Re: [musl] Can’t build musl with lto=thin

[Jiahao XU <Jiahao_XU@outlook.com>]

[-- Attachment #1: Type: text/plain, Size: 2249 bytes --]

My hello.c:

    #include <stdio.h>
    int main()
    {
        puts(“Hello, world!”);
        return 0;
    }

And if I changed puts to printf and uses -fno-builtin, it also generates a 16KB executable.

Jiahao XU
________________________________
From: Fangrui Song <emacsray@gmail.com>
Sent: Tuesday, February 2, 2021 5:06:26 AM
To: Jiahao XU <Jiahao_XU@outlook.com>
Cc: musl@lists.openwall.com <musl@lists.openwall.com>
Subject: Re: [musl] Can’t build musl with lto=thin

On Mon, Feb 1, 2021 at 9:53 AM Rich Felker <dalias@libc.org> wrote:
>
> On Mon, Feb 01, 2021 at 12:31:47AM +0000, Jiahao XU wrote:
> > Interesting enough, I found —gc-section used along with -flto can cut the size of final hello_world executable from 48KB to 5KB.
> >
> > After investigating with bloaty, I found that —gc-section along with -flto is able to cut .text from 25.4 KiB to 3.04 KiB, and cut the .rodata from 19.5 KiB to 120 bytes.
> > ..data section however, seen an increase from 316 bytes to 372 bytes, but the VM size is cut from 252 to 244 bytes.
> >
> >
> > $ bloaty gc-section-a.out -- no-gc-section.a.out
> >
> >     FILE SIZE        VM SIZE
> >
> >  --------------  --------------
> >
> >    +18%     +56  -3.2%      -8    .data
> >
> >   [NEW]      +6  [NEW]      +6    [LOAD #2 [RX]]
> >
> >   [DEL]      -4 -66.7%      -8    [LOAD #4 [RW]]
> >
> >  -72.7%      -8  [ = ]       0    [Unmapped]
> >
> >  -32.0%     -64  [ = ]       0    .comment
> >
> >  -99.4% -19.4Ki -99.7% -19.4Ki    .rodata
> >
> >  -88.0% -22.3Ki -88.2% -22.3Ki    .text
> >
> >  -89.4% -41.8Ki -88.5% -41.8Ki    TOTAL
>
> What is included in your hello world? Mine, static linked normally (no
> LTO) is around 4k of text and virtually no rodata. (This is with GCC;
> I'm not using clang.) If I compile with -fno-builtin so printf doesn't
> get transformed to puts, there's about 16k of text and 3k of rodata,
> but still nowhere near the ~20k you saw.
>
> Rich

In LLD and LLVMgold.so's LTO configuration, -ffunction-sections &
-fdata-sections are automatically enabled.
Without them --gc-sections is not useful.

-ffunction-sections & -fdata-sections are code generation options and
not encoded in bitcode files.

[-- Attachment #2: Type: text/html, Size: 4339 bytes --]