[InstCombine] Handle ssubo always overflow

[nikic]

Summary

Followup to D60483 and D60497: Handle always overflow for ssubo. This it the last case we can handle right now.

[lebedev.ri]

LG

[nikic]

llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp

4016 ↗

(On Diff #194536)

Not related to this patch, but this fold looks buggy to me. X * undef should be {0, false}, as that's the only results that's possible for all values of X (including X=0). IIRC not even undef * undef can result in {undef, undef} for all bitwidths.

[lebedev.ri]

Hmm

llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp

4016 ↗

(On Diff #194536)

----------------------------------------
Name: D60518
  %r = mul i8 %X, undef
=>
  %r = undef

ERROR: Value mismatch for i8 %r

Example:
i8 %X = #x00 (0)
Source value: #x00 (0)
Target value: #x80 (128)


----------------------------------------
Name: D60518
  %r = mul i8 %X, undef
=>
  %r = i8 0

Done: 1
Optimization is correct!

----------------------------------------
Name: D60518
  %r = mul i8 %X, undef
=>
  %r = i8 %X

Done: 1
Optimization is correct!

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

if (match(RHS, m_Zero()))
       return SetResult(LHS, Builder.getFalse(), false);
 
     OverflowResult OR;
     if (OCF == OCF_UNSIGNED_SUB) {
       OR = computeOverflowForUnsignedSub(LHS, RHS, &OrigI);
       if (OR == OverflowResult::NeverOverflows)
         return SetResult(Builder.CreateNUWSub(LHS, RHS), Builder.getFalse(),
                          true);
-
-      if (OR == OverflowResult::AlwaysOverflows)
-        return SetResult(Builder.CreateSub(LHS, RHS), Builder.getTrue(), true);
     } else {
       OR = computeOverflowForSignedSub(LHS, RHS, &OrigI);
       if (OR == OverflowResult::NeverOverflows)
         return SetResult(Builder.CreateNSWSub(LHS, RHS), Builder.getFalse(),
                          true);
     }
+
+    if (OR == OverflowResult::AlwaysOverflows)
+      return SetResult(Builder.CreateSub(LHS, RHS), Builder.getTrue(), true);
     break;
   }
 
   case OCF_UNSIGNED_MUL:
   case OCF_SIGNED_MUL: {
     // X * undef -> undef
     if (isa<UndefValue>(RHS))
       return SetResult(RHS, UndefValue::get(Builder.getInt1Ty()), false);

llvm/test/Transforms/InstCombine/with_overflow.ll

;
   %a = call { i8, i1 } @llvm.sadd.with.overflow.i8(i8 %y, i8 28)
   ret { i8, i1 } %a
 }
 
 define { i8, i1 } @ssub_always_overflow(i8 %x) nounwind {
 ; CHECK-LABEL: @ssub_always_overflow(
 ; CHECK-NEXT:    [[C:%.*]] = icmp sgt i8 [[X:%.*]], 29
 ; CHECK-NEXT:    [[Y:%.*]] = select i1 [[C]], i8 [[X]], i8 29
-; CHECK-NEXT:    [[A:%.*]] = call { i8, i1 } @llvm.ssub.with.overflow.i8(i8 -100, i8 [[Y]])
-; CHECK-NEXT:    ret { i8, i1 } [[A]]
+; CHECK-NEXT:    [[A:%.*]] = sub nuw i8 -100, [[Y]]
+; CHECK-NEXT:    [[TMP1:%.*]] = insertvalue { i8, i1 } { i8 undef, i1 true }, i8 [[A]], 0
+; CHECK-NEXT:    ret { i8, i1 } [[TMP1]]
 ;
   %c = icmp sgt i8 %x, 29
   %y = select i1 %c, i8 %x, i8 29
   %a = call { i8, i1 } @llvm.ssub.with.overflow.i8(i8 -100, i8 %y)
   ret { i8, i1 } %a
 }
 
 define { i8, i1 } @smul_always_overflow(i8 %x) nounwind {