Re: [PATCH] tools/nolibc: sh: enter _start_c with jsr instead of bsr

[Willy Tarreau <w@1wt.eu>]

Hi guys,

On Thu, Oct 22, 2026 at 06:12:27PM +0200, Thomas Weißschuh wrote:
> Hi Daniel!
> 
> On 2026-10-22 19:01:59+0900, Daniel Palmer wrote:
> > bsr has limited displacement and cannot reach _start_c if its
> > more than +/-4KiB away.
> > 
> > This starts to show up if you have big programs (static binary
> > of the game Quake in my case) and/or use LTO and things get moved
> > around.
> 
> That is certainly a usecase that justifies making the two other SH4
> nolibc users' binaries a bit larger :-)

Indeed!

> > Replace bsr with a calculation of the effective address of
> > _start_c and jsr.
> > 
> > Tested on QEMU and real SH4 and SH4a machines.
> > 
> > Signed-off-by: Daniel Palmer <daniel@0x0f.com>
> > ---
> > 
> > I'm no SuperH asm expert but this works for me. :)
> > Maybe there is a way in C to get _start and _start_c to always
> > be close together that would avoid this entirely?
> 
> That would be my preference, too.
> 
> Maybe one of these works:
> 
> 1) Put both _start() and _start_c() in a dedicated .text subsection.
> 
> Apparently the default GNU ld linkerscript contains '*(SORT(.text.sorted.*))'.
> So if we use .text.sorted.nolibc_start for both functions they should
> end up next to each other.
> See https://sourceware.org/binutils/docs/ld/Input-Section-Wildcards.html.
> 
> 2) Put _start() *into* _start_c:
> 
> diff --git a/tools/include/nolibc/crt.h b/tools/include/nolibc/crt.h
> index 714256228914..ebcd0ea2b9f2 100644
> --- a/tools/include/nolibc/crt.h
> +++ b/tools/include/nolibc/crt.h
> @@ -115,6 +115,19 @@ void _start_c(long *sp)
>                 (*--dtor_func)();
> 
>         exit(exitcode);
> +
> +       __asm__ volatile (
> +               ".global _start\n"           /* The C function will have a prologue,         */
> +               ".type _start, @function\n"  /* corrupting "sp"                              */
> +               ".weak _start\n"
> +               "_start:\n"
> +
> +               "mov sp, r4\n"               /* save argc pointer to r4, as arg1 of _start_c */
> +               "bsr _start_c\n"             /* transfer to c runtime                        */
> +               "nop\n"                      /* delay slot                                   */
> +
> +               ".size _start, .-_start\n"
> +       );
>  }
> 
>  #endif /* NOLIBC_NO_RUNTIME */
> 
> But that for all architectures, using macros etc.
> 
> If the .text.sorted. solution works, I'd prefer that.

Do be honest, I think that Daniel's proposal remains the most maintainable
one. We rarely touch the start code, and having only a few do arch-
specific tricks is likely less of a burden for the long term than
relying on linker's ordering of sections, or starting to play macro
games. And it gets a chance that someone more familiar with that arch
improves it later if needed. But I'm not opiniated on this at all :-)

Thanks,
Willy