[ARM] Mark -1 as cheap in xor's for thumb1

We can always convert xor %a, -1 into MVN, even in thumb 1 where the -1
would not otherwise be considered a cheap constant. This prevents the
-1's from being pulled out into constants and potentially hoisted.

Differential Revision: https://reviews.llvm.org/D43451

llvm-svn: 325573

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
index 43d7888075b..02c6eb925ca 100644
--- a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -126,6 +126,13 @@ int ARMTTIImpl::getIntImmCost(unsigned Opcode, unsigned Idx, const APInt &Imm,
       return 0;
   }
 
+  // xor a, -1 can always be folded to MVN
+  if (Opcode == Instruction::Xor) {
+    int64_t NegImm = Imm.getSExtValue();
+    if (NegImm == -1)
+      return 0;
+  }
+
   return getIntImmCost(Imm, Ty);
 }