[InstSimplify] shl nuw C, %x -> C iff signbit is set on C.

Summary:
`%r = shl nuw i8 C, %x`

As per langref:
```
If the nuw keyword is present, then the shift produces
a poison value if it shifts out any non-zero bits.
```
Thus, if the sign bit is set on `C`, then `%x` can only be `0`,
which means that `%r` can only be `C`.
Or in other words, set sign bit means that the signed value
is negative, so the constant is `<= 0`.

https://rise4fun.com/Alive/WMk
https://rise4fun.com/Alive/udv

Was mentioned in D47428 review.

We already handle the `0` constant, https://godbolt.org/g/UZq1sJ, so this only handles negative constants.

Could use computeKnownBits() / LazyValueInfo,
but the cost-benefit analysis (https://reviews.llvm.org/D47891)
suggests it isn't worth it.

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47883

llvm-svn: 334222

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index cdcb81b8060..0b150f86a32 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -1226,6 +1226,13 @@ static Value *SimplifyShlInst(Value *Op0, Value *Op1, bool isNSW, bool isNUW,
   Value *X;
   if (match(Op0, m_Exact(m_Shr(m_Value(X), m_Specific(Op1)))))
     return X;
+
+  // shl nuw i8 C, %x  ->  C  iff C has sign bit set.
+  if (isNUW && match(Op0, m_Negative()))
+    return Op0;
+  // NOTE: could use computeKnownBits() / LazyValueInfo,
+  // but the cost-benefit analysis suggests it isn't worth it.
+
   return nullptr;
 }