On Tue, 2015-04-28 at 21:01 +0200, christophe leroy wrote: > > > Le 25/03/2015 02:30, Scott Wood a écrit : > > > On Tue, Feb 03, 2015 at 12:39:27PM +0100, LEROY Christophe wrote: > > > The C version of csum_add() as defined in include/net/checksum.h gives the > > > following assembly: > > > 0: 7c 04 1a 14 add r0,r4,r3 > > > 4: 7c 64 00 10 subfc r3,r4,r0 > > > 8: 7c 63 19 10 subfe r3,r3,r3 > > > c: 7c 63 00 50 subf r3,r3,r0 > > > > > > include/net/checksum.h also offers the possibility to define an arch > > > specific > > > function. > > > This patch provides a ppc32 specific csum_add() inline function. > > What makes it 32-bit specific? > > > > > As far as I understand, the 64-bit will do a 64 bit addition, so we > will have to handle differently the carry, can't just be an addze like > in 32-bit.
OK. Before I couldn't find where this was ifdeffed to 32-bit, but it's in patch 1/2. > The generated code is most likely different on ppc64. I have no ppc64 > compiler so I can't check what gcc generates for the following code: > > __wsum csum_add(__wsum csum, __wsum addend) > { > u32 res = (__force u32)csum; > res += (__force u32)addend; > return (__force __wsum)(res + (res < (__force u32)addend)); > } > > Can someone with a ppc64 compiler tell what we get ? With CONFIG_GENERIC_CPU: 0xc000000000001af8 <+0>: add r3,r3,r4 0xc000000000001afc <+4>: cmplw cr7,r3,r4 0xc000000000001b00 <+8>: mfcr r4 0xc000000000001b04 <+12>: rlwinm r4,r4,29,31,31 0xc000000000001b08 <+16>: add r3,r4,r3 0xc000000000001b0c <+20>: clrldi r3,r3,32 0xc000000000001b10 <+24>: blr The mfcr is particularly nasty, at least on our chips. With CONFIG_CPU_E6500: 0xc000000000001b30 <+0>: add r3,r3,r4 0xc000000000001b34 <+4>: cmplw cr7,r3,r4 0xc000000000001b38 <+8>: mfocrf r4,1 0xc000000000001b3c <+12>: rlwinm r4,r4,29,31,31 0xc000000000001b40 <+16>: add r3,r4,r3 0xc000000000001b44 <+20>: clrldi r3,r3,32 0xc000000000001b48 <+24>: blr Ideal (short of a 64-bit __wsum) would probably be something like (untested): add r3,r3,r4 srdi r5,r3,32 add r3,r3,r5 clrldi r3,r3,32 Or in C code (which would let the compiler schedule it better): static inline __wsum csum_add(__wsum csum, __wsum addend) { u64 res = (__force u64)csum; res += (__force u32)addend; return (__force __wsum)((u32)res + (res >> 32)); } -Scott -- 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/