[ARM] Clarify legal addressing modes for ARM and Thumb2. NFC

The existing code is very clever, but not clear, which seems like the wrong tradeoff here. Differential Revision: https://reviews.llvm.org/D36559 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310653 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Eli Friedman

lib/Target/ARM/ARMISelLowering.cpp

@@ -12357,8 +12357,13 @@ bool ARMTargetLowering::isLegalT2ScaledAddressingMode(const AddrMode &AM,
  Scale = Scale & ~1;
  return Scale == 2 || Scale == 4 || Scale == 8;
  case MVT::i64:
+ // FIXME: What are we trying to model here? ldrd doesn't have an r + r
+ // version in Thumb mode.
  // r + r
- if (((unsigned)AM.HasBaseReg + Scale) <= 2)
+ if (Scale == 1)
+ return true;
+ // r * 2 (this can be lowered to r + r).
+ if (!AM.HasBaseReg && Scale == 2)
  return true;
  return false;
  case MVT::isVoid:
@@ -12416,8 +12421,11 @@ bool ARMTargetLowering::isLegalAddressingMode(const DataLayout &DL,
  return isPowerOf2_32(Scale & ~1);
  case MVT::i16:
  case MVT::i64:
- // r + r
- if (((unsigned)AM.HasBaseReg + Scale) <= 2)
+ // r +/- r
+ if (Scale == 1 || (AM.HasBaseReg && Scale == -1))
+ return true;
+ // r * 2 (this can be lowered to r + r).
+ if (!AM.HasBaseReg && Scale == 2)
  return true;
  return false;