Values of WIDE_INT_MAX_ELTS in gcc11 and gcc12 are different

Andrew Pinski pinskia@gmail.com
Fri Nov 5 06:05:35 GMT 2021


On Thu, Nov 4, 2021 at 1:38 PM Qing Zhao via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
>
> I noticed that the macro “WIDE_INT_MAX_ELTS” has different values in GCC11 and GCC12 (on the same X86 machine)
>
> For gcc11:
>
> wide int max elts =3
>
> For gcc12:
>
> wide int max elts =9
>
> Does anyone know what’s the reason for this difference?
>
> Thanks a lot for any help.

Yes originally, the x86 backend only used OI and XI modes for vectors
during data movement.
This changed with r10-5741-gc57b4c22089 which added the use of OI mode
for TImode adding with overflow and then MAX_BITSIZE_MODE_ANY_INT
changed from 128 to 160 (in r10-6178-gc124b345e46078) to fix the ICE
introduced by that change .
And then with r12-979-g782e57f2c09 removed the define of
MAX_BITSIZE_MODE_ANY_INT.
Now what was not mentioned in r12-979-g782e57f2c09 (or before) of why
MAX_BITSIZE_MODE_ANY_INT was defined in the first place for x86. HJL
assumed there was some problem of why it was defined that way but not
realizing memory usage was the reason.
It was defined to keep the memory usage down as you see that it is now
almost a 3x memory increase for all wi::wide_int.
I do think r12-979-g782e57f2c09 should be reverted with an added
comment on saying defining MAX_BITSIZE_MODE_ANY_INT here is to
decrease the memory footprint.


Thanks,
Andrew Pinski

>
> Qing


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