On Fri, Aug 28, 2015 at 03:02:54PM -0700, Jarno Rajahalme wrote:
>
> > On Aug 28, 2015, at 10:54 AM, Ben Pfaff <b...@nicira.com> wrote:
> > However, GCC is almost criminally bad at optimizing it:
> >
> > blp@sigabrt:~/nicira/ovs/_build(0)$ cat tmp.c
> > struct x {
> > union {
> > unsigned char b[6];
> > unsigned short w[3];
> > };
> > };
> > void g(struct x);
> > void f(void)
> > {
> > struct x y = { { { 1, 2, 3, 4, 5, 6 } } };
> > g(y);
> > }
> >
> > blp@sigabrt:~/nicira/ovs/_build(0)$ gcc -O2 -g -m64 -c tmp.c
> > blp@sigabrt:~/nicira/ovs/_build(0)$ objdump -dr tmp.o
> >
> > tmp.o: file format elf64-x86-64
> >
> >
> > Disassembly of section .text:
> >
> > 0000000000000000 <f>:
> > 0: 48 83 ec 18 sub $0x18,%rsp
> > 4: c6 04 24 01 movb $0x1,(%rsp)
> > 8: c6 44 24 01 02 movb $0x2,0x1(%rsp)
> > d: c6 44 24 02 03 movb $0x3,0x2(%rsp)
> > 12: c6 44 24 03 04 movb $0x4,0x3(%rsp)
> > 17: c6 44 24 04 05 movb $0x5,0x4(%rsp)
> > 1c: c6 44 24 05 06 movb $0x6,0x5(%rsp)
> > 21: 48 8b 3c 24 mov (%rsp),%rdi
> > 25: e8 00 00 00 00 callq 2a <f+0x2a>
> > 26: R_X86_64_PC32 g-0x4
> > 2a: 48 83 c4 18 add $0x18,%rsp
> > 2e: c3 retq
>
> One would hope that GCC would do a better job when inlining, though?
I hope so!
> > Acked-by: Ben Pfaff <b...@nicira.com <mailto:b...@nicira.com>>
>
> Thanks for the review. Unfortunately I forgot to add your Acked-by to
> the commit message, and I realized that just after pushing to github -
> now I don’t have anyone to shift blame to if builds start failing…
;-)
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