[InstSimplify] Simplify uadd/sadd/umul/smul with overflow intrinsics when the Zero or Undef is on the LHS.

[craig.topper]

Summary

This code was migrated from InstCombine a few years ago. InstCombine had nearby code that would move Constants to the RHS for these, but InstSimplify doesn't have such code on this path.

[spatel]

LGTM.

I acknowledge that there are concerns about division-of-labor between InstSimplify and InstCombine as well as the combination of those two with other passes (see current llvm-dev threads about compile-time), but handling commuted variants of existing patterns is always good (makes the pass behavior more consistent) and cheap (if it's not, then that's an independent problem).

llvm/trunk/lib/Analysis/InstructionSimplify.cpp

case Intrinsic::ssub_with_overflow: {
       if (isa<UndefValue>(LHS) || isa<UndefValue>(RHS))
         return UndefValue::get(ReturnType);
 
       return nullptr;
     }
     case Intrinsic::uadd_with_overflow:
     case Intrinsic::sadd_with_overflow: {
       // X + undef -> undef
-      if (isa<UndefValue>(RHS))
+      if (isa<UndefValue>(LHS) || isa<UndefValue>(RHS))
         return UndefValue::get(ReturnType);
 
       return nullptr;
     }
     case Intrinsic::umul_with_overflow:
     case Intrinsic::smul_with_overflow: {
+      // 0 * X -> { 0, false }
       // X * 0 -> { 0, false }
-      if (match(RHS, m_Zero()))
+      if (match(LHS, m_Zero()) || match(RHS, m_Zero()))
         return Constant::getNullValue(ReturnType);
 
+      // undef * X -> { 0, false }
       // X * undef -> { 0, false }
-      if (match(RHS, m_Undef()))
+      if (match(LHS, m_Undef()) || match(RHS, m_Undef()))
         return Constant::getNullValue(ReturnType);
 
       return nullptr;
     }
     case Intrinsic::load_relative: {
       Constant *C0 = dyn_cast<Constant>(LHS);
       Constant *C1 = dyn_cast<Constant>(RHS);
       if (C0 && C1)

llvm/trunk/test/Transforms/InstSimplify/call.ll

 define {i8, i1} @test_uadd3(i8 %v) {
 ; CHECK-LABEL: @test_uadd3(
 ; CHECK-NEXT:    ret { i8, i1 } undef
 ;
   %result = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 %v, i8 undef)
   ret {i8, i1} %result
 }
 
+define {i8, i1} @test_uadd4(i8 %v) {
+; CHECK-LABEL: @test_uadd4(
+; CHECK-NEXT:    ret { i8, i1 } undef
+;
+  %result = call {i8, i1} @llvm.uadd.with.overflow.i8(i8 undef, i8 %v)
+  ret {i8, i1} %result
+}
+
 define i1 @test_sadd1() {
 ; CHECK-LABEL: @test_sadd1(
 ; CHECK-NEXT:    ret i1 true
 ;
   %x = call {i8, i1} @llvm.sadd.with.overflow.i8(i8 126, i8 3)
   %overflow = extractvalue {i8, i1} %x, 1
   ret i1 %overflow
 }

 define {i8, i1} @test_sadd3(i8 %v) {
 ; CHECK-LABEL: @test_sadd3(
 ; CHECK-NEXT:    ret { i8, i1 } undef
 ;
   %result = call {i8, i1} @llvm.sadd.with.overflow.i8(i8 %v, i8 undef)
   ret {i8, i1} %result
 }
 
+define {i8, i1} @test_sadd4(i8 %v) {
+; CHECK-LABEL: @test_sadd4(
+; CHECK-NEXT:    ret { i8, i1 } undef
+;
+  %result = call {i8, i1} @llvm.sadd.with.overflow.i8(i8 undef, i8 %v)
+  ret {i8, i1} %result
+}
+
 define {i8, i1} @test_usub1(i8 %V) {
 ; CHECK-LABEL: @test_usub1(
 ; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
 ;
   %x = call {i8, i1} @llvm.usub.with.overflow.i8(i8 %V, i8 %V)
   ret {i8, i1} %x
 }
 

 define {i8, i1} @test_umul2(i8 %V) {
 ; CHECK-LABEL: @test_umul2(
 ; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
 ;
   %x = call {i8, i1} @llvm.umul.with.overflow.i8(i8 %V, i8 undef)
   ret {i8, i1} %x
 }
 
+define {i8, i1} @test_umul3(i8 %V) {
+; CHECK-LABEL: @test_umul3(
+; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
+;
+  %x = call {i8, i1} @llvm.umul.with.overflow.i8(i8 0, i8 %V)
+  ret {i8, i1} %x
+}
+
+define {i8, i1} @test_umul4(i8 %V) {
+; CHECK-LABEL: @test_umul4(
+; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
+;
+  %x = call {i8, i1} @llvm.umul.with.overflow.i8(i8 undef, i8 %V)
+  ret {i8, i1} %x
+}
+
 define {i8, i1} @test_smul1(i8 %V) {
 ; CHECK-LABEL: @test_smul1(
 ; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
 ;
   %x = call {i8, i1} @llvm.smul.with.overflow.i8(i8 %V, i8 0)
   ret {i8, i1} %x
 }
 
 define {i8, i1} @test_smul2(i8 %V) {
 ; CHECK-LABEL: @test_smul2(
 ; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
 ;
   %x = call {i8, i1} @llvm.smul.with.overflow.i8(i8 %V, i8 undef)
   ret {i8, i1} %x
 }
 
+define {i8, i1} @test_smul3(i8 %V) {
+; CHECK-LABEL: @test_smul3(
+; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
+;
+  %x = call {i8, i1} @llvm.smul.with.overflow.i8(i8 0, i8 %V)
+  ret {i8, i1} %x
+}
+
+define {i8, i1} @test_smul4(i8 %V) {
+; CHECK-LABEL: @test_smul4(
+; CHECK-NEXT:    ret { i8, i1 } zeroinitializer
+;
+  %x = call {i8, i1} @llvm.smul.with.overflow.i8(i8 undef, i8 %V)
+  ret {i8, i1} %x
+}
+
 declare i256 @llvm.cttz.i256(i256 %src, i1 %is_zero_undef)
 
 define i256 @test_cttz() {
 ; CHECK-LABEL: @test_cttz(
 ; CHECK-NEXT:    ret i256 1
 ;
   %x = call i256 @llvm.cttz.i256(i256 10, i1 false)
   ret i256 %x