On 10/7/23 12:53, Richard Sandiford wrote:
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
As Richard Earnshaw mentioned, this does break the use of `static inline
__attribute__(("always_inline"))', something I had found out in my
testing. My chosen implementation was indeed, to quote Richard
Sandiford, a case of "picking your poison" to have things line up with
Clang and behaving consistently across optimization levels.
Relaxing the the use of `TARGET_CHECK_BUILTIN_CALL' meant optimizations
were letting too many things through. Example:
const char *regname = "amcgcr_el0";
long long a = __builtin_aarch64_rsr64 (regname);
gets folded to
long long a = __builtin_aarch64_rsr64 ("amcgcr_el0");
and compilation passes at -01 even though it fails at -O0.
I had, however, not given any thought to the use of a template as a
valid C++ alternative.
I will evaluate the use of templates and add tests accordingly.
Cheers,
Victor
