[llvm] [GlobalIsel] Combine select to integer minmax (second attemnt). (PR #77520)

[Thorsten Schütt via llvm-commits]

================
@@ -6548,6 +6548,87 @@ bool CombinerHelper::tryFoldBoolSelectToLogic(GSelect *Select,
   return false;
 }
 
+bool CombinerHelper::tryFoldSelectToIntMinMax(GSelect *Select,
+                                              BuildFnTy &MatchInfo) {
+  Register DstReg = Select->getReg(0);
+  Register Cond = Select->getCondReg();
+  Register True = Select->getTrueReg();
+  Register False = Select->getFalseReg();
+  LLT DstTy = MRI.getType(DstReg);
+
+  // We need an G_ICMP on the condition register.
+  GICmp *Cmp = getOpcodeDef<GICmp>(Cond, MRI);
+  if (!Cmp)
+    return false;
+
+  // We want to fold the icmp and replace the select.
+  if (!MRI.hasOneNonDBGUse(Cmp->getReg(0)))
+    return false;
+
+  CmpInst::Predicate Pred = Cmp->getCond();
+  // We need a larger or smaller predicate for
+  // canonicalization.
+  if (CmpInst::isEquality(Pred))
+    return false;
+
+  Register CmpLHS = Cmp->getLHSReg();
+  Register CmpRHS = Cmp->getRHSReg();
+
+  // We can swap CmpLHS and CmpRHS for higher hitrate.
+  if (True == CmpRHS && False == CmpLHS) {
+    std::swap(CmpLHS, CmpRHS);
+    Pred = CmpInst::getSwappedPredicate(Pred);
+  }
+
+  // (icmp X, Y) ? X : Y -> integer minmax.
+  // see matchSelectPattern in ValueTracking.
+  // Legality between G_SELECT and integer minmax can differ.
+  if (True == CmpLHS && False == CmpRHS) {
+    switch (Pred) {
+    case ICmpInst::ICMP_UGT:
+    case ICmpInst::ICMP_UGE: {
+      if (!isLegalOrBeforeLegalizer({TargetOpcode::G_UMAX, DstTy}))
+        return false;
+      MatchInfo = [=](MachineIRBuilder &B) {
+        B.buildUMax(DstReg, True, False);
+      };
+      return true;
+    }
+    case ICmpInst::ICMP_SGT:
+    case ICmpInst::ICMP_SGE: {
+      if (!isLegalOrBeforeLegalizer({TargetOpcode::G_SMAX, DstTy}))
----------------
tschuett wrote:

The API works for combining selects. The result has the same type as the select. It fails for e.g. combining and/or of 2 icmps into one icmp.
https://github.com/llvm/llvm-project/blob/fa9284589f111cfd3614a75bfbe0709db39a8f15/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp#L1264

https://github.com/llvm/llvm-project/pull/77520