[llvm] [GlobalIsel] Combine select to integer minmax. (PR #77213)

[Matt Arsenault 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;
+
+  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}))
----------------
arsenm wrote:

I think we need a smarter set of legality predicates. It also makes sense if we're going to legalize this type to a type where it's going to be legal. Doesn't really need to be part of this patch, it's an existing issue scattered around 

https://github.com/llvm/llvm-project/pull/77213