Evgeny Karpov <[email protected]> writes:
> Thursday, September 19, 2024
> Richard Sandiford <[email protected]> wrote:
>
>>> For instance:
>>> float __attribute__((aligned (32))) large_aligned_array[3];
>>>
>>> BIGGEST_ALIGNMENT could be up to 512 bits on x64.
>>> This patch has been added to cover this case without needing to
>>> change the FFmpeg code.
>>
>> What goes wrong if we don't do this? I'm not sure from the description
>> whether it's a correctness fix, a performance fix, or whether it's about
>> avoiding wasted space.
>
> It is a correctness fix.
But you could you explain what goes wrong if you don't do this?
(I realise it might be very obvious when you've seen it happen :)
But I'm genuinely unsure.)
Why do we ignore the alignment if it is less than BIGGEST_ALIGNMENT?
>>> +#define ASM_OUTPUT_ALIGNED_LOCAL(FILE, NAME, SIZE, ALIGNMENT) \
>>> + { \
>>> + unsigned HOST_WIDE_INT rounded = MAX ((SIZE), 1); \
>>> + unsigned HOST_WIDE_INT alignment = MAX ((ALIGNMENT),
>>> BIGGEST_ALIGNMENT); \
>>> + rounded += (alignment / BITS_PER_UNIT) - 1; \
>>> + rounded = (rounded / (alignment / BITS_PER_UNIT) \
>>> + * (alignment / BITS_PER_UNIT)); \
>>
>> There's a ROUND_UP macro that could be used here.
>
> Here is the patch after applying ROUND_UP.
>
> Regards,
> Evgeny
>
> diff --git a/gcc/config/aarch64/aarch64-coff.h
> b/gcc/config/aarch64/aarch64-coff.h
> index 3c8aed806c9..1a45256ebfe 100644
> --- a/gcc/config/aarch64/aarch64-coff.h
> +++ b/gcc/config/aarch64/aarch64-coff.h
> @@ -58,6 +58,13 @@
> assemble_name ((FILE), (NAME)), \
> fprintf ((FILE), "," HOST_WIDE_INT_PRINT_DEC "\n", (ROUNDED)))
>
> +#define ASM_OUTPUT_ALIGNED_LOCAL(FILE, NAME, SIZE, ALIGNMENT) \
> + { \
> + unsigned rounded = ROUND_UP (MAX ((SIZE), 1), \
> + MAX ((ALIGNMENT), BIGGEST_ALIGNMENT) / BITS_PER_UNIT); \
Better to use "auto" rather than "unsigned".
Otherwise it looks good modulo the questions above.
Thanks,
Richard
> + ASM_OUTPUT_LOCAL (FILE, NAME, SIZE, rounded); \
> + }
> +
> #define ASM_OUTPUT_SKIP(STREAM, NBYTES) \
> fprintf (STREAM, "\t.space\t%d // skip\n", (int) (NBYTES))
