On Thu, Jul 24, 2014 at 10:33 PM, Linus Torvalds <torva...@linux-foundation.org> wrote: > On Thu, Jul 24, 2014 at 6:25 PM, Michel Dänzer <mic...@daenzer.net> wrote: >> >> Attached is fair.s from Debian gcc 4.8.3-5. Does that look better? I'm >> going to try reproducing the problem with a kernel built by that now. > > This looks better. For roughly that same code sequence it does > (ignoring the debug line and cfi information): > > subq $184, %rsp #, > movq (%r12), %rax # sd_22(D)->parent, sd_parent > movl %edi, -156(%rbp) # this_cpu, %sfp > movl %ecx, -160(%rbp) # idle, %sfp > movq %r8, -184(%rbp) # continue_balancing, %sfp > movq %rax, -176(%rbp) # sd_parent, %sfp > movq $load_balance_mask, %rax #, tcp_ptr__ > #APP > add %gs:this_cpu_off, %rax # this_cpu_off, tcp_ptr__ > #NO_APP > > so it updates the stack pointer before any spills, and it also doesn't > spill that constant value. > > I still have no idea why it does the 4-byte rep stosl/movsl thing, but > that's a whole separate guessing game and might have something to do > with the fact that you do CONFIG_CC_OPTIMIZE_FOR_SIZE and the 4-byte > form is one byte smaller. > > I'm a big believer in not blowing up the I$ footprint, and I have to > admit to pushing that myself a few years ago, but gcc does some rather > bad things with '-Os', so it's not actually suggested for the kernel > any more. I wish there was some middle ground model that cared about > size, but not to exclusion of everything else. The string instructions > are not good for performance when it's a compile-time known small > size. > > Linus > -- > 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/
Seems better after looking at it too, seems I don't need to test this and this bug is in gcc 4.9 related versions. Cheers Nick -- To UNSUBSCRIBE, email to debian-kernel-requ...@lists.debian.org with a subject of "unsubscribe". Trouble? Contact listmas...@lists.debian.org Archive: https://lists.debian.org/capdomviurywj_26subougdvwfpeoamqc3_po3ewhhrqybj6...@mail.gmail.com
