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

[Andre McCurdy <armccurdy@gmail.com>]

On Wed, Jan 15, 2020 at 11:24 AM Rich Felker <dalias@libc.org> wrote:
> On Wed, Jan 15, 2020 at 10:49:03AM -0800, Andre McCurdy wrote:
> > 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.
>
> Thanks.
>
> > > 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...
>
> I wonder if there was some magical mechanism by which the anticipated
> crash failed to trigger. It certainly triggered in the other affected
> arch, sh2, though. If you happen to look at it and find out what was
> going on, let us know on the list.

For ARM, testing libc.auxv is guarded by a test on __hwcap, so if
__hwcap is also being initialised after __set_thread_area() then
__set_thread_area() will never access libc.auxv ?