[clang] [llvm] [AArch64] Add -mlr-for-calls-only to replace the now removed -ffixed-x30 flag. (PR #98073)

[Amara Emerson via cfe-commits]

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@@ -3267,10 +3267,13 @@ bool AArch64FrameLowering::spillCalleeSavedRegisters(
         InsertSEH(MIB, TII, MachineInstr::FrameSetup);
     } else { // The code when the pair of ZReg is not present
       MachineInstrBuilder MIB = BuildMI(MBB, MI, DL, TII.get(StrOpc));
-      if (!MRI.isReserved(Reg1))
+      const AArch64RegisterInfo *RegInfo =
+          static_cast<const AArch64RegisterInfo *>(
+              MF.getSubtarget().getRegisterInfo());
+      if (!RegInfo->isStrictlyReservedReg(MF, Reg1))
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aemerson wrote:

> We could unreserve LR after register allocation, I guess? That would have roughly the right semantics.

Having `getReservedRegs(MF)` return different things depending on where it's running seems worse than the bug itself.

With shrink-wrapping the prolog can be in successor blocks, so LR can be live on some CFG paths and not others.

https://github.com/llvm/llvm-project/pull/98073