InstCombine: Fold X-C1 <u 2 -> (X & -2) == C1

Back in r179493 we determined that two transforms collided with each
other.  The fix back then was to reorder the transforms so that the
preferred transform would give it a try and then we would try the
secondary transform.  However, it was noted that the best approach would
canonicalize one transform into the other, removing the collision and
allowing us to optimize IR given to us in that form.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185808 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 18 insertions, 0 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCompares.cpp b/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 0f1576050f..f4734461a6 100644
--- a/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1539,6 +1539,14 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
                               Builder->getInt(CR.getLower()));
         }
       }
+
+      // X-C1 <u 2 -> (X & -2) == C1
+      //   iff C1 & 1 == 0
+      if (ICI.getPredicate() == ICmpInst::ICMP_ULT && LHSI->hasOneUse() &&
+          LHSV[0] == 0 && RHSV == 2)
+        return new ICmpInst(ICmpInst::ICMP_EQ,
+                            Builder->CreateAnd(LHSI->getOperand(0), -RHSV),
+                            ConstantExpr::getNeg(LHSC));
     }
     break;
   }
diff --git a/test/Transforms/InstCombine/icmp.ll b/test/Transforms/InstCombine/icmp.ll
index 929688195f..cf8305df2a 100644
--- a/test/Transforms/InstCombine/icmp.ll
+++ b/test/Transforms/InstCombine/icmp.ll
@@ -1113,3 +1113,13 @@ define i1 @or_icmp_eq_B_0_icmp_ult_A_B(i64 %a, i64 %b) {
   %3 = or i1 %1, %2
   ret i1 %3
 }
+
+; CHECK: @icmp_add_ult_2
+; CHECK-NEXT: [[AND:%[a-z0-9]+]] = and i32 %X, -2
+; CHECK-NEXT: [[CMP:%[a-z0-9]+]] = icmp eq i32 [[AND]], 14
+; CHECK-NEXT: ret i1 [[CMP]]
+define i1 @icmp_add_ult_2(i32 %X) {
+  %add = add i32 %X, -14
+  %cmp = icmp ult i32 %add, 2
+  ret i1 %cmp
+}