[InstCombine] add (sext i1 X), 1 --> zext (not X)

http://rise4fun.com/Alive/i8Q

A narrow bitwise logic op is obviously better than math for value tracking, 
and zext is better than sext. Typically, the 'not' will be folded into an 
icmp predicate.

The IR difference would even survive through codegen for x86, so we would see 
worse code:

https://godbolt.org/g/C14HMF

one_or_zero(int, int):                      # @one_or_zero(int, int)
        xorl    %eax, %eax
        cmpl    %esi, %edi
        setle   %al
        retq

one_or_zero_alt(int, int):                  # @one_or_zero_alt(int, int)
        xorl    %ecx, %ecx
        cmpl    %esi, %edi
        setg    %cl
        movl    $1, %eax
        subl    %ecx, %eax
        retq

llvm-svn: 306243

1 files changed, 18 insertions, 9 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
index 287a5167fe2..d5f0dd19141 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp
@@ -988,15 +988,24 @@ static Instruction *foldAddWithConstant(BinaryOperator &Add,
     return new ZExtInst(Builder.CreateNUWAdd(X, NewC), Ty);
   }
 
-  // Shifts and add used to flip and mask off the low bit:
-  // add (ashr (shl i32 X, 31), 31), 1 --> and (not X), 1
-  const APInt *C3;
-  if (C->isOneValue() &&
-      match(Op0,
-            m_OneUse(m_AShr(m_Shl(m_Value(X), m_APInt(C2)), m_APInt(C3)))) &&
-      C2 == C3 && *C2 == Ty->getScalarSizeInBits() - 1) {
-    Value *NotX = Builder.CreateNot(X);
-    return BinaryOperator::CreateAnd(NotX, ConstantInt::get(Ty, 1));
+  if (C->isOneValue() && Op0->hasOneUse()) {
+    // add (sext i1 X), 1 --> zext (not X)
+    // TODO: The smallest IR representation is (select X, 0, 1), and that would
+    // not require the one-use check. But we need to remove a transform in
+    // visitSelect and make sure that IR value tracking for select is equal or
+    // better than for these ops.
+    if (match(Op0, m_SExt(m_Value(X))) &&
+        X->getType()->getScalarSizeInBits() == 1)
+      return new ZExtInst(Builder.CreateNot(X), Ty);
+
+    // Shifts and add used to flip and mask off the low bit:
+    // add (ashr (shl i32 X, 31), 31), 1 --> and (not X), 1
+    const APInt *C3;
+    if (match(Op0, m_AShr(m_Shl(m_Value(X), m_APInt(C2)), m_APInt(C3))) &&
+        C2 == C3 && *C2 == Ty->getScalarSizeInBits() - 1) {
+      Value *NotX = Builder.CreateNot(X);
+      return BinaryOperator::CreateAnd(NotX, ConstantInt::get(Ty, 1));
+    }
   }
 
   return nullptr;