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


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.

> 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.

> 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;

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.

Thanks,
Richard.


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