[llvm] [AArch64][SME] Don't mark 'smstart za' as using/defining VG. (PR #84775)

[Paul Walker via llvm-commits]

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@@ -7751,13 +7751,23 @@ void AArch64TargetLowering::AdjustInstrPostInstrSelection(MachineInstr &MI,
   // register allocator to pass call args in callee saved regs, without extra
   // copies to avoid these fake clobbers of actually-preserved GPRs.
   if (MI.getOpcode() == AArch64::MSRpstatesvcrImm1 ||
-      MI.getOpcode() == AArch64::MSRpstatePseudo)
+      MI.getOpcode() == AArch64::MSRpstatePseudo) {
     for (unsigned I = MI.getNumOperands() - 1; I > 0; --I)
       if (MachineOperand &MO = MI.getOperand(I);
           MO.isReg() && MO.isImplicit() && MO.isDef() &&
           (AArch64::GPR32RegClass.contains(MO.getReg()) ||
            AArch64::GPR64RegClass.contains(MO.getReg())))
         MI.removeOperand(I);
+    // If the instruction or pseudo changes PSTATE.SM, then add VG as a Use and
+    // Def.
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paulwalker-arm wrote:

Whilst true it's effectively what the code says.  Perhaps "The SVE vector length can change when entering/leaving streaming mode."?

https://github.com/llvm/llvm-project/pull/84775