From 94fc18d0400d0e779226c4ee065032f0e3b25c15 Mon Sep 17 00:00:00 2001
From: Benjamin Kramer <benny.kra@googlemail.com>
Date: Tue, 11 Feb 2014 21:09:03 +0000
Subject: [PATCH] InstCombine: Teach icmp merging about the equivalence of bit
 tests and UGE/ULT with a power of 2.

This happens in bitfield code. While there reorganize the existing code
a bit.

llvm-svn: 201176
---
 .../InstCombine/InstCombineAndOrXor.cpp       | 61 ++++++++++++-------
 .../InstCombine/sign-test-and-or.ll           | 38 ++++++++++++
 2 files changed, 76 insertions(+), 23 deletions(-)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index a2875e2f538e..d768903f5f8c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -503,31 +503,46 @@ static unsigned conjugateICmpMask(unsigned Mask) {
 /// decomposition fails.
 static bool decomposeBitTestICmp(const ICmpInst *I, ICmpInst::Predicate &Pred,
                                  Value *&X, Value *&Y, Value *&Z) {
-  // X < 0 is equivalent to (X & SignBit) != 0.
-  if (I->getPredicate() == ICmpInst::ICMP_SLT)
-    if (ConstantInt *C = dyn_cast<ConstantInt>(I->getOperand(1)))
-      if (C->isZero()) {
-        X = I->getOperand(0);
-        Y = ConstantInt::get(I->getContext(),
-                             APInt::getSignBit(C->getBitWidth()));
-        Pred = ICmpInst::ICMP_NE;
-        Z = C;
-        return true;
-      }
+  ConstantInt *C = dyn_cast<ConstantInt>(I->getOperand(1));
+  if (!C)
+    return false;
 
-  // X > -1 is equivalent to (X & SignBit) == 0.
-  if (I->getPredicate() == ICmpInst::ICMP_SGT)
-    if (ConstantInt *C = dyn_cast<ConstantInt>(I->getOperand(1)))
-      if (C->isAllOnesValue()) {
-        X = I->getOperand(0);
-        Y = ConstantInt::get(I->getContext(),
-                             APInt::getSignBit(C->getBitWidth()));
-        Pred = ICmpInst::ICMP_EQ;
-        Z = ConstantInt::getNullValue(C->getType());
-        return true;
-      }
+  switch (I->getPredicate()) {
+  default:
+    return false;
+  case ICmpInst::ICMP_SLT:
+    // X < 0 is equivalent to (X & SignBit) != 0.
+    if (!C->isZero())
+      return false;
+    Y = ConstantInt::get(I->getContext(), APInt::getSignBit(C->getBitWidth()));
+    Pred = ICmpInst::ICMP_NE;
+    break;
+  case ICmpInst::ICMP_SGT:
+    // X > -1 is equivalent to (X & SignBit) == 0.
+    if (!C->isAllOnesValue())
+      return false;
+    Y = ConstantInt::get(I->getContext(), APInt::getSignBit(C->getBitWidth()));
+    Pred = ICmpInst::ICMP_EQ;
+    break;
+  case ICmpInst::ICMP_ULT:
+    // X <u 2^n is equivalent to (X & ~(2^n-1)) == 0.
+    if (!C->getValue().isPowerOf2())
+      return false;
+    Y = ConstantInt::get(I->getContext(), -C->getValue());
+    Pred = ICmpInst::ICMP_EQ;
+    break;
+  case ICmpInst::ICMP_UGT:
+    // X >u 2^n-1 is equivalent to (X & ~(2^n-1)) != 0.
+    if (!(C->getValue() + 1).isPowerOf2())
+      return false;
+    Y = ConstantInt::get(I->getContext(), ~C->getValue());
+    Pred = ICmpInst::ICMP_NE;
+    break;
+  }
 
-  return false;
+  X = I->getOperand(0);
+  Z = ConstantInt::getNullValue(C->getType());
+  return true;
 }
 
 /// foldLogOpOfMaskedICmpsHelper:
diff --git a/llvm/test/Transforms/InstCombine/sign-test-and-or.ll b/llvm/test/Transforms/InstCombine/sign-test-and-or.ll
index 95ed9b976ba3..aa23d933b09e 100644
--- a/llvm/test/Transforms/InstCombine/sign-test-and-or.ll
+++ b/llvm/test/Transforms/InstCombine/sign-test-and-or.ll
@@ -177,3 +177,41 @@ if.then:
 if.end:
   ret void
 }
+
+define void @test10(i32 %a) nounwind {
+  %1 = and i32 %a, 2
+  %2 = icmp eq i32 %1, 0
+  %3 = icmp ult i32 %a, 4
+  %or.cond = and i1 %2, %3
+  br i1 %or.cond, label %if.then, label %if.end
+
+; CHECK-LABEL: @test10(
+; CHECK-NEXT: %1 = icmp ult i32 %a, 2
+; CHECK-NEXT: br i1 %1, label %if.then, label %if.end
+
+if.then:
+  tail call void @foo() nounwind
+  ret void
+
+if.end:
+  ret void
+}
+
+define void @test11(i32 %a) nounwind {
+  %1 = and i32 %a, 2
+  %2 = icmp ne i32 %1, 0
+  %3 = icmp ugt i32 %a, 3
+  %or.cond = or i1 %2, %3
+  br i1 %or.cond, label %if.then, label %if.end
+
+; CHECK-LABEL: @test11(
+; CHECK-NEXT: %1 = icmp ugt i32 %a, 1
+; CHECK-NEXT: br i1 %1, label %if.then, label %if.end
+
+if.then:
+  tail call void @foo() nounwind
+  ret void
+
+if.end:
+  ret void
+}