Re: [PATCH v1 03/13] aarch64: Mark x18 register as a fixed register for MS ABI

["Richard Earnshaw (lists)" <Richard.Earnshaw@arm.com>]

On 21/02/2024 18:30, Evgeny Karpov wrote:
> 
+/* X18 reserved for the TEB on Windows.  */
+#ifdef TARGET_ARM64_MS_ABI
+# define FIXED_X18 1
+# define CALL_USED_X18 0
+#else
+# define FIXED_X18 0
+# define CALL_USED_X18 1
+#endif

I'm not overly keen on ifdefs like this (and the one below), it can get quite confusing if we have to support more than a couple of ABIs.  Perhaps we could create a couple of new headers, one for the EABI (which all existing targets would then need to include) and one for the MS ABI.  Then the mingw port would use that instead of the EABI header.

An alternative is to make all this dynamic, based on the setting of the aarch64_calling_abi enum and to make the adjustments in aarch64_conditional_register_usage.

+# define CALL_USED_X18 0

Is that really correct?  If the register is really reserved, but some code modifies it anyway, this will cause the compiler to restore the old value at the end of a function; generally, for a reserved register, code that knows what it's doing would want to make permanent changes to this value.

+#ifdef TARGET_ARM64_MS_ABI
+# define STATIC_CHAIN_REGNUM		R17_REGNUM
+#else
+# define STATIC_CHAIN_REGNUM		R18_REGNUM
+#endif

If we went the enum way, we'd want something like

#define STATIC_CHAIN_REGNUM (calling_abi == AARCH64_CALLING_ABI_MS ? R17_REGNUM : R18_REGNUM)

R.