On Mon, Dec 17, 2018 at 11:16 AM Richard Biener
<richard.guenther@xxxxxxxxx> wrote:
>
> On Mon, Dec 17, 2018 at 8:34 AM Thomas Koenig <tkoenig@xxxxxxxxxxxxx> wrote:
> >
> > Hi Harald,
> >
> > > If there's interest, I can create a bugzilla with test program
> > > and test data.
> >
> > Please do.
> >
> > In an ideal world, people would use always use bounds checking,
> > with almost zero overhead. This is not realistic, but we should
> > not regress on our way there :-)
>
> GCC 9 IL looks saner than the GCC 7/8 one. Note both compilers
> have bound checks inside the innermost loop. The main difference
> seems to be in loop header copying where GCC 9 is behaving
> much "better" IMHO. It would be interesting to see whether
> -fno-tree-ch brings results of the compilers in-line again (even
> if it causes the code to run even more slow).
Oh, and -funroll-loops might be an issue as well given the large
number of branches inside the loop body. Citing non-unrolled
innermost loop body from GCC 9:
.L19:
testq %r8, %r8
jle .L25
cmpq %r8, %r12
jl .L26
movslq (%rax), %rdx
testq %rdx, %rdx
jle .L27
cmpq %rdx, %r14
jl .L28
cmpq %r8, %rcx
jl .L29
cmpq %r11, %r13
jl .L30
imulq %r10, %rdx
vxorpd %xmm0, %xmm0, %xmm0
vcvtss2sd (%rsi), %xmm0, %xmm0
subq %r10, %rdx
leaq (%r15,%rdx,8), %rdx
vmovsd (%rdx), %xmm1
incq %r8
vfmadd132sd (%r9), %xmm1, %xmm0
addq %rbp, %rsi
addq %rbx, %rax
vmovsd %xmm0, (%rdx)
cmpl %r8d, %edi
jg .L19
branch density of the bounds-checking code is quite dense
and I suspect predictors don't like that very much. You
might want to look at perf output with counting branch mispredicts.
For GCC it might make sense to more (read: very) aggressively
combine test&branches to abort()s. Maybe the FE can already
do this for bound checks from a single statement?
Richard.
> Richard.
>
> > Regards
> >
> > Thomas
