[InstCombine] Simplify shift-by-sext to shift-by-zext

Summary:
This is valid for any `sext` bitwidth pair:
```
Processing /tmp/opt.ll..

----------------------------------------
  %signed = sext %y
  %r = shl %x, %signed
  ret %r
=>
  %unsigned = zext %y
  %r = shl %x, %unsigned
  ret %r
  %signed = sext %y

Done: 2016
Optimization is correct!
```

(This isn't so for funnel shifts, there it's illegal for e.g. i6->i7.)

Main motivation is the C++ semantics:
```
int shl(int a, char b) {
    return a << b;
}
```
ends as
```
  %3 = sext i8 %1 to i32
  %4 = shl i32 %0, %3
```
https://godbolt.org/z/0jgqUq
which is, as this shows, too pessimistic.

There is another problem here - we can only do the fold
if sext is one-use. But we can trivially have cases
where several shifts have the same sext shift amount.
This should be resolved, later.

Reviewers: spatel, nikic, RKSimon

Reviewed By: spatel

Subscribers: efriedma, hiraditya, nlopes, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68103

llvm-svn: 373106

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
index c88827f916c..ab1e484df93 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -241,6 +241,13 @@ Instruction *InstCombiner::commonShiftTransforms(BinaryOperator &I) {
   Value *Op0 = I.getOperand(0), *Op1 = I.getOperand(1);
   assert(Op0->getType() == Op1->getType());
 
+  // If the shift amount is a one-use `sext`, we can demote it to `zext`.
+  Value *Y;
+  if (match(Op1, m_OneUse(m_SExt(m_Value(Y))))) {
+    Value *NewExt = Builder.CreateZExt(Y, I.getType(), Op1->getName());
+    return BinaryOperator::Create(I.getOpcode(), Op0, NewExt);
+  }
+
   // See if we can fold away this shift.
   if (SimplifyDemandedInstructionBits(I))
     return &I;