Richard Earnshaw <richard.earns...@foss.arm.com> writes:
> On 03/10/2023 16:18, Victor Do Nascimento wrote:
>> In implementing the ACLE read/write system register builtins it was
>> observed that leaving argument type checking to be done at expand-time
>> meant that poorly-formed function calls were being "fixed" by certain
>> optimization passes, meaning bad code wasn't being properly picked up
>> in checking.
>>
>> Example:
>>
>> const char *regname = "amcgcr_el0";
>> long long a = __builtin_aarch64_rsr64 (regname);
>>
>> is reduced by the ccp1 pass to
>>
>> long long a = __builtin_aarch64_rsr64 ("amcgcr_el0");
>>
>> As these functions require an argument of STRING_CST type, there needs
>> to be a check carried out by the front-end capable of picking this up.
>>
>> The introduced `check_general_builtin_call' function will be called by
>> the TARGET_CHECK_BUILTIN_CALL hook whenever a call to a builtin
>> belonging to the AARCH64_BUILTIN_GENERAL category is encountered,
>> carrying out any appropriate checks associated with a particular
>> builtin function code.
>
> Doesn't this prevent reasonable wrapping of the __builtin... names with
> something more palatable? Eg:
>
> static inline __attribute__(("always_inline")) long long get_sysreg_ll
> (const char *regname)
> {
> return __builtin_aarch64_rsr64 (regname);
> }
>
> ...
> long long x = get_sysreg_ll("amcgcr_el0");
> ...
I think it's case of picking your poison. If we didn't do this,
and only checked later, then it's unlikely that GCC and Clang would
be consistent about when a constant gets folded soon enough.
But yeah, it means that the above would need to be a macro in C.
Enlightened souls using C++ could instead do:
template<const char *regname>
long long get_sysreg_ll()
{
return __builtin_aarch64_rsr64(regname);
}
... get_sysreg_ll<"amcgcr_el0">() ...
Or at least I hope so. Might be nice to have a test for this.
Thanks,
Richard
