Fix clobber_abi and disallow SVE-related registers in Arm64EC inline assembly

[taiki-e]

Currently clobber_abi in Arm64EC inline assembly is implemented using InlineAsmClobberAbi::AArch64NoX18, but broken since it attempts to clobber registers that cannot be used in Arm64EC: https://godbolt.org/z/r3PTrGz5r

error: cannot use register `x13`: x13, x14, x23, x24, x28, v16-v31 cannot be used for Arm64EC
 --> <source>:6:14
  |
6 |     asm!("", clobber_abi("C"), options(nostack, nomem, preserves_flags));
  |              ^^^^^^^^^^^^^^^^

error: cannot use register `x14`: x13, x14, x23, x24, x28, v16-v31 cannot be used for Arm64EC
 --> <source>:6:14
  |
6 |     asm!("", clobber_abi("C"), options(nostack, nomem, preserves_flags));
  |              ^^^^^^^^^^^^^^^^

<omitted the same errors for v16-v31>

Additionally, this disallows SVE-related registers per #131332 (comment).

cc @dpaoliello

r? @Amanieu

@rustbot label O-windows O-AArch64 +A-inline-assembly

[taiki-e]

Given the reason why v16-v31 are blocked in this target, I suspect that SVE-related registers (z*, p*, ffr) may actually be unavailable in Arm64EC.
Although AFAIK they have not documented anything about it in https://learn.microsoft.com/en-us/windows/arm/arm64ec-abi#register-mapping-and-blocked-registers
(I don't think this question is a blocker for this PR, but I think it could be a blocker for stabilization.)

[dpaoliello]

@pmsjt can we get the docs updated to indicate if SVE-related registers (z*, p*, ffr) are available or not on ARM64EC?

[dpaoliello]

@pmsjt can we get the docs updated to indicate if SVE-related registers (z*, p*, ffr) are available or not on ARM64EC?

[pmsjt]

@dpaoliello SVE use in Windows hasn't been documented in either Arm64 classic or Arm64EC. That is coming soon and, when it does, it'll cover both.

To answer your most immediate question: No, SVE should not be used by Arm64EC code. No Arm64EC code should ever access SVE state or ISA regardless of the CPU supporting it. Just as for the presently reserved GP and Neon state in EC [1], SVE will also be reserved and not mapped to any x86-64 state.

[1] https://learn.microsoft.com/en-us/cpp/build/arm64ec-windows-abi-conventions

[rustbot]

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

[taiki-e]

@pmsjt Thanks for the clarification!

I updated PR to disallow SVE-related registers.

[taiki-e]

Unfortunately, I could not keep the approach of using the aarch64 implementation to support asm for arm64ec.

If I wanted to just reject p*/ffr, it worked with the previous file structure (955d8c6), but I couldn't figure out how to reject z0-z15, which are defined as super registers for v0-v15.
(Filter in def_regs! (like % restricted_for_arm64ec) could not be used for this purpose because it did not know what the original register name was. Also, z* could not be defined as a separate clobber-only register class because it can accept values on aarch64.)