[llvm] Reland [InstCombine] Teach foldSelectOpOp about samesign (PR #124320)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Inter-diff:

```diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index d5d9a829c306..635782407704 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -428,16 +428,18 @@ Instruction *InstCombinerImpl::foldSelectOpOp(SelectInst &SI, Instruction *TI,
     CmpPredicate TPred, FPred;
     if (match(TI, m_ICmp(TPred, m_Value(), m_Value())) &&
         match(FI, m_ICmp(FPred, m_Value(), m_Value()))) {
-      bool Swapped = ICmpInst::isRelational(FPred) &&
-                     CmpPredicate::getMatching(
+      auto P = CmpPredicate::getMatching(TPred, FPred);
+      bool Swapped = !P;
+      if (Swapped)
+        P = CmpPredicate::getMatching(
             TPred, ICmpInst::getSwappedCmpPredicate(FPred));
-      if (CmpPredicate::getMatching(TPred, FPred) || Swapped) {
+      if (P) {
         if (Value *MatchOp =
-                getCommonOp(TI, FI, ICmpInst::isEquality(TPred), Swapped)) {
+                getCommonOp(TI, FI, ICmpInst::isEquality(*P), Swapped)) {
           Value *NewSel = Builder.CreateSelect(Cond, OtherOpT, OtherOpF,
                                                SI.getName() + ".v", &SI);
           return new ICmpInst(
-              MatchIsOpZero ? TPred : ICmpInst::getSwappedCmpPredicate(TPred),
+              MatchIsOpZero ? *P : ICmpInst::getSwappedCmpPredicate(*P),
               MatchOp, NewSel);
         }
       }
diff --git a/llvm/test/Transforms/InstCombine/select-cmp.ll b/llvm/test/Transforms/InstCombine/select-cmp.ll
index 7e5d5821d9f6..b1bd7a0ecc8a 100644
--- a/llvm/test/Transforms/InstCombine/select-cmp.ll
+++ b/llvm/test/Transforms/InstCombine/select-cmp.ll
@@ -161,7 +161,7 @@ define i1 @icmp_slt_common(i1 %c, i6 %x, i6 %y, i6 %z) {
 define i1 @icmp_slt_samesign_common(i1 %c, i6 %x, i6 %y, i6 %z) {
 ; CHECK-LABEL: @icmp_slt_samesign_common(
 ; CHECK-NEXT:    [[R_V:%.*]] = select i1 [[C:%.*]], i6 [[Y:%.*]], i6 [[Z:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ult i6 [[X:%.*]], [[R_V]]
+; CHECK-NEXT:    [[R:%.*]] = icmp slt i6 [[X:%.*]], [[R_V]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %cmp1 = icmp samesign ult i6 %x, %y
@@ -185,7 +185,7 @@ define i1 @icmp_sgt_common(i1 %c, i6 %x, i6 %y, i6 %z) {
 define i1 @icmp_sgt_samesign_common(i1 %c, i6 %x, i6 %y, i6 %z) {
 ; CHECK-LABEL: @icmp_sgt_samesign_common(
 ; CHECK-NEXT:    [[R_V:%.*]] = select i1 [[C:%.*]], i6 [[Y:%.*]], i6 [[Z:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ugt i6 [[X:%.*]], [[R_V]]
+; CHECK-NEXT:    [[R:%.*]] = icmp sgt i6 [[X:%.*]], [[R_V]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %cmp1 = icmp samesign ugt i6 %x, %y
@@ -257,7 +257,7 @@ define i1 @icmp_slt_sgt_common(i1 %c, i6 %x, i6 %y, i6 %z) {
 define i1 @icmp_slt_sgt_samesign_common(i1 %c, i6 %x, i6 %y, i6 %z) {
 ; CHECK-LABEL: @icmp_slt_sgt_samesign_common(
 ; CHECK-NEXT:    [[R_V:%.*]] = select i1 [[C:%.*]], i6 [[Y:%.*]], i6 [[Z:%.*]]
-; CHECK-NEXT:    [[R:%.*]] = icmp ult i6 [[X:%.*]], [[R_V]]
+; CHECK-NEXT:    [[R:%.*]] = icmp slt i6 [[X:%.*]], [[R_V]]
 ; CHECK-NEXT:    ret i1 [[R]]
 ;
   %cmp1 = icmp samesign ult i6 %x, %y
@@ -796,5 +796,17 @@ define i1 @sel_icmp_cmp_and_no_simplify_comm(i1 %c, i32 %a1, i32 %a2, i8 %b) {
   ret i1 %cmp
 }

+define i1 @icmp_lt_slt(i1 %c, i32 %arg) {
+; CHECK-LABEL: @icmp_lt_slt(
+; CHECK-NEXT:    [[SELECT_V:%.*]] = select i1 [[C:%.*]], i32 131072, i32 0
+; CHECK-NEXT:    [[SELECT:%.*]] = icmp slt i32 [[ARG:%.*]], [[SELECT_V]]
+; CHECK-NEXT:    ret i1 [[SELECT]]
+;
+  %cmp1 = icmp samesign ult i32 %arg, 131072
+  %cmp2 = icmp slt i32 %arg, 0
+  %select = select i1 %c, i1 %cmp1, i1 %cmp2
+  ret i1 %select
+}
+
 declare void @use(i1)
 declare void @use.i8(i8)
```

https://github.com/llvm/llvm-project/pull/124320