[ARM] Treat cmn immediates as legal in isLegalICmpImmediate.

The original code attempted to do this, but the std::abs() call didn't
actually do anything due to implicit type conversions.  Fix the type
conversions, and perform the correct check for negative immediates.

This probably has very little practical impact, but it's worth fixing
just to avoid confusion in the future, I think.

Differential Revision: https://reviews.llvm.org/D48907

llvm-svn: 336742

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 9b143dcff73..a57bb567b8c 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/lib/Target/ARM/ARMISelLowering.cpp
@@ -3857,8 +3857,8 @@ SDValue ARMTargetLowering::getARMCmp(SDValue LHS, SDValue RHS, ISD::CondCode CC,
                                      const SDLoc &dl) const {
   if (ConstantSDNode *RHSC = dyn_cast<ConstantSDNode>(RHS.getNode())) {
     unsigned C = RHSC->getZExtValue();
-    if (!isLegalICmpImmediate(C)) {
-      // Constant does not fit, try adjusting it by one?
+    if (!isLegalICmpImmediate((int32_t)C)) {
+      // Constant does not fit, try adjusting it by one.
       switch (CC) {
       default: break;
       case ISD::SETLT:
@@ -13208,9 +13208,11 @@ bool ARMTargetLowering::isLegalAddressingMode(const DataLayout &DL,
 bool ARMTargetLowering::isLegalICmpImmediate(int64_t Imm) const {
   // Thumb2 and ARM modes can use cmn for negative immediates.
   if (!Subtarget->isThumb())
-    return ARM_AM::getSOImmVal(std::abs(Imm)) != -1;
+    return ARM_AM::getSOImmVal((uint32_t)Imm) != -1 ||
+           ARM_AM::getSOImmVal(-(uint32_t)Imm) != -1;
   if (Subtarget->isThumb2())
-    return ARM_AM::getT2SOImmVal(std::abs(Imm)) != -1;
+    return ARM_AM::getT2SOImmVal((uint32_t)Imm) != -1 ||
+           ARM_AM::getT2SOImmVal(-(uint32_t)Imm) != -1;
   // Thumb1 doesn't have cmn, and only 8-bit immediates.
   return Imm >= 0 && Imm <= 255;
 }