Hello,
On 19 May 09:58, H.J. Lu wrote:
> On Mon, May 19, 2014 at 9:45 AM, Uros Bizjak <ubiz...@gmail.com> wrote:
> > On Mon, May 19, 2014 at 6:42 PM, H.J. Lu <hjl.to...@gmail.com> wrote:
> >
> >>>> Uros,
> >>>> I am looking into libreoffice size and the data alignment seems to make
> >>>> huge
> >>>> difference. Data section has grown from 5.8MB to 6.3MB in between GCC
> >>>> 4.8 and 4.9,
> >>>> while clang produces 5.2MB.
> >>>>
> >>>> The two patches I posted to not align vtables and RTTI reduces it to
> >>>> 5.7MB, but
> >>>> But perhaps we want to revisit the alignment rules. The optimization
> >>>> manuals
> >>>> usually care only about performance critical loops. Perhaps we can make
> >>>> the
> >>>> rules to align only bigger datastructures, or so at least for -O2.
> >>>
> >>> Based on the above quote, "Misaligned data access can incur
> >>> significant performance penalties." and the fact that this particular
> >>> alignment rule has some compatibility issues with previous versions of
> >>> gcc (these were later fixed by Jakub), I'd rather leave this rule as
> >>> is. However, if the access is from the cold section, we can perhaps
> >>> avoid extra alignment, while avoiding those compatibility issues.
> >>>
> >>
> >> It is excessive to align
> >>
> >> struct foo
> >> {
> >> int x1;
> >> int x2;
> >> char x3;
> >> int x4;
> >> int x5;
> >> char x6;
> >> int x7;
> >> int x8;
> >> };
> >>
> >> to 32 bytes and align
> >>
> >> struct foo
> >> {
> >> int x1;
> >> int x2;
> >> char x3;
> >> int x4;
> >> int x5;
> >> char x6;
> >> int x7[9];
> >> int x8;
> >> };
> >>
> >> to 64 bytes. What performance gain does it provide?
> >
> > Avoids "significant performance penalties," perhaps?
> >
>
> Kirill, do we have performance data for excessive alignment
> vs ABI alignment?
Nope, we have no actual data showing performance impact on such changes,
sorry.
We may try such a change on HSW machine (on Spec 2006), will it be useful?
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -26576,7 +26576,7 @@ ix86_data_alignment (tree type, int align, bool opt)
used to assume. */
int max_align_compat
- = optimize_size ? BITS_PER_WORD : MIN (256, MAX_OFILE_ALIGNMENT);
+ = optimize_size ? BITS_PER_WORD : MIN (128, MAX_OFILE_ALIGNMENT);
/* A data structure, equal or greater than the size of a cache line
(64 bytes in the Pentium 4 and other recent Intel processors, including
> --
> H.J.
--
Thanks, K
