[llvm] [ARM] Honour -mno-movt in stack protector handling (PR #109022)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-arm

Author: Ard Biesheuvel (ardbiesheuvel)

<details>
<summary>Changes</summary>

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.

Suggestions welcome for how/where to add a test case for this.

---
Full diff: https://github.com/llvm/llvm-project/pull/109022.diff


1 Files Affected:

- (modified) llvm/lib/Target/ARM/Thumb2InstrInfo.cpp (+2-1) 


``````````diff
diff --git a/llvm/lib/Target/ARM/Thumb2InstrInfo.cpp b/llvm/lib/Target/ARM/Thumb2InstrInfo.cpp
index d1e07b6703a5e6..ce6bc0681a5028 100644
--- a/llvm/lib/Target/ARM/Thumb2InstrInfo.cpp
+++ b/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);

``````````

</details>


https://github.com/llvm/llvm-project/pull/109022