[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

llvm-svn: 310653

1 files changed, 11 insertions, 3 deletions

diff --git a/llvm/lib/Target/ARM/ARMISelLowering.cpp b/llvm/lib/Target/ARM/ARMISelLowering.cpp
index 9746490988e..753008588fc 100644
--- a/llvm/lib/Target/ARM/ARMISelLowering.cpp
+++ b/llvm/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;