[clang] [llvm] [AArch64][SME] Save VG for unwind info when changing streaming-mode (PR #83301)

[Kerry McLaughlin via cfe-commits]

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@@ -8287,6 +8289,13 @@ AArch64TargetLowering::LowerCall(CallLoweringInfo &CLI,
 
   SDValue InGlue;
   if (RequiresSMChange) {
+
+    if (Subtarget->hasSVE()) {
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kmclaughlin-arm wrote:

If we still emit the spill of VG with `-fno-asynchronous-unwind-tables`, I don't know how we would be able to recover the correct value without the per-call CFI saves and restores. And without the correct VG value, I don't think it will be possible to recover any VG based values in the stack frame.

I'm still not sure why this wouldn't require a diagnostic, because if there is not enough information to recover VG then I don't think we can unwind correctly?

https://github.com/llvm/llvm-project/pull/83301