[ARM] Honour -mno-movt in stack protector handling

When -mno-movt is passed to Clang, the ARM codegen correctly avoids
movt/movw pairs to take the address of __stack_chk_guard in the stack
protector code emitted into the function pro- and epilogues. However,
the Thumb2 codegen fails to do so, and happily emits movw/movt pairs
unless it is generating an ELF binary and the symbol might be in a
different DSO. Let's incorporate a check for useMovt() in the logic
here, so movt/movw are never emitted when -mno-movt is specified.

Signed-off-by: Ard Biesheuvel <[email protected]>

Author: ardbiesheuvel

llvm/lib/Target/ARM/Thumb2InstrInfo.cpp

@@ -264,7 +264,8 @@ void Thumb2InstrInfo::expandLoadStackGuard(
   }
 
   const auto *GV = cast<GlobalValue>((*MI->memoperands_begin())->getValue());
-  if (MF.getSubtarget<ARMSubtarget>().isTargetELF() && !GV->isDSOLocal())
+  const ARMSubtarget &Subtarget = MF.getSubtarget<ARMSubtarget>();
+  if ((Subtarget.isTargetELF() && !GV->isDSOLocal()) || !Subtarget.useMovt())
     expandLoadStackGuardBase(MI, ARM::t2LDRLIT_ga_pcrel, ARM::t2LDRi12);
   else if (MF.getTarget().isPositionIndependent())
     expandLoadStackGuardBase(MI, ARM::t2MOV_ga_pcrel, ARM::t2LDRi12);