[llvm] [AArch64][SME] Fix restoring callee-saves from FP with hazard padding (PR #143371)

[Sander de Smalen via llvm-commits]

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@@ -2535,27 +2535,69 @@ void AArch64FrameLowering::emitEpilogue(MachineFunction &MF,
                     DeallocateAfter, TII, MachineInstr::FrameDestroy, false,
                     NeedsWinCFI, &HasWinCFI);
   } else if (SVEStackSize) {
-    // If we have stack realignment or variable sized objects on the stack,
-    // restore the stack pointer from the frame pointer prior to SVE CSR
-    // restoration.
-    if (AFI->isStackRealigned() || MFI.hasVarSizedObjects()) {
-      if (int64_t CalleeSavedSize = AFI->getSVECalleeSavedStackSize()) {
-        // Set SP to start of SVE callee-save area from which they can
-        // be reloaded. The code below will deallocate the stack space
-        // space by moving FP -> SP.
-        emitFrameOffset(MBB, RestoreBegin, DL, AArch64::SP, AArch64::FP,
-                        StackOffset::getScalable(-CalleeSavedSize), TII,
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sdesmalen-arm wrote:

My understanding is that the only thing that is missing here is that this should have been: `StackOffset::getScalable(-CalleeSavedSize) - StackOffset::getFixed(getStackHazardSize(MF))`.

`emitFrameOffset` is written such that it takes a `StackOffset` object and should be able to [de]allocate both the fixed *and* scalable parts of the offset. If the offset is negative and consists of two components, the code doesn't correctly implement this correctly using a temporary register like you've done here. My recommendation would be to update `emitFrameOffset` to handle negative offsets properly, rather than adding that logic here for this particular case.

https://github.com/llvm/llvm-project/pull/143371