[llvm] [AArch64] Don't emit Neon in streaming[-compatible] functions with -fzero-call-used-regs (PR #116995)

[David Green via llvm-commits]

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@@ -9694,19 +9694,26 @@ void AArch64InstrInfo::buildClearRegister(Register Reg, MachineBasicBlock &MBB,
                                           MachineBasicBlock::iterator Iter,
                                           DebugLoc &DL,
                                           bool AllowSideEffects) const {
-  const MachineFunction &MF = *MBB.getParent();
+  MachineFunction &MF = *MBB.getParent();
   const AArch64Subtarget &STI = MF.getSubtarget<AArch64Subtarget>();
   const AArch64RegisterInfo &TRI = *STI.getRegisterInfo();
 
   if (TRI.isGeneralPurposeRegister(MF, Reg)) {
     BuildMI(MBB, Iter, DL, get(AArch64::MOVZXi), Reg).addImm(0).addImm(0);
-  } else if (STI.hasSVE()) {
+  } else if (STI.isSVEorStreamingSVEAvailable()) {
     BuildMI(MBB, Iter, DL, get(AArch64::DUP_ZI_D), Reg)
       .addImm(0)
       .addImm(0);
-  } else {
+  } else if (STI.isNeonAvailable()) {
     BuildMI(MBB, Iter, DL, get(AArch64::MOVIv2d_ns), Reg)
       .addImm(0);
+  } else {
+    // This is a streaming-compatible function without SVE. We don't have full
+    // Neon (just FPRs), so we can at most use the first 64-bit sub-register.
+    // So given `movi v..` would be illegal use `fmov d..` instead.
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davemgreen wrote:

fmovd (and all other instructions that set a `d` register) will implicitly zero the upper bits of the register too, so it should be OK even if other parts of the register do get used.

https://github.com/llvm/llvm-project/pull/116995