[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

diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index 9746490988e99f460f6848516437a5c0d847fc20..753008588fc851dafc15c8273797a4ca3a2032df 100644 (file)
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/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;