Messages in this thread | | | Date | Thu, 21 Feb 2013 11:40:54 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [RFC] arm: use built-in byte swap function |
| |
On Thu, 21 Feb 2013, Kim Phillips wrote:
> On Wed, 20 Feb 2013 23:29:58 -0500 > Nicolas Pitre <nico@fluxnic.net> wrote: > > > On Wed, 20 Feb 2013, Kim Phillips wrote: > > > > > On Wed, 20 Feb 2013 10:43:18 -0500 > > > Nicolas Pitre <nico@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
| |