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

[Ulrich Weigand via llvm-commits]

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@@ -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"))
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uweigand wrote:

Why do we need the `createExternalSymbolName` here?   Shouldn't pointing to a string constant directly be just fine?

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