[AArch64][SME] Fix iterator to fixupCalleeSaveRestoreStackOffset (#110855)

The iterator passed to `fixupCalleeSaveRestoreStackOffset` may be
incorrect when it tries to skip over the instructions that get the
current value of 'vg', when there is a 'rdsvl' instruction straight
after the prologue. That's because it doesn't check that the instruction
is still a 'frame-setup' instruction.

Author: sdesmalen-arm

Diff for: llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -1947,12 +1947,9 @@ void AArch64FrameLowering::emitPrologue(MachineFunction &MF,
   // pointer bump above.
   while (MBBI != End && MBBI->getFlag(MachineInstr::FrameSetup) &&
          !IsSVECalleeSave(MBBI)) {
-    // Move past instructions generated to calculate VG
-    if (requiresSaveVG(MF))
-      while (isVGInstruction(MBBI))
-        ++MBBI;
-
-    if (CombineSPBump)
+    if (CombineSPBump &&
+        // Only fix-up frame-setup load/store instructions.
+        (!requiresSaveVG(MF) || !isVGInstruction(MBBI)))
       fixupCalleeSaveRestoreStackOffset(*MBBI, AFI->getLocalStackSize(),
                                         NeedsWinCFI, &HasWinCFI);
     ++MBBI;

Diff for: llvm/test/CodeGen/AArch64/sme-vg-to-stack.ll

@@ -1102,6 +1102,44 @@ define void @streaming_compatible_no_sve(i32 noundef %x) #4 {
   ret void
 }
 
+; The algorithm that fixes up the offsets of the callee-save/restore
+; instructions must jump over the instructions that instantiate the current
+; 'VG' value. We must make sure that it doesn't consider any RDSVL in
+; user-code as if it is part of the frame-setup when doing so.
+define void @test_rdsvl_right_after_prologue(i64 %x0) nounwind {
+; NO-SVE-CHECK-LABEL: test_rdsvl_right_after_prologue:
+; NO-SVE-CHECK:     // %bb.0:
+; NO-SVE-CHECK-NEXT: stp     d15, d14, [sp, #-96]!           // 16-byte Folded Spill
+; NO-SVE-CHECK-NEXT: stp     d13, d12, [sp, #16]             // 16-byte Folded Spill
+; NO-SVE-CHECK-NEXT: mov     x9, x0
+; NO-SVE-CHECK-NEXT: stp     d11, d10, [sp, #32]             // 16-byte Folded Spill
+; NO-SVE-CHECK-NEXT: stp     d9, d8, [sp, #48]               // 16-byte Folded Spill
+; NO-SVE-CHECK-NEXT: stp     x29, x30, [sp, #64]             // 16-byte Folded Spill
+; NO-SVE-CHECK-NEXT: bl      __arm_get_current_vg
+; NO-SVE-CHECK-NEXT: str     x0, [sp, #80]                   // 8-byte Folded Spill
+; NO-SVE-CHECK-NEXT: mov     x0, x9
+; NO-SVE-CHECK-NEXT: rdsvl   x8, #1
+; NO-SVE-CHECK-NEXT: add     x29, sp, #64
+; NO-SVE-CHECK-NEXT: lsr     x8, x8, #3
+; NO-SVE-CHECK-NEXT: mov     x1, x0
+; NO-SVE-CHECK-NEXT: smstart sm
+; NO-SVE-CHECK-NEXT: mov     x0, x8
+; NO-SVE-CHECK-NEXT: bl      bar
+; NO-SVE-CHECK-NEXT: smstop  sm
+; NO-SVE-CHECK-NEXT: ldp     x29, x30, [sp, #64]             // 16-byte Folded Reload
+; NO-SVE-CHECK-NEXT: ldp     d9, d8, [sp, #48]               // 16-byte Folded Reload
+; NO-SVE-CHECK-NEXT: ldp     d11, d10, [sp, #32]             // 16-byte Folded Reload
+; NO-SVE-CHECK-NEXT: ldp     d13, d12, [sp, #16]             // 16-byte Folded Reload
+; NO-SVE-CHECK-NEXT: ldp     d15, d14, [sp], #96             // 16-byte Folded Reload
+; NO-SVE-CHECK-NEXT: ret
+  %some_alloc = alloca i64, align 8
+  %rdsvl = tail call i64 @llvm.aarch64.sme.cntsd()
+  call void @bar(i64 %rdsvl, i64 %x0) "aarch64_pstate_sm_enabled"
+  ret void
+}
+
+declare void @bar(i64, i64)
+
 ; Ensure we still emit async unwind information with -fno-asynchronous-unwind-tables
 ; if the function contains a streaming-mode change.