[InstCombine] foldICmpBinOp(): consider inverted check in 'unsigned sub overflow' check
A follow-up for r329011.
This may be changed to produce @llvm.sub.with.overflow in a later patch,
but for now just make things more consistent overall.
A few observations stem from this:
* There does not seem to be a similar one-instruction fold for uadd-overflow
* I'm not sure we'll want to canonicalize `B u> A` as `usub.with.overflow`,
so since the `icmp` here no longer refers to `sub`,
reconstructing `usub.with.overflow` will be problematic,
and will likely require standalone pass (similar to DivRemPairs).
https://rise4fun.com/Alive/Zqs
Name: (A - B) u> A --> B u> A
%t0 = sub i8 %A, %B
%r = icmp ugt i8 %t0, %A
=>
%r = icmp ugt i8 %B, %A
Name: (A - B) u<= A --> B u<= A
%t0 = sub i8 %A, %B
%r = icmp ule i8 %t0, %A
=>
%r = icmp ule i8 %B, %A
Name: C u< (C - D) --> C u< D
%t0 = sub i8 %C, %D
%r = icmp ult i8 %C, %t0
=>
%r = icmp ult i8 %C, %D
Name: C u>= (C - D) --> C u>= D
%t0 = sub i8 %C, %D
%r = icmp uge i8 %C, %t0
=>
%r = icmp uge i8 %C, %D
llvm-svn: 371101
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 65c0d2d..cef8e05 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3791,12 +3791,13 @@
if (C == Op0 && NoOp1WrapProblem)
return new ICmpInst(Pred, D, Constant::getNullValue(Op0->getType()));
- // (A - B) >u A --> A <u B
- if (A == Op1 && Pred == ICmpInst::ICMP_UGT)
- return new ICmpInst(ICmpInst::ICMP_ULT, A, B);
- // C <u (C - D) --> C <u D
- if (C == Op0 && Pred == ICmpInst::ICMP_ULT)
- return new ICmpInst(ICmpInst::ICMP_ULT, C, D);
+ // Convert sub-with-unsigned-overflow comparisons into a comparison of args.
+ // (A - B) u>/u<= A --> B u>/u<= A
+ if (A == Op1 && (Pred == ICmpInst::ICMP_UGT || Pred == ICmpInst::ICMP_ULE))
+ return new ICmpInst(Pred, B, A);
+ // C u</u>= (C - D) --> C u</u>= D
+ if (C == Op0 && (Pred == ICmpInst::ICMP_ULT || Pred == ICmpInst::ICMP_UGE))
+ return new ICmpInst(Pred, C, D);
// icmp (Y-X), (Z-X) -> icmp Y, Z for equalities or if there is no overflow.
if (B && D && B == D && NoOp0WrapProblem && NoOp1WrapProblem &&