On Thu, 21 Feb 2013, Kim Phillips wrote: > On Wed, 20 Feb 2013 23:29:58 -0500 > Nicolas Pitre <n...@fluxnic.net> wrote: > > > On Wed, 20 Feb 2013, Kim Phillips wrote: > > > > > On Wed, 20 Feb 2013 10:43:18 -0500 > > > Nicolas Pitre <n...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/