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Date: Mon, 1 Feb 2021 10:06:26 -0800
From: Fangrui Song <emacsray@...il.com>
To: Jiahao XU <Jiahao_XU@...look.com>
Cc: musl@...ts.openwall.com
Subject: Re: Can’t build musl with lto=thin
On Mon, Feb 1, 2021 at 9:53 AM Rich Felker <dalias@...c.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.
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