Re: [PATCH] Fix pthread_arch.h for microMIPS

["dalias@libc.org" <dalias@libc.org>]

On Mon, Mar 21, 2016 at 10:01:02AM +0000, Jaydeep Patil wrote:
> Hi Rich,
> 
> The patch fixes a link time error when compiled for microMIPS. The
> pthread_self() function has been modified to use rdhwr instruction
> instead of .word directive.
> The change has been done for both clang and gcc. Functions
> containing .word are not compiled for microMIPS.
> 
> Please refer to https://github.com/JaydeepIMG/musl-1/tree/fix_rdhwr_for_umips for details.
> 
> 
> 
> >From 09e4e395d9f1538edb548ffaa02db74e8e11701e Mon Sep 17 00:00:00 2001
> From: Jaydeep Patil <jaydeep.patil@imgtec.com>
> Date: Mon, 21 Mar 2016 09:53:37 +0000
> Subject: [PATCH] Use rdhwr insn instead of .word for microMIPS
> 
> ---
> arch/mips/pthread_arch.h   | 10 ++--------
> arch/mips64/pthread_arch.h |  9 ++-------
> 2 files changed, 4 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/mips/pthread_arch.h b/arch/mips/pthread_arch.h
> index 8a49965..30e2394 100644
> --- a/arch/mips/pthread_arch.h
> +++ b/arch/mips/pthread_arch.h
> @@ -1,13 +1,7 @@
> static inline struct pthread *__pthread_self()
> {
> -#ifdef __clang__
> -       char *tp;
> -       __asm__ __volatile__ (".word 0x7c03e83b ; move %0, $3" : "=r" (tp) : : "$3" );
> -#else
> -       register char *tp __asm__("$3");
> -       /* rdhwr $3,$29 */
> -       __asm__ __volatile__ (".word 0x7c03e83b" : "=r" (tp) );
> -#endif
> +       register char *tp;
> +       __asm__ __volatile__ ("rdhwr %0,$29" : "=r" (tp));
>         return (pthread_t)(tp - 0x7000 - sizeof(struct pthread));
> }

You can't remove the register constraint to use $3 here; the reason
for the constraint is not that the opcode is hard-coded, but that the
kernel's fast-path emulation for MIPS-I, MIPS-II, and MIPS32r1 cpus
that lack support for this hardware register only works when $3 is
used as the destination register. Otherwise a very slow path for
emulation is taken. (On our part, this probably should be documented
in a comment -- sorry it's not.)

There are probably other reasons we're using .word instead of the
mnemonic here too; I suspect it fails to assemble without .set to a
proper ISA level or sufficient -march. This needs to be checked. Is
there a reason the .word doesn't work on microMIPS? I thought the
32-bit opcodes were the same but maybe I'm mistaken.

Rich