[InstCombine] Avoid folding select(umin(X, Y), X) with min/max values in false arm

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -1719,6 +1719,13 @@ Instruction *InstCombinerImpl::FoldOpIntoSelect(Instruction &Op, SelectInst *SI,
   if (SI->getType()->isIntOrIntVectorTy(1))
     return nullptr;
 
+  if (isa<MinMaxIntrinsic>(&Op))
+    for (Value *IntrinOp : Op.operands())
+      if (auto *PN = dyn_cast<PHINode>(IntrinOp))
+        for (Value *PhiOp : PN->operands())
+          if (PhiOp == &Op)
+            return nullptr;
+
   // Test if a FCmpInst instruction is used exclusively by a select as
   // part of a minimum or maximum operation. If so, refrain from doing
   // any other folding. This helps out other analyses which understand

llvm/test/Transforms/InstCombine/select.ll

@@ -5047,3 +5047,18 @@ define <2 x ptr> @select_freeze_constant_expression_vector_gep(i1 %cond, <2 x pt
   %sel = select i1 %cond, <2 x ptr> %y, <2 x ptr> %freeze
   ret <2 x ptr> %sel
 }
+
+; declare i8 @llvm.umin.i8(i8, i8)
+;
+; define i8 @no_fold_masked_min(i8 %acc, i8 %val, i8 %mask) {
+; ; CHECK-LABEL: @no_fold_masked_min(
+; ; CHECK-NEXT:  [[COND:%.*]] = icmp eq i8 [[MASK:%.*]], 0
+; ; CHECK-NEXT:  [[MASKED_VAL:%.*]] = select i1 [[COND:%.*]], i8 [[VAL:%.*]], i8 -1
+; ; CHECK-NEXT:  [[RES:%.*]] = call i8 @llvm.umin.i8(i8 [[ACC:%.*]], i8 [[MASKED_VAL:%.*]])
+; ; CHECK-NEXT:  ret i8 [[RES]]
+; ;
+;   %cond = icmp eq i8 %mask, 0
+;   %masked_val = select i1 %cond, i8 %val, i8 -1
+;   %res = call i8 @llvm.umin.i8(i8 %acc, i8 %masked_val)
+;   ret i8 %res
+; }