Re: RFC [1/2] divmod transform

[Richard Biener <rguenther@suse.de>]

On Tue, 24 May 2016, Prathamesh Kulkarni wrote:

> On 23 May 2016 at 17:35, Richard Biener <richard.guenther@gmail.com> wrote:
> > On Mon, May 23, 2016 at 10:58 AM, Prathamesh Kulkarni
> > <prathamesh.kulkarni@linaro.org> wrote:
> >> Hi,
> >> I have updated my patch for divmod (attached), which was originally
> >> based on Kugan's patch.
> >> The patch transforms stmts with code TRUNC_DIV_EXPR and TRUNC_MOD_EXPR
> >> having same operands to divmod representation, so we can cse computation of mod.
> >>
> >> t1 = a TRUNC_DIV_EXPR b;
> >> t2 = a TRUNC_MOD_EXPR b
> >> is transformed to:
> >> complex_tmp = DIVMOD (a, b);
> >> t1 = REALPART_EXPR (complex_tmp);
> >> t2 = IMAGPART_EXPR (complex_tmp);
> >>
> >> * New hook divmod_expand_libfunc
> >> The rationale for introducing the hook is that different targets have
> >> incompatible calling conventions for divmod libfunc.
> >> Currently three ports define divmod libfunc: c6x, spu and arm.
> >> c6x and spu follow the convention of libgcc2.c:__udivmoddi4:
> >> return quotient and store remainder in argument passed as pointer,
> >> while the arm version takes two arguments and returns both
> >> quotient and remainder having mode double the size of the operand mode.
> >> The port should hence override the hook expand_divmod_libfunc
> >> to generate call to target-specific divmod.
> >> Ports should define this hook if:
> >> a) The port does not have divmod or div insn for the given mode.
> >> b) The port defines divmod libfunc for the given mode.
> >> The default hook default_expand_divmod_libfunc() generates call
> >> to libgcc2.c:__udivmoddi4 provided the operands are unsigned and
> >> are of DImode.
> >>
> >> Patch passes bootstrap+test on x86_64-unknown-linux-gnu and
> >> cross-tested on arm*-*-*.
> >> Bootstrap+test in progress on arm-linux-gnueabihf.
> >> Does this patch look OK ?
> >
> > diff --git a/gcc/targhooks.c b/gcc/targhooks.c
> > index 6b4601b..e4a021a 100644
> > --- a/gcc/targhooks.c
> > +++ b/gcc/targhooks.c
> > @@ -1965,4 +1965,31 @@ default_optab_supported_p (int, machine_mode,
> > machine_mode, optimization_type)
> >    return true;
> >  }
> >
> > +void
> > +default_expand_divmod_libfunc (bool unsignedp, machine_mode mode,
> > +                              rtx op0, rtx op1,
> > +                              rtx *quot_p, rtx *rem_p)
> >
> > functions need a comment.
> >
> > ISTR it was suggested that ARM change to libgcc2.c__udivmoddi4 style?  In that
> > case we could avoid the target hook.
> Well I would prefer adding the hook because that's more easier -;)
> Would it be ok for now to go with the hook ?
> >
> > +      /* If target overrides expand_divmod_libfunc hook
> > +        then perform divmod by generating call to the target-specifc divmod
> > libfunc.  */
> > +      if (targetm.expand_divmod_libfunc != default_expand_divmod_libfunc)
> > +       return true;
> > +
> > +      /* Fall back to using libgcc2.c:__udivmoddi4.  */
> > +      return (mode == DImode && unsignedp);
> >
> > I don't understand this - we know optab_libfunc returns non-NULL for 'mode'
> > but still restrict this to DImode && unsigned?  Also if
> > targetm.expand_divmod_libfunc
> > is not the default we expect the target to handle all modes?
> Ah indeed, the check for DImode is unnecessary.
> However I suppose the check for unsignedp should be there,
> since we want to generate call to __udivmoddi4 only if operand is unsigned ?

The optab libfunc for sdivmod should be NULL in that case.

> >
> > That said - I expected the above piece to be simply a 'return true;' ;)
> >
> > Usually we use some can_expand_XXX helper in optabs.c to query if the target
> > supports a specific operation (for example SImode divmod would use DImode
> > divmod by means of widening operands - for the unsigned case of course).
> Thanks for pointing out. So if a target does not support divmod
> libfunc for a mode
> but for a wider mode, then we could zero-extend operands to the wider-mode,
> perform divmod on the wider-mode, and then cast result back to the
> original mode.
> I haven't done that in this patch, would it be OK to do that as a follow up ?

I think that you should conservatively handle the div_optab query, thus if
the target has a HW division in a wider mode don't use the divmod IFN.
You'd simply iterate over GET_MODE_WIDER_MODE and repeat the
if (optab_handler (div_optab, mode) != CODE_FOR_nothing) check, bailing
out if that is available.

> > +  /* Disable the transform if either is a constant, since
> > division-by-constant
> > +     may have specialized expansion.  */
> > +  if (TREE_CONSTANT (op1) || TREE_CONSTANT (op2))
> > +    return false;
> >
> > please use CONSTANT_CLASS_P (op1) || CONSTANT_CLASS_P (op2)
> >
> > +  if (TYPE_OVERFLOW_TRAPS (type))
> > +    return false;
> >
> > why's that?  Generally please first test cheap things (trapping, constant-ness)
> > before checking expensive stuff (target_supports_divmod_p).
> I added TYPE_OVERFLOW_TRAPS (type) based on your suggestion in:
> https://www.mail-archive.com/gcc@gcc.gnu.org/msg78534.html
> "When looking at TRUNC_DIV_EXPR you should also exclude
> the case where TYPE_OVERFLOW_TRAPS (type) as that should
> expand using the [su]divv optabs (no trapping overflow
> divmod optab exists)."

Ok, didn't remember that.

> >
> > +static bool
> > +convert_to_divmod (gassign *stmt)
> > +{
> > +  if (!divmod_candidate_p (stmt))
> > +    return false;
> > +
> > +  tree op1 = gimple_assign_rhs1 (stmt);
> > +  tree op2 = gimple_assign_rhs2 (stmt);
> > +
> > +  vec<gimple *> stmts = vNULL;
> >
> > use an auto_vec <gimple *> - you currently leak it in at least one place.
> >
> > +      if (maybe_clean_or_replace_eh_stmt (use_stmt, use_stmt))
> > +       cfg_changed = true;
> >
> > note that this suggests you should check whether any of the stmts may throw
> > internally as you don't update / transfer EH info correctly.  So for 'stmt' and
> > all 'use_stmt' check stmt_can_throw_internal and if so do not add it to
> > the list of stmts to modify.
> >
> > Btw, I think you should not add 'stmt' immediately but when iterating over
> > all uses also gather uses in TRUNC_MOD_EXPR.
> >
> > Otherwise looks ok.
> Done changes in this version. I am gathering mod uses same time as div uses,
> so this imposes a constraint that mod dominates mod. I am not sure if
> that's desirable.

I think you also need a mod_seen variable now that you don't necessarily
end up with 'stmt' in the vector of stmts.  I don't see how there is a
constraint that mod dominates mod - it's just that the top_stmt needs
to dominate all other uses that can be replaced with replacing top_stmt
with a divmod.  It's just that the actual stmt set we choose may now
depend on immediate uses order which on a second thought is bad
as immediate uses order could be affected by debug stmts ... hmm.

To avoid this please re-add the code adding 'stmt' to stmts immediately
and add a use_stmt != stmt check to the immediate use processing loop
so that we don't end up adding it twice.

Richard.