llvm dev - Mar 2021 - Why the binary size in trunk are smaller significantly?

Hi Chuanqi,

On Tue, Mar 30, 2021 at 9:47 AM chuanqi.xcq via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> In a private code bases, I find that the binary size produced in llvm-11 is
60M while the size produced in trunk is 51M.
>
> I tried to find the patch who made this change in git log or in phabricator
by keywords like `reduce`, 'size' or 'binary'. But I find
nothing.
Interesting -- which sections reduce in size between llvm-11 and
trunk? Assuming you're using ELF/DWARF binaries, `readelf -W -S` will
give you sections and sizes.

Since llvm-11 branched, there were some patches [0] that improved how
variable locations are represented in DWARF, I've seen some binaries
where that reduced file size by 10%. That would be reflected in the
.debug_loc section. There's also the constructor homing flag, passed
to clang with "-Xclang -fuse-ctor-homing", I'm not sure whether
it's
on by default in trunk. That would show a significant reduction in the
.debug_info section.

[0] https://reviews.llvm.org/rG0b5a8050ea39355a3876cc6bba9383d91e224e1f

--
Thanks,
Jeremy

Yep, I'd be curious to see more data - comparisons of sections sizes (Bloaty
<https://github.com/google/bloaty> is a handy tool to make comparisons
simpler (though for a one off might not be worth the hassle of going and
downloading/building it/etc)) & any work that could be done
isolating/reducing the test case (hopefully enough that it can be shared)
and verifying what differences might exist between the two compilations if
there's anything other than the compiler version.

On Tue, Mar 30, 2021 at 3:33 AM Jeremy Morse <jeremy.morse.llvm at
gmail.com>
wrote:
> Hi Chuanqi,
>
> On Tue, Mar 30, 2021 at 9:47 AM chuanqi.xcq via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > In a private code bases, I find that the binary size produced in
llvm-11
> is 60M while the size produced in trunk is 51M.
> >
> > I tried to find the patch who made this change in git log or in
> phabricator by keywords like `reduce`, 'size' or 'binary'.
But I find
> nothing.
>
> Interesting -- which sections reduce in size between llvm-11 and
> trunk? Assuming you're using ELF/DWARF binaries, `readelf -W -S` will
> give you sections and sizes.
>
> Since llvm-11 branched, there were some patches [0] that improved how
> variable locations are represented in DWARF, I've seen some binaries
> where that reduced file size by 10%. That would be reflected in the
> .debug_loc section. There's also the constructor homing flag, passed
> to clang with "-Xclang -fuse-ctor-homing", I'm not sure
whether it's
> on by default in trunk. That would show a significant reduction in the
> .debug_info section.
>
> [0] https://reviews.llvm.org/rG0b5a8050ea39355a3876cc6bba9383d91e224e1f
>
> --
> Thanks,
> Jeremy
>
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