[llvm] [DAGCombiner] Freeze maybe poison operands when folding select to logic (PR #84924)

[Björn Pettersson via llvm-commits]

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@@ -229,10 +229,41 @@ define i32 @combine_pmaddubsw_constant_sat() {
 
 ; Constant folding PMADDWD was causing an infinite loop in the PCMPGT commuting between 2 constant values.
 define i1 @pmaddwd_pcmpgt_infinite_loop() {
-; CHECK-LABEL: pmaddwd_pcmpgt_infinite_loop:
-; CHECK:       # %bb.0:
-; CHECK-NEXT:    movb $1, %al
-; CHECK-NEXT:    retq
+; SSE-LABEL: pmaddwd_pcmpgt_infinite_loop:
----------------
bjope wrote:

Looks like we get:
```
   t3: v8i16 = BUILD_VECTOR Constant:i16<-32768>, Constant:i16<-32768>, Constant:i16<-32768>, Constant:i16<-32768>, Constant:i16<-32768>, Constant:i16<-32768>, Constant:i16<-32768>, Constant:i16<-32768>
   t4: v4i32 = llvm.x86.sse2.pmadd.wd TargetConstant:i64<13393>, t3, t3
   t62: v4i32 = freeze t4
```

X86 can perhaps model that intrinsic as not creating poison. But a more general solution could be to let SelectionDAG::computeKnownBits look through FREEZE (and then I guess the same should be done for SelectionDAG::ComputeNumSignBits as well).


https://github.com/llvm/llvm-project/pull/84924