[dtcxzyw]

LGTM.