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Re: [RFC PR43721] Optimize a/b and a%b to single divmod call


On Wed, 11 Nov 2015, Prathamesh Kulkarni wrote:

> On 11 November 2015 at 16:03, Richard Biener <rguenther@suse.de> wrote:
> > On Wed, 11 Nov 2015, Prathamesh Kulkarni wrote:
> >
> >> On 10 November 2015 at 20:11, Richard Biener <rguenther@suse.de> wrote:
> >> > On Mon, 9 Nov 2015, Prathamesh Kulkarni wrote:
> >> >
> >> >> On 4 November 2015 at 20:35, Richard Biener <rguenther@suse.de> wrote:
> >> >> >
> >> >> > Btw, did you investigate code gen differences on x86_64/i586?  That
> >> >> > target expands all divisions/modulo ops via divmod, relying on CSE
> >> >> > solely as the HW always computes both div and mod (IIRC).
> >> >> x86_64 has optab_handler for divmod defined, so the transform won't
> >> >> take place on x86.
> >> >
> >> > Ok.
> >> >
> >> >> > +
> >> >> > +        gassign *assign_stmt = gimple_build_assign (gimple_assign_lhs
> >> >> > (use_stmt), rhs);
> >> >> > +        gimple_stmt_iterator gsi = gsi_for_stmt (use_stmt);
> >> >> >
> >> >> > Ick.  Please use
> >> >> >
> >> >> >     gimple_set_rhs_from_tree (use_stmt, res);
> >> >> Um there doesn't seem to be gimple_set_rhs_from_tree.
> >> >> I used gimple_assign_set_rhs_from_tree which requires gsi for use_stmt.
> >> >> Is that OK ?
> >> >
> >> > Yes.
> >> >
> >> >> >     update_stmt (use_stmt);
> >> >> >     if (maybe_clean_or_replace_eh_stmt (use_stmt, use_stmt))
> >> >> >       cfg_changed = true;
> >> >> >
> >> >> > +  free_dominance_info (CDI_DOMINATORS);
> >> >> >
> >> >> > do not free dominators.
> >> >>
> >> >> I have done the suggested changes in the attached patch.
> >> >> I have a few questions:
> >> >>
> >> >> a) Does the change to insert DIVMOD call before topmost div or mod
> >> >> stmt with matching operands
> >> >> look correct ?
> >> >
> >> > +  /* Insert call-stmt just before the topmost div/mod stmt.
> >> > +     top_bb dominates all other basic blocks containing div/mod stms
> >> > +     so, the topmost stmt would be the first div/mod stmt with matching
> >> > operands
> >> > +     in top_bb.  */
> >> > +
> >> > +  gcc_assert (top_bb != 0);
> >> > +  gimple_stmt_iterator gsi;
> >> > +  for (gsi = gsi_after_labels (top_bb); !gsi_end_p (gsi); gsi_next
> >> > (&gsi))
> >> > +    {
> >> > +      gimple *g = gsi_stmt (gsi);
> >> > +      if (is_gimple_assign (g)
> >> > +         && (gimple_assign_rhs_code (g) == TRUNC_DIV_EXPR
> >> > +            || gimple_assign_rhs_code (g) == TRUNC_MOD_EXPR)
> >> > +         && operand_equal_p (op1, gimple_assign_rhs1 (g), 0)
> >> > +         && operand_equal_p (op2, gimple_assign_rhs2 (g), 0))
> >> > +       break;
> >> >
> >> > Looks overly complicated to me.  Just remember "topmost" use_stmt
> >> > alongside top_bb (looks like you'll no longer need top_bb if you
> >> > retail top_stmt).  And then do
> >> >
> >> >    gsi = gsi_for_stmt (top_stmt);
> >> >
> >> > and insert before that.
> >> Thanks, done in this patch. Does it look OK ?
> >> IIUC gimple_uid (stmt1) < gimple_uid (stmt2) can be used to check if
> >> stmt1 occurs before stmt2
> >> only if stmt1 and stmt2 are in the same basic block ?
> >> >
> >> >> b) Handling constants - I dropped handling constants in the attached
> >> >> patch. IIUC we don't want
> >> >> to enable this transform if there's a specialized expansion for some
> >> >> constants for div or mod ?
> >> >
> >> > See expand_divmod which has lots of special cases for constant operands
> >> > not requiring target support for div or mod.
> >> Thanks, would it be OK if I do this in follow up patch ?
> >
> > Well, just not handle them like in your patch is fine.
> >
> >> >
> >> >> I suppose this would also be target dependent and require a target hook ?
> >> >> For instance arm defines modsi3 pattern to expand mod when 2nd operand
> >> >> is constant and <= 0 or power of 2,
> >> >> while for other cases goes the expand_divmod() route to generate call
> >> >> to __aeabi_idivmod libcall.
> >> >
> >> > Ok, so it lacks a signed mod instruction.
> >> >
> >> >> c) Gating the divmod transform -
> >> >> I tried gating it on checks for optab_handlers on div and mod, however
> >> >> this doesn't enable transform for arm cortex-a9
> >> >> anymore (cortex-a9 doesn't have hardware instructions for integer div and mod).
> >> >> IIUC for cortex-a9,
> >> >> optab_handler (sdivmod_optab, SImode) returns CODE_FOR_nothing because
