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Date: Sun, 26 Aug 2018 02:47:52 +0100
From: Jon Chesterfield <jonathanchesterfield@...il.com>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] _start_c does more work than is necessary
That's interesting context, thanks. rcrt1.c is ofc similar.
I'd be happy to see the crt files go. The patch was motivated by debugging
the
consequences of importing crtbegin from bsd (as broadly recommended online)
while trying to placate lld. Said crtbegin installed a function that walks
init_array into .init so all the constructors fired twice.
crt1.o appears to be the only necessary crt file for llvm/x86-64/c++.
.ctor/.dtor is still implemented in lld along with a comment about legacy :)
Cheers
Jon
On Sun, Aug 26, 2018 at 2:24 AM Rich Felker <dalias@...c.org> wrote:
> On Sun, Aug 26, 2018 at 01:56:30AM +0100, Jon Chesterfield wrote:
> > The init sequence in musl is _start calls _start_c which calls
> > __libc_start_main.
> >
> > _start_c passes pointers to the _init and _fini functions, and also a
> > trailing zero, to __libc_start_main.
> >
> > __libc_start_main currently takes exactly three arguments. I'd like to
> > simplify crt1.c by only passing main, argc, argv.
> >
> > This is worth a few lines of C and three instructions in the startup
> > sequence. E.g. x86-64 this removes mov, mov, xor for fourteen bytes.
> >
> > It also removes uses of _init() and _free() which I'm considering
> deleting
> > from a musl/llvm toolchain which makes no use of either, slightly
> > decreasing the size of my out of tree patch.
> >
> > Thanks,
> >
> > Jon
> >
> > ---
> > diff --git a/crt/crt1.c b/crt/crt1.c
> > index af02af9..f27c949 100644
> > --- a/crt/crt1.c
> > +++ b/crt/crt1.c
> > @@ -5,14 +5,11 @@
> > #include "crt_arch.h"
> >
> > int main();
> > -void _init() __attribute__((weak));
> > -void _fini() __attribute__((weak));
> > -_Noreturn int __libc_start_main(int (*)(), int, char **,
> > - void (*)(), void(*)(), void(*)());
> > +_Noreturn int __libc_start_main(int (*)(), int, char **);
> >
> > void _start_c(long *p)
> > {
> > int argc = p[0];
> > char **argv = (void *)(p+1);
> > - __libc_start_main(main, argc, argv, _init, _fini, 0);
> > + __libc_start_main(main, argc, argv);
> > }
>
> It may be reasonable to make this change now, but it should be
> reviewed. Read the git log for __libc_start_main.c and in particular
> commit 7586360badcae6e73f04eb1b8189ce630281c4b2 which explains the
> history and non-obvious reason musl does not have any reason to use
> these arguments.
>
> Ultimately all crt files except crt1 should be removed at some point.
> The _init/_fini machinery (crti/crtn) is purely legacy mess from
> before init/fini arrays were used, and the gcc crtbegin/crtend mess is
> just for wacky language runtimes like java (gcj, obsolete and removed
> from gcc). I'm not sure if there are other compilers out there that
> still don't know how to do the arrays, though (pcc? firm/cparser?).
>
> Rich
>
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