[llvm] [InstCombine] Improve select equiv fold for plain condition (PR #83405)

[via llvm-commits]

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@@ -500,6 +500,44 @@ static bool isSelect01(const APInt &C1I, const APInt &C2I) {
   return C1I.isOne() || C1I.isAllOnes() || C2I.isOne() || C2I.isAllOnes();
 }
 
+/// Try to simplify a select instruction when the user of its select user
+/// indicates the condition.
+static bool simplifySeqSelectWithSameCond(SelectInst &SI,
+                                          const SimplifyQuery &SQ,
+                                          InstCombinerImpl &IC) {
+  Value *CondVal = SI.getCondition();
+  if (match(CondVal, m_ImmConstant()))
+    return false;
+
+  auto trySimplifySeqSelect = [=, &SI, &IC](unsigned OpIndex) {
+    assert((OpIndex == 1 || OpIndex == 2) && "Unexpected operand index");
+    SelectInst *SINext = &SI;
+    Value *ValOp = SINext->getOperand(OpIndex);
+    Value *CondNext;
+    while (match(ValOp, m_Select(m_Value(CondNext), m_Value(), m_Value()))) {
+      if (CondNext == CondVal) {
+        IC.replaceOperand(*SINext, OpIndex,
----------------
vfdff wrote:

I think we don't need support the case with multi-arms because it may be not profitable  to build a new select
```
define i32 @sequence_select_with_same_cond_extra_use(i1 %c1, i1 %c2){
; CHECK-LABEL: @sequence_select_with_same_cond_extra_use(
; CHECK-NEXT:    [[S1:%.*]] = select i1 [[C1:%.*]], i32 23, i32 45
; CHECK-NEXT:    [[S2_NEW:%.*]] = select i1 [[C2:%.*]], i32 666, i32 45
; CHECK-NEXT:    [[S2:%.*]] = select i1 [[C2]], i32 666, i32 [[S1]]
; CHECK-NEXT:    call void @use32(i32 [[S2]])
; CHECK-NEXT:    [[S3:%.*]] = select i1 [[C1]], i32 789, i32 [[S2_NEW]]
; CHECK-NEXT:    ret i32 [[S3]]
;
  %s1 = select i1 %c1, i32 23, i32 45
  %s2 = select i1 %c2, i32 666, i32 %s1
  call void @use32(i32 %s2)
  %s3 = select i1 %c1, i32 789, i32 %s2
  ret i32 %s3
}
```

https://github.com/llvm/llvm-project/pull/83405