> >> >> HAVE_divsi3 is 0.
> >> >> However optab_handler (smod_optab, SImode) matches since optab_handler
> >> >> only checks for existence of pattern
> >> >> (and not whether the pattern gets matched).
> >> >> I suppose we should enable the transform only if the divmod, div, and
> >> >> mod pattern do not match rather than checking
> >> >> if the patterns exist via optab_handler ? For a general x % y, modsi3
> >> >> would fail to match but optab_handler(smod_optab, SImode ) still
> >> >> says it's matched.
> >> >
> >> > Ah, of course.  Querying for an optab handler is just a cheap
> >> > guesstimate...  Not sure how to circumvent this best (sub-target
> >> > enablement of patterns).  RTL expansion just goes ahead (of course)
> >> > and sees if expansion eventually fails.  Richard?
> >> >
> >> >> Should we define a new target hook combine_divmod, which returns true
> >> >> if transforming to divmod is desirable for that
> >> >> target ?
> >> >> The default definition could be:
> >> >> bool default_combine_divmod (enum machine_mode mode, tree op1, tree op2)
> >> >> {
> >> >>   // check for optab_handlers for div/mod/divmod and libfunc for divmod
> >> >> }
> >> >>
> >> >> And for arm, it could be over-ridden to return false if op2 is
> >> >> constant and <= 0 or power of 2.
> >> >> I am not really sure if this is a good idea since I am replicating
> >> >> information from modsi3 pattern.
> >> >> Any change to the pattern may require corresponding change to the hook :/
> >> >
> >> > Yeah, I don't think that is desirable.  Ideally we'd have a way
> >> > to query HAVE_* for CODE_FOR_* which would mean target-insns.def
> >> > support for all div/mod/divmod patterns(?) and queries...
> >> >
> >> > Not sure if what would be enough though.
> >> >
> >> > Note that the divmod check is equally flawed.
> >> >
> >> > I think with the above I'd enable the transform when
> >> >
> >> > +  if (optab_handler (divmod_optab, mode) != CODE_FOR_nothing
> >> > +      || (optab_libfunc (divmod_optab, mode) != NULL_RTX
> >> >            && optab_handler ([su]div_optab, mode) == CODE_FOR_nothing))
> >> > +    return false;
> >> Um this fails for the arm backend (for cortex-a9) because
> >> optab_handler (divmod_optab, mode) != CODE_FOR_nothing is false
> >> optab_libfunc (divmod_optab, mode) != NULL_RTX is true.
> >> optab_handler (div_optab, mode) == CODE_FOR_nothing is true.
> >> which comes down to false || (true && true) which is true and we hit
> >> return false.
> >
> > Oh, sorry to mess up the test - it was supposed to be inverted.
> >
> >> AFAIU, we want the transform to be disabled if:
> >> a) optab_handler exists for divmod.
> >> b) optab_handler exists for div.
> >> c) optab_libfunc does not exist for divmod.  */
> >>
> >> +  if (optab_handler (divmod_optab, mode) != CODE_FOR_nothing
> >> +      || optab_handler (div_optab, mode) != CODE_FOR_nothing
> >> +      || optab_libfunc (divmod_optab, mode) == NULL_RTX)
> >> +    return false;
> >> Does that look correct ?
> >
> > No.  That will disable if we have a divmod optab.  Instead try
> >
> >  if (! (optab_handler (divmod_optab, mode) != CODE_FOR_nothing
> >         || (optab_libfunc (divmod_optab, mode) != NULL_RTX
> >             && optab_handler ([su]div_optab, mode) == CODE_FOR_nothing)))
> >    return false;
> >
> > which is what I intended.  If we have a divmod optab go ahead.
> > If we have a libfunc and not a div optab then as well.
> Oops, I assumed that we only wanted this transform if optab_libfunc existed.
> Modified the test in the attached patch.
> Well this does affect x86_64 and i?86 -;)
> 
> I added the following hunk back to expand_DIVMOD, since if optab_handler exists
> we want to use it for expansion. Does it look OK ?
> 
> +  /* Check if optab handler exists for udivmod/sdivmod.  */
> +  if (optab_handler (tab, mode) != CODE_FOR_nothing)
> +    {
> +      rtx quotient = gen_reg_rtx (mode);
> +      rtx remainder = gen_reg_rtx (mode);
> +      expand_twoval_binop (tab, op0, op1, quotient, remainder,
> TYPE_UNSIGNED (type));
> +
> +      /* Wrap the return value (quotient, remaineder) within COMPLEX_EXPR */
> +      expand_expr (build2 (COMPLEX_EXPR, TREE_TYPE (lhs),
> +                  make_tree (TREE_TYPE (arg0), quotient),
> +                  make_tree (TREE_TYPE (arg1), remainder)),
> +                  target, VOIDmode, EXPAND_NORMAL);
> +
> +      return;
> +    }

