On Thu, Nov 04, 2021 at 11:05:35PM -0700, Andrew Pinski via Gcc-patches wrote:
> > 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.
I completely agree.
Jakub
