[PATCH] InstCombineCompare with constant return false if we know it is never going to be equal

[suyog]

Hi David,

Thanks for your valuable comment. 
As per your suggestions, i have now moved this code to InstSimplify. Also added code for ICMP_NE + test cases for both icmp instructions.

I will try to come up with logic for other icmp instructions in subsequent patches. I have for now added a TODO item for it. 

Please help in reviewing the updated patch if it looks good to you.

http://reviews.llvm.org/D3868

Files:
  lib/Analysis/InstructionSimplify.cpp
  test/Transforms/InstSimplify/compare.ll

Index: lib/Analysis/InstructionSimplify.cpp
===================================================================
--- lib/Analysis/InstructionSimplify.cpp
+++ lib/Analysis/InstructionSimplify.cpp
@@ -1990,6 +1990,31 @@
     }
   }
 
+  // (icmp eq (a * ~1) 1) -> will always evaluate to false
+  // TODO: generalize this for all constants whose trailing zero's are less
+  // than known trailing zero's of LHS
+  if (match(RHS, m_One())) {
+    if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
+      uint32_t BitWidth = CI->getBitWidth();
+      APInt LHSKnownZero(BitWidth, 0, 1);
+      APInt LHSKnownOne(BitWidth, 0, 1);
+      computeKnownBits(LHS, LHSKnownZero, LHSKnownOne);
+      switch (Pred) {
+      // TODO: Handle for other icmp instructions.
+      default:
+        break;
+      case ICmpInst::ICMP_EQ:
+        if (LHSKnownZero.countTrailingOnes() >= 1)
+          return ConstantInt::getFalse(CI->getContext());
+        break;
+      case ICmpInst::ICMP_NE:
+        if (LHSKnownZero.countTrailingOnes() >= 1)
+          return ConstantInt::getTrue(CI->getContext());
+        break;
+      }
+    }
+  }
+
   // See if we are doing a comparison with a constant integer.
   if (ConstantInt *CI = dyn_cast<ConstantInt>(RHS)) {
     // Rule out tautological comparisons (eg., ult 0 or uge 0).
Index: test/Transforms/InstSimplify/compare.ll
===================================================================
--- test/Transforms/InstSimplify/compare.ll
+++ test/Transforms/InstSimplify/compare.ll
@@ -817,3 +817,28 @@
 ; CHECK-LABEL: @compare_always_false_ne
 ; CHECK-NEXT: ret i1 true
 }
+
+ at a = common global i32 0, align 4
+ at b = common global i32 0, align 4
+
+; CHECK-LABEL: @icmp_eq_const
+; CHECK-NEXT: store i32 0, i32* @b
+define void @icmp_eq_const() nounwind {
+  %1 = load i32* @a, align 4
+  %2 = mul nsw i32 %1, -2
+  %3 = icmp eq i32 %2, 1
+  %4 = zext i1 %3 to i32
+  store i32 %4, i32* @b, align 4
+  ret void
+}
+
+; CHECK-LABEL: @icmp_ne_const
+; CHECK-NEXT: store i32 1, i32* @b
+define void @icmp_ne_const() nounwind {
+  %1 = load i32* @a, align 4
+  %2 = mul nsw i32 %1, -2
+  %3 = icmp ne i32 %2, 1
+  %4 = zext i1 %3 to i32
+  store i32 %4, i32* @b, align 4
+  ret void
+}
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