[PATCH] [InstCombine/InstSimplify] Move ashr optimization from Instcombine to Instsimplify

[suyog]

Hi Nick,

Added supporting testcase.

Please if you could commit it for me if it looks 
good to you.

Thanks,
Suyog

http://reviews.llvm.org/D4102

Files:
  lib/Analysis/InstructionSimplify.cpp
  lib/Transforms/InstCombine/InstCombineShifts.cpp
  test/Transforms/InstSimplify/ashr-nop.ll

Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -1436,6 +1436,11 @@
       cast<OverflowingBinaryOperator>(Op0)->hasNoSignedWrap())
     return X;
 
+  // Arithmetic shifting an all-sign-bit value is a no-op.
+  unsigned NumSignBits = ComputeNumSignBits(Op0);
+  if (NumSignBits == Op0->getType()->getScalarSizeInBits())
+    return Op0;
+
   return nullptr;
 }
 
Index: lib/Transforms/InstCombine/InstCombineShifts.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineShifts.cpp
+++ lib/Transforms/InstCombine/InstCombineShifts.cpp
@@ -820,10 +820,5 @@
                         APInt::getSignBit(I.getType()->getScalarSizeInBits())))
     return BinaryOperator::CreateLShr(Op0, Op1);
 
-  // Arithmetic shifting an all-sign-bit value is a no-op.
-  unsigned NumSignBits = ComputeNumSignBits(Op0);
-  if (NumSignBits == Op0->getType()->getScalarSizeInBits())
-    return ReplaceInstUsesWith(I, Op0);
-
   return nullptr;
 }
Index: test/Transforms/InstSimplify/ashr-nop.ll
===================================================================
--- test/Transforms/InstSimplify/ashr-nop.ll
+++ test/Transforms/InstSimplify/ashr-nop.ll
@@ -0,0 +1,8 @@
+; RUN: opt < %s -instsimplify -S | not grep ashr
+
+define i32 @foo(i32 %x) {
+  %o = and i32 %x, 1
+  %n = add i32 %o, -1
+  %t = ashr i32 %n, 17
+  ret i32 %t
+}
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