Ah, sure.

> I verified the code generated for x86_64 and i?86 and it's same for my
> test-cases.
> However during a clean build of gcc for x86_64, I am getting segfault
> in bid64_div.c:
> In file included from
> /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/libgcc/config/libbid/bid_internal.h:27:0,
>                  from
> /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/libgcc/config/libbid/bid64_div.c:56:
> /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/libgcc/config/libbid/bid64_div.c:
> In function â__bid64_divâ:
> /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/libgcc/config/libbid/bid_conf.h:36:19:
> internal compiler error: in expand_DIVMOD, at internal-fn.c:2099
>  #define bid64_div __bid64_div
>                    ^
> /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/libgcc/config/libbid/bid64_div.c:80:1:
> note: in expansion of macro âbid64_divâ
>  bid64_div (UINT64 x,
>  ^
> 0x8e101f expand_DIVMOD
>         /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/gcc/internal-fn.c:2099
> 0x705927 expand_call_stmt
>         /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/gcc/cfgexpand.c:2549
> 0x705927 expand_gimple_stmt_1
>         /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/gcc/cfgexpand.c:3509
> 0x705927 expand_gimple_stmt
>         /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/gcc/cfgexpand.c:3672
> 0x708d65 expand_gimple_basic_block
>         /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/gcc/cfgexpand.c:5676
> 0x70e696 execute
>         /home/bilbo/gnu-toolchain/src/gcc.git~tcwg-72/gcc/cfgexpand.c:6288
> 
> It looks like in the following code in expand_DIVMOD:
> 
> +  rtx quotient = simplify_gen_subreg (mode, libval, libval_mode, 0);
> +  rtx remainder = simplify_gen_subreg (mode, libval, libval_mode,
> +                                      GET_MODE_SIZE (mode));
> 
> remainder is (nil) and hence the segfault in make_tree (I added the
> asserts for quotient and remainder later).
> I am not sure why it's happening  though, investigating it.

No idea - as said, RTL expansion isn't my area of expertise.  I would
expect that offsetted subregs shouldn't be used for complex components.
Maybe gen_lowpart / gen_highpart will work better which abstracts
the subregging.

Richard.

> Thanks,
> Prathamesh
> >
> >> >
> >> > so we either will have a divmod instruction (hopefully not sub-target
> >> > disabled for us) or a libfunc for divmod and for sure no HW divide
> >> > instruction (HW mod can be emulated by HW divide but not the other
> >> > way around).
> >> >
> >> >> d) Adding effective-target-check for divmod: I just enabled it for
> >> >> arm*-*-* for now. I could additionally append more targets,
> >> >> not sure if this is the right approach.
> >> >
> >> > Looks good to me.
> >> Is this version OK if bootstrap/testing passes ?
> >
> > Ok with adjusting the optab check like above.
> >
> > Thanks,
> > Richard.

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