There is a claim in linux asm-i386/byteswap.h that:
/* Do not define swab16. Gcc is smart enough to recognize "C" version and
convert it into rotation or exhange. */
Not really. Consider these two testcases:
--cut here--
unsigned short bad(unsigned short a)
{
return ((a & 0xff00) >> 8 | (a & 0x00ff) << 8);
}
unsigned short good(unsigned short a)
{
return (a >> 8 | a << 8);
}
--cut here--
gcc -O2 -S -m32 -fomit-frame-pointer
bad:
movzwl 4(%esp), %edx
movl %edx, %eax
sall $8, %eax
shrl $8, %edx
orl %edx, %eax
movzwl %ax, %eax
ret
good:
movzwl 4(%esp), %eax
rolw $8, %ax
movzwl %ax, %eax
ret
IMO both forms should produce equal asm.
Unfortunatelly, first form is usually used:
drivers/net/sk98lin/skxmac2.c:
SWord = ((SWord & 0xff00) >> 8) | ((SWord & 0x00ff) << 8);
drivers/atm/iphase.c:
#define swap(x) (((x & 0xff) << 8) | ((x & 0xff00) >> 8))
drivers/atm/iphase.c:
trailer->length = ((skb->len & 0xff) << 8) | ((skb->len & 0xff00) >> 8);
Ideally, this construct should be compiled using (not yet introduced...)
bswaphi2 pattern to generate "xchgb %ah,%al" instead of rolw insn. Accordint to
pentopt.pdf, xchg is faster, 1.5 vs 4 clks.
Maybe this transformation could also be used for 32bit and 64bit data, to
automatically convert open coded shift series into bswapsi2 and bswapdi2
patterns.
--
Summary: Missing byte swap optimizations
Product: gcc
Version: 4.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: ubizjak at gmail dot com
GCC host triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29749
