[InstCombine] Do not keep samesign when speculatively executing icmps (#127007)

Closes #126974.

Author: dtcxzyw

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -5637,6 +5637,11 @@ Instruction *InstCombinerImpl::foldICmpWithMinMax(Instruction &I,
       return false;
     return std::nullopt;
   };
+  // Remove samesign here since it is illegal to keep it when we speculatively
+  // execute comparisons. For example, `icmp samesign ult umax(X, -46), -32`
+  // cannot be decomposed into `(icmp samesign ult X, -46) or (icmp samesign ult
+  // -46, -32)`. `X` is allowed to be non-negative here.
+  Pred = static_cast<CmpInst::Predicate>(Pred);
   auto CmpXZ = IsCondKnownTrue(simplifyICmpInst(Pred, X, Z, Q));
   auto CmpYZ = IsCondKnownTrue(simplifyICmpInst(Pred, Y, Z, Q));
   if (!CmpXZ.has_value() && !CmpYZ.has_value())

llvm/test/Transforms/InstCombine/umax-icmp.ll

@@ -804,4 +804,28 @@ end:
   ret void
 }
 
+define i1 @pr126974(i8 %x) {
+; CHECK-LABEL: @pr126974(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[COND:%.*]] = icmp sgt i8 [[X:%.*]], -2
+; CHECK-NEXT:    br i1 [[COND]], label [[IF_THEN:%.*]], label [[IF_ELSE:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[X]], -1
+; CHECK-NEXT:    ret i1 [[CMP]]
+; CHECK:       if.else:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %cond = icmp sgt i8 %x, -2
+  br i1 %cond, label %if.then, label %if.else
+
+if.then:
+  %umax = call i8 @llvm.umax.i8(i8 %x, i8 -46)
+  %cmp = icmp samesign ult i8 %umax, -32
+  ret i1 %cmp
+
+if.else:
+  ret i1 false
+}
+
 declare i32 @llvm.umax.i32(i32, i32)