Re: [musl] [PATCH 1/2] Add Thumb2 support to ARM assembler memcpy

[Andre McCurdy <armccurdy@gmail.com>]

On Wed, Jan 15, 2020 at 8:36 AM Rich Felker <dalias@libc.org> wrote:
> On Fri, Sep 13, 2019 at 11:44:31AM -0700, Andre McCurdy wrote:
> > For Thumb2 compatibility, replace two instances of a single
> > instruction "orr with a variable shift" with the two instruction
> > equivalent. Neither of the replacements are in a performance critical
> > loop.
> > ---
> >  src/string/arm/memcpy.c    |  2 +-
> >  src/string/arm/memcpy_le.S | 17 ++++++++++-------
> >  2 files changed, 11 insertions(+), 8 deletions(-)
> >
> > diff --git a/src/string/arm/memcpy.c b/src/string/arm/memcpy.c
> > index f703c9bd..041614f4 100644
> > --- a/src/string/arm/memcpy.c
> > +++ b/src/string/arm/memcpy.c
> > @@ -1,3 +1,3 @@
> > -#if __ARMEB__ || __thumb__
> > +#if __ARMEB__
> >  #include "../memcpy.c"
> >  #endif
> > diff --git a/src/string/arm/memcpy_le.S b/src/string/arm/memcpy_le.S
> > index 9cfbcb2a..64bc5f9e 100644
> > --- a/src/string/arm/memcpy_le.S
> > +++ b/src/string/arm/memcpy_le.S
> > @@ -1,4 +1,4 @@
> > -#if !__ARMEB__ && !__thumb__
> > +#if !__ARMEB__
> >
> >  /*
> >   * Copyright (C) 2008 The Android Open Source Project
> > @@ -40,8 +40,9 @@
> >   * This file has been modified from the original for use in musl libc.
> >   * The main changes are: addition of .type memcpy,%function to make the
> >   * code safely callable from thumb mode, adjusting the return
> > - * instructions to be compatible with pre-thumb ARM cpus, and removal
> > - * of prefetch code that is not compatible with older cpus.
> > + * instructions to be compatible with pre-thumb ARM cpus, removal of
> > + * prefetch code that is not compatible with older cpus and support for
> > + * building as thumb 2.
> >   */
> >
> >  .syntax unified
> > @@ -241,8 +242,9 @@ non_congruent:
> >       beq     2f
> >       ldr     r5, [r1], #4
> >       sub     r2, r2, #4
> > -     orr     r4, r3, r5,             lsl lr
> > -     mov     r3, r5,                 lsr r12
> > +     mov     r4, r5, lsl lr
> > +     orr     r4, r4, r3
> > +     mov     r3, r5, lsr r12
> >       str     r4, [r0], #4
> >       cmp     r2, #4
> >       bhs     1b
>
> This is outside of loops and not a hot path,
>
> > @@ -348,8 +350,9 @@ less_than_thirtytwo:
> >
> >  1:      ldr     r5, [r1], #4
> >       sub     r2, r2, #4
> > -     orr     r4, r3, r5,             lsl lr
> > -     mov     r3,     r5,                     lsr r12
> > +     mov     r4, r5, lsl lr
> > +     orr     r4, r4, r3
> > +     mov     r3, r5, lsr r12
> >       str     r4, [r0], #4
> >       cmp     r2, #4
> >       bhs     1b
>
> This one is in a loop, but perhaps not terribly critical to
> performance.

Yes, it's in a loop, but I can confirm it's not a performance critical one.

> We could keep old version with #if !__thumb__ but I doubt
> it matters, and it looks like hardly anyone is using pre-thumb2 ARM
> anymore anyway; a show-stopping bug went uncaught for over a year in
> other things for v6.

I was meaning to ask about that after seeing your recent commit in
master. My primary target is pre-thumb2 armv6 and I hadn't noticed any
problems...

> One cosmetic fix I'd like to make when applying this is keeping the
> old gratuitously-ugly formatting just so the actual change isn't
> obscured by the formatting-only change on an adjacent line. I can
> handle that though.
>
> Rich