[llvm-dev] Why the binary size in trunk are smaller significantly?

[David Blaikie via llvm-dev]

On Tue, Mar 30, 2021 at 7:16 PM chuanqi.xcq <yedeng.yd at linux.alibaba.com>
wrote:

> Hi Jeremy and David,
>
>      The most significant reduction comes from the section `.debug_loc`
> and `.debug_info` section.
>      The size of `.debug_info` reduced about 14% and the size of of
> `.debug_loc` reduced to even 69%!
>

Surprises me a bit, but yeah, that could be debug info related, or
unrelated - optimizations might make fewer variable locations because we've
lost track of more variables so there are no locations - or locations might
be better tracked so they're more contiguously described (so less small
slices of variable locations - each slice having some overhead in its
description)... but hard to say. llvm-dwarfdump --statistics might be
insightful as to whether it's improvement in locations or not. But I'd make
an educated guess that this probably isn't reflective of any bugs and is
just roughly expected change from optimization changes and maybe debug info
location improvements.

But I could be wrong - it is a pretty big swing, for sure.

- Dave


>
>      I would try the patches in the link.
>
> Thanks,
> Chuanqi
>
>
>
> ------------------------------------------------------------------
> From:David Blaikie <dblaikie at gmail.com>
> Send Time:2021年3月31日(星期三) 00:24
> To:Jeremy Morse <jeremy.morse.llvm at gmail.com>
> Cc:chuanqi.xcq <yedeng.yd at linux.alibaba.com>; llvm-dev at lists.llvm.org <
> llvm-dev at lists.llvm.org>
> Subject:Re: [llvm-dev] Why the binary size in trunk are smaller
> significantly?
>
> 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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