[llvm-bugs] [Bug 34106] New: ARMTargetLowering::isLegalAddressingMode can accept incorrect addressing modes for Thumb1 target

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=34106

            Bug ID: 34106
           Summary: ARMTargetLowering::isLegalAddressingMode can accept
                    incorrect addressing modes for Thumb1 target
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: eastig at yandex.ru
                CC: llvm-bugs at lists.llvm.org

This bug report is a result of investigation of performance regressions on
Cortex-M0+ and Cortex-M23 triggered by the changes:
https://reviews.llvm.org/D34583

See a discussion thread
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20170731/475927.html
for more information about the investigation.

The current code of ARMTargetLowering::isLegalAddressingMode:

bool ARMTargetLowering::isLegalAddressingMode(const DataLayout &DL,
                                              const AddrMode &AM, Type *Ty,
                                              unsigned AS, Instruction *I)
const {
  EVT VT = getValueType(DL, Ty, true);
  if (!isLegalAddressImmediate(AM.BaseOffs, VT, Subtarget))
    return false;

  // Can never fold addr of global into load/store.
  if (AM.BaseGV)
    return false;

  switch (AM.Scale) {
  case 0:  // no scale reg, must be "r+i" or "r", or "i".
    break;
  case 1:
    if (Subtarget->isThumb1Only())
      return false;
    LLVM_FALLTHROUGH;
  default:
    // ARM doesn't support any R+R*scale+imm addr modes.
    if (AM.BaseOffs)
      return false;

    if (!VT.isSimple())
      return false;

    if (Subtarget->isThumb2())
      return isLegalT2ScaledAddressingMode(AM, VT);


For the Thumb1 target the code returns false when AM.Scale equals 1. It returns
true when AM.Scale equals 4, for example. AM.Scale == 1 which actually means no
scaling so the Thumb1 target can accept it. Other Scale values can be accepted
by the Thumb1 target because its addressing modes do not support scaling.

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