Re: [PATCH v2] elf: Avoid nested functions in the loader (all ports) [BZ #27220]

["H.J. Lu" <hjl.tools@gmail.com>]

On Thu, Sep 23, 2021 at 12:23 AM Fangrui Song via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> [Alternative to https://sourceware.org/pipermail/libc-alpha/2021-August/130340.html
> This version fixes all ports and doesn't add NESTING dispatches.]
> [Available at
> https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/maskray/unnest ]
>
> dynamic-link.h is included more than once in some elf/ files (rtld.c,
> dl-conflict.c, dl-reloc.c, dl-reloc-static-pie.c) and uses GCC nested
> functions. This harms readability and the nested functions usage
> is the biggest obstacle prevents CC=clang (which doesn't support the
> feature).
>
> To un-nest elf_machine_rela, the key idea is to pass the variable in
> the containing scope as extra arguments.
> Stan Shebs implemented ppc64/x86-64 parts in the google/grte/v5-2.27/master
> branch by using static variables.
> This patch is inspired by that but takes an approach avoiding static variables.
>
> Tested on aarch64-linux-gnu, powerpc64le-linux-gnu and x86_64-linux-gnu.
> On x86_64, ld.so size does not change (.text is slightly larger, .rodata
> is slightly smaller).

Please disassemble and show what additional/changed instructions are.

> On aarch64, .text is 64 bytes larger but .rodata is 240 bytes smaller.
> On ppc64le, both .text and .rodata are smaller.
> I think the performance does not matter because most cycles are spent in symbol
> lookup.
>
> Tested build-many-glibcs.py compilers with
> {alpha,arc,csky,hppa,ia64,microblaze,s390,s390x,sh,sparc64,sparcv9}-linux-gnu,
> arm-linux-gnueabi{,hf}, mips64{,el}-linux-gnu{,-n32,-n64}, and
> riscv64-linux-gnu-rv64imafdc-lp64d.

H.J.