On Thu, 24 Nov 2016, Senthil Kumar Selvaraj wrote:
>
> Richard Biener writes:
>
> > On Thu, 24 Nov 2016, Richard Biener wrote:
> >
> >> On Thu, 24 Nov 2016, Senthil Kumar Selvaraj wrote:
> >>
> >> > Hi,
> >> >
> >> > I've been analyzing a failing regtest (gcc.dg/strlenopt-8.c) for the
> >> > avr
> >> > target. I found that the (dump) failure is because there are 4
> >> > instances of memcpy, while the testcase expects only 2 for a
> >> > non-strict align target like the avr.
> >> >
> >> > Comparing that with a dump generated by x64_64-pc-linux, I found that
> >> > the extra memcpy's come from the forwprop pass, when it replaces
> >> > strcat with strlen and memcpy. For x86_64, the memcpy generated gets
> >> > folded into a load/store in gimple_fold_builtin_memory_op. That
> >> > doesn't happen for the avr because len (2) happens to be bigger than
> >> > MOVE_MAX (1).
> >> >
> >> > The avr can only move 1 byte efficiently from reg <-> memory, but it's
> >> > more efficient to load and store 2 bytes than to call memcpy, so
> >> > MOVE_MAX_PIECES is set to 2.
> >> >
> >> > Given that gimple_fold_builtin_memory_op gets to choose between
> >> > leaving the memcpy call as is, or breaking it down to a by-pieces
> >> > move, shouldn't it use MOVE_MAX_PIECES instead of
> >> > MOV_MAX?
> >> >
> >> > That is what the below patch does, and that makes the test
> >> > pass. Does this sound right?
> >>
> >> No, as we handle both memcpy and memmove this way we rely on
> >> the whole storage fit in a single register so we do the
> >> right thing for overlapping memory.
> >
> > So actually your patch doesn't chnage that, the ordering is ensured
> > by emitting a single GIMPLE load/store pair. There are only
> > four targets defining MOVE_MAX_PIECES, and one (s390) even has
> > a smaller MOVE_MAX_PIECES than MOVE_MAX (huh). AVR has larger
> > MOVE_MAX_PIECES than MOVE_MAX, but that seems to not make much
> > sense to me given their very similar description plus the
> > fact that AVR can only load a single byte at a time...
> >
> > The x86 comment says
> >
> > /* MOVE_MAX_PIECES is the number of bytes at a time which we can
> > move efficiently, as opposed to MOVE_MAX which is the maximum
> > number of bytes we can move with a single instruction.
> >
> > which doesn't match up with
> >
> > @defmac MOVE_MAX
> > The maximum number of bytes that a single instruction can move quickly
> > between memory and registers or between two memory locations.
> > @end defmac
> >
> > note "quickly" here. But OTOH
> >
> > @defmac MOVE_MAX_PIECES
> > A C expression used by @code{move_by_pieces} to determine the largest unit
> > a load or store used to copy memory is. Defaults to @code{MOVE_MAX}.
> > @end defmac
> >
> > here the only difference is "copy memory". But we try to special
> > case the one load - one store case, not generally "copy memory".
> >
> > So I think MOVE_MAX matches my intent when writing the code.
>
> Ok, but isn't that inconsistent with tree-inline.c:estimate_move_cost, which
> considers MOVE_MAX_PIECES and MOVE_RATIO to decide between a libcall and
> by-pieces move?
Well, I don't understand why we have both MOVE_MAX and MOVE_MAX_PIECES.
There are exactly two uses of MOVE_MAX in GCC AFAICS, the gimple-fold.c
one and caller-save.c which derives MOVE_MAX_WORDS from it.
MOVE_MAX_PIECES has the only use in block move expansion plus the
single use in tree-inline.c.
So I can't give a reason why one or the other should be more valid
but the tree-inline.c one tries to match memcpy expansion (obviously),
while the gimple-fold.c one tries to get at the maximum possible
single-insn move amount (and AVR is odd here having that lower than
MOVE_MAX_PIECES, compared to say s390 which has it the opposite way
around).
Richard.
> Regards
> Senthil
>
> >
> > Richard.
> >
> >> Richard.
> >>
> >> > Regards
> >> > Senthil
> >> >
> >> > Index: gcc/gimple-fold.c
> >> > ===================================================================
> >> > --- gcc/gimple-fold.c (revision 242741)
> >> > +++ gcc/gimple-fold.c (working copy)
> >> > @@ -703,7 +703,7 @@
> >> > src_align = get_pointer_alignment (src);
> >> > dest_align = get_pointer_alignment (dest);
> >> > if (tree_fits_uhwi_p (len)
> >> > - && compare_tree_int (len, MOVE_MAX) <= 0
> >> > + && compare_tree_int (len, MOVE_MAX_PIECES) <= 0
> >> > /* ??? Don't transform copies from strings with known length
> >> > this
> >> > confuses the tree-ssa-strlen.c. This doesn't handle
> >> > the case in gcc.dg/strlenopt-8.c which is XFAILed for that
> >> >
> >> >
> >>
> >>
>
>
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
