[llvm] [SelectionDAG] Treat CopyFromReg as freezing the value (PR #85932)

[Björn Pettersson via llvm-commits]

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@@ -202,13 +202,27 @@ define <4 x i32> @freeze_add_vec(<4 x i32> %a0) nounwind {
 define <4 x i32> @freeze_add_vec_undef(<4 x i32> %a0) nounwind {
 ; X86-LABEL: freeze_add_vec_undef:
 ; X86:       # %bb.0:
+; X86-NEXT:    pushl %ebp
+; X86-NEXT:    movl %esp, %ebp
+; X86-NEXT:    andl $-16, %esp
+; X86-NEXT:    subl $32, %esp
+; X86-NEXT:    movl %eax, {{[0-9]+}}(%esp)
+; X86-NEXT:    movl $3, {{[0-9]+}}(%esp)
+; X86-NEXT:    movl $2, {{[0-9]+}}(%esp)
+; X86-NEXT:    movl $1, (%esp)
+; X86-NEXT:    paddd (%esp), %xmm0
 ; X86-NEXT:    paddd {{\.?LCPI[0-9]+_[0-9]+}}, %xmm0
-; X86-NEXT:    paddd {{\.?LCPI[0-9]+_[0-9]+}}, %xmm0
+; X86-NEXT:    movl %ebp, %esp
+; X86-NEXT:    popl %ebp
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bjope wrote:

Seems like it can be avoided by a patch like this (and suddenly there are floats in the constant pool, but the asm instructions no longer diff):
```
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 35f756ea5e1d..d7539b2d942d 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -8771,7 +8771,7 @@ X86TargetLowering::LowerBUILD_VECTOR(SDValue Op, SelectionDAG &DAG) const {
   // and blend the FREEZE-UNDEF operands back in.
   // FIXME: is this worthwhile even for a single FREEZE-UNDEF operand?
   if (unsigned NumFrozenUndefElts = FrozenUndefMask.popcount();
-      NumFrozenUndefElts >= 2 && NumFrozenUndefElts < NumElems) {
+      NumFrozenUndefElts >= 1 && NumFrozenUndefElts < NumElems) {
     SmallVector<int, 16> BlendMask(NumElems, -1);
     SmallVector<SDValue, 16> Elts(NumElems, DAG.getUNDEF(OpEltVT));
     for (unsigned i = 0; i < NumElems; ++i) {

```

https://github.com/llvm/llvm-project/pull/85932