[RFC] arm: use built-in byte swap function

[Nicolas Pitre]

On Thu, 21 Feb 2013, Kim Phillips wrote:

> On Wed, 20 Feb 2013 23:29:58 -0500
> Nicolas Pitre <nico at fluxnic.net> wrote:
> 
> > On Wed, 20 Feb 2013, Kim Phillips wrote:
> > 
> > > On Wed, 20 Feb 2013 10:43:18 -0500
> > > Nicolas Pitre <nico at fluxnic.net> wrote:
> > > 
> > > > On Wed, 20 Feb 2013, Woodhouse, David wrote:
> > > > > On Wed, 2013-02-20 at 09:06 -0500, Nicolas Pitre wrote:
> > > > > > ... in which case there is no harm shipping a .c file and trivially 
> > > > > > enforcing -O2, the rest being equal.
> > > > > 
> > > > > For today's compilers, unless the wind changes.
> > > > 
> > > > We'll adapt if necessary.  Going with -O2 should remain pretty safe anyway.
> > > 
> > > Alas, not so for gcc 4.4 - I had forgotten I had tested
> > > Ubuntu/Linaro 4.4.7-1ubuntu2 here:
> > > 
> > > https://patchwork.kernel.org/patch/2101491/
> > > 
> > > add -O2 to that test script and gcc 4.4 *always* emits calls to
> > > __bswap[sd]i2, even with -march=armv6k+.
> 
> argh, sorry - that script was testing support for 
> __builtin_bswap{16,32,64} directly, which isn't the same as testing
> code generation of a byte swap pattern in C.

Still, I'm not as confident as I was about this.

> I'll still try the assembly approach - gcc 4.4's armv6 output looks
> worse than both the pre-armv6 and post-armv6 __arch_swab32
> implementations currently in use:
> 
> mov     ip, sp
> push    {fp, ip, lr, pc}
> sub     fp, ip, #4

You should use -fomit-frame-pointer to compile this.  We don't need a 
frame pointer here, especially for a leaf function that the compiler 
decides to call on its own.

> and     r2, r0, #65280  ; 0xff00
> lsl     ip, r0, #24
> orr     r1, ip, r0, lsr #24
> and     r0, r0, #16711680       ; 0xff0000
> orr     r3, r1, r2, lsl #8
> orr     r0, r3, r0, lsr #8

Other than that, it is true that the above is slightly suboptimal.


Nicolas