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[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
This commit is contained in:
Eli Friedman 2017-08-10 19:31:27 +00:00
parent dbbe5756c1
commit e9499dd4c7
1 changed files with 11 additions and 3 deletions
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14
llvm/lib/Target/ARM/ARMISelLowering.cpp
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@ -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;