On Wed, May 10, 2017 at 12:59:28PM -0500, Steven Munroe wrote:
> > That is just for the testsuite; I meant what happens if a user tries
> > to use it with an older target (or BE, or 32-bit)? Is there a useful,
> > obvious error message?
> >
> So looking at the X86 headers, their current practice falls into two two
> areas.
>
> 1) guard 64-bit dependent intrinsic functions with:
>
> #ifdef __x86_64__
> #endif
>
> But they do not provide any warnings. I assume that attempting to use an
> intrinsic of this class would result in an implicit function declaration
> and a link-time failure.
Yeah probably. Which is fine -- it does not silently do the wrong thing,
and it is easy to find where the problem is.
> If GCC does support that target then the '#pragma GCC target("avx")'
> will enable code generation, but the user might get a SIGILL if the
> hardware they have does not support those instructions.
That is less friendly, but it still does not silently generate bad code.
> In the BMI headers I already guard with:
>
> #ifdef __PPC64__
> #endif
>
> This means that like x86_64, attempting to use _pext_u64 on a 32-bit
> compiler will result in an implicit function declaration and cause a
> linker error.
Yup, that's fine.
> This is sufficient for most of BMI and BMI2 (registers only / endian
> agnostic). But this does not address the larger issues (for SSE/SSE2+)
> which needing VXS implementation or restricting to LE.
Right.
> So should I check for:
>
> #ifdef __VSX__
> #endif
>
> or
>
> #ifdef __POWER8_VECTOR__
>
> or
>
> #ifdef _ARCH_PWR8
>
> and perhaps:
>
> #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
>
> as well to enforce this.
>
> And are you suggesting I add an #else clause with #warning or #error? Or
> is the implicit function and link failure sufficient?
The first is friendlier, the second is sufficient I think.
Maybe it is good enough to check for LE only? Most unmodified code
written for x86 (using intrinsics etc.) will not work correctly on BE.
And if you restrict to LE you get 64-bit and POWER8 automatically.
So maybe just require LE?
Segher
