[llvm] [SystemZ] Avoid modifying IR in mcount instrumentation. (PR #152298)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-systemz

Author: Dominik Steenken (dominik-steenken)

<details>
<summary>Changes</summary>

This PR changes how the call to `mcount` is inserted in `emitPrologue`. It is now emitted as an external symbol rather than a global variable, preventing potentially unexpected IR modification.

---
Full diff: https://github.com/llvm/llvm-project/pull/152298.diff


1 Files Affected:

- (modified) llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp (+1-3) 


``````````diff
diff --git a/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp b/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
index 6297916310803..ed953fef8c1f1 100644
--- a/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
+++ b/llvm/lib/Target/SystemZ/SystemZFrameLowering.cpp
@@ -574,13 +574,11 @@ void SystemZELFFrameLowering::emitPrologue(MachineFunction &MF,
 
     // Call mcount (Regmask from CC AnyReg since mcount preserves all normal
     // argument registers).
-    FunctionCallee FC = MF.getFunction().getParent()->getOrInsertFunction(
-        "mcount", Type::getVoidTy(MF.getFunction().getContext()));
     const uint32_t *Mask = MF.getSubtarget<SystemZSubtarget>()
                                .getSpecialRegisters()
                                ->getCallPreservedMask(MF, CallingConv::AnyReg);
     BuildMI(MBB, MBBI, DL, ZII->get(SystemZ::CallBRASL))
-        .addGlobalAddress(dyn_cast<Function>(FC.getCallee()))
+        .addExternalSymbol(MF.createExternalSymbolName("mcount"))
         .addRegMask(Mask);
 
     // Reload return address from 8 bytes above stack pointer.

``````````

</details>


https://github.com/llvm/llvm-project/pull/152298