[PATCH] D154206: [InstCombine] Fold comparison of usub.sat

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

Could you please add alive2 proofs for these transforms?



================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:3440
+          Pred == ICmpInst::ICMP_EQ ? ICmpInst::ICMP_UGE : ICmpInst::ICMP_ULT;
+      if (match(II1, m_Constant()))
+        return BinaryOperator::CreateAnd(
----------------
m_ImmConstant


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:3672
+    Value *II0 = II->getOperand(0);
+    Value *II1 = II->getOperand(1);
+    if (match(II1, m_Constant())) {
----------------
Add comment explaining the transform, something like this:
```
//    usub.sat(X, C) pred C2
// -> (X < C ? 0 : X - C) pred C2
// -> X >= C && (X - C) pred C2 (if 0 pred C2 is false)
// -> X < C || (X - C) pred C2 (if 0 pred C2 is true)
```


================
Comment at: llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp:3682
+          Builder.CreateICmp(Pred, Builder.CreateSub(II0, II1), Builder.getInt(C)));
+    }
+    break;
----------------
Some of these cases will end up producing 2 instructions (add + icmp), so I think we should add a one-use limitation on the intrinsic.


================
Comment at: llvm/test/Transforms/InstCombine/icmp-usub-sat.ll:120
+
+declare i8 @llvm.usub.sat.i8(i8, i8)
----------------
Add a multi-use and vector test.


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  https://reviews.llvm.org/D154206/new/

https://reviews.llvm.org/D154206