[llvm] [AArch64][SME] Support `aarch64-split-sve-objects` with VLAs/realignment (PR #163816)

[Sander de Smalen via llvm-commits]

================
@@ -1501,28 +1519,26 @@ void AArch64EpilogueEmitter::emitEpilogue() {
         (AFI->isStackRealigned() || MFI.hasVarSizedObjects()) ? AArch64::FP
                                                               : AArch64::SP;
     if (SVECalleeSavesSize && BaseForSVEDealloc == AArch64::FP) {
-      // TODO: Support stack realigment and variable-sized objects.
-      assert(
-          SVELayout != SVEStackLayout::Split &&
-          "unexpected stack realignment or variable sized objects with split "
-          "SVE stack objects");
-
-      Register CalleeSaveBase = AArch64::FP;
-      if (int64_t CalleeSaveBaseOffset =
-              AFI->getCalleeSaveBaseToFrameRecordOffset()) {
-        // If we have have an non-zero offset to the non-SVE CS base we need to
-        // compute the base address by subtracting the offest in a temporary
-        // register first (to avoid briefly deallocating the SVE CS).
-        CalleeSaveBase = MBB.getParent()->getRegInfo().createVirtualRegister(
-            &AArch64::GPR64RegClass);
-        emitFrameOffset(MBB, RestoreBegin, DL, CalleeSaveBase, AArch64::FP,
-                        StackOffset::getFixed(-CalleeSaveBaseOffset), TII,
+      // The offset from the frame-pointer to the start of the ZPR saves.
+      StackOffset FPOffsetZPR =
+          -SVECalleeSavesSize -
+          StackOffset::getFixed(AFI->getCalleeSaveBaseToFrameRecordOffset());
+
+      // With split SVE, the PPR locals are above the ZPR callee-saves.
+      if (ZPR.CalleeSavesSize && SVELayout == SVEStackLayout::Split)
+        FPOffsetZPR -= PPR.LocalsSize;
+
+      // Deallocate the stack space space by moving the SP to the start of the
+      // ZPR/PPR callee-save area.
+      moveSPBelowFP(ZPRRange.Begin, FPOffsetZPR);
----------------
sdesmalen-arm wrote:

If the code below uses FP as the base register, then this can be predicated under `if (ZPR.CalleeSavesSize) `.

https://github.com/llvm/llvm-project/pull/163816