On 24 May 2016 at 19:39, Richard Biener <rguent...@suse.de> wrote:
> On Tue, 24 May 2016, Prathamesh Kulkarni wrote:
>
>> On 24 May 2016 at 17:42, Richard Biener <rguent...@suse.de> wrote:
>> > On Tue, 24 May 2016, Prathamesh Kulkarni wrote:
>> >
>> >> On 23 May 2016 at 17:35, Richard Biener <richard.guent...@gmail.com>
>> >> wrote:
>> >> > On Mon, May 23, 2016 at 10:58 AM, Prathamesh Kulkarni
>> >> > <prathamesh.kulka...@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.
>> Ah indeed, thanks.
>> >
>> >> >
>> >> > 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.
>> Done.
>> >
>> >> > + /* 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.
>> Well I wonder what will happen for the following case:
>> t1 = x / y;
>> if (cond)
>> t2 = x % y;
>> else
>> t3 = x % y;
>>
>> Assuming stmt == top_stmt is "t2 = x % y" and use_stmt is "t3 = x % y",
>> use_stmt will not get added to stmts vector, since top_stmt and
>> use_stmt are not in same bb,
>> and bb's containing top_stmt and use_stmt don't dominate each other.
>> Not sure if this is practical case (I assume fre will hoist mod
>> outside if-else?)
>>
>> Now that we immediately add stmt to stmts vector, I suppose mod_seen
>> shall not be required ?
>
> In that case mod_seen is not needed. But the situation you say will
> still happen so I wonder if we'd need a better way of iterating over
> immediate uses, like first pushing all candidates into a worklist
> vector and then iterating over that until we find no more candidates.
>
> You can then also handle the case of more than one group of stmts
> (the pass currently doesn't iterate in any particular useful order
> over BBs).
IIUC, we want to perform the transform if:
i) there exists top_stmt with code trunc_div_expr/trunc_mod_expr and
having same operands as stmt.
ii) top_stmt dominates all other stmts with code
trunc_div_expr/trunc_mod_expr and having same operands as top_stmt.
Firstly, we try to get to top_stmt if it exists, by iterating over uses of stmt,
and then iterate over all uses of top_stmt and add them to stmts vector
only if top_stmt dominates all the stmts with same operands as top_stmt
and have code trunc_div_expr/trunc_mod_expr.
/* Get to top_stmt. */
top_stmt = stmt;
top_bb = gimple_bb (stmt);
FOR_EACH_IMM_USE_STMT (use_stmt, use_iter, op1)
{
if (use_stmt code is TRUNC_DIV_EXPR or TRUNC_MOD_EXPR
&& use_stmt has same operands as stmt)
{
if (gimple_bb (use_stmt) dominates top_bb)
{
top_bb = gimple_bb (use_stmt);
top_stmt = use_stmt;
}
else if (gimple_bb (use_stmt) == top_stmt
&& gimple_uid (use_stmt) < top_stmt)
top_stmt = use_stmt;
}
}
/* Speculatively consider top_stmt as dominating all other
div_expr/mod_expr stmts with same operands as stmt. */
stmts.safe_push (top_stmt);
/* Walk uses of top_stmt to ensure that all stmts are dominated by top_stmt. */
top_op1 = gimple_assign_rhs1 (top_stmt);
FOR_EACH_IMM_USE_STMT (use_stmt, use_iter, top_op1)
{
if (use_stmt code is TRUNC_DIV_EXPR or TRUNC_MOD_EXPR
&& use_stmt has same operands as top_stmt)
{
if (use_stmt == top_stmt)
continue;
/* No top_stmt exits, do not proceed with transform */
if (top_bb does not dominate gimple_bb (use_stmt))
return false;
stmts.safe_push (use_stmt);
}
}
For the case:
t1 = x / y;
if (cond)
t2 = x % y;
else
t3 = x % y;
Assuming stmt is "t2 = x % y", it will walk uses of stmt and set
top_stmt to "t1 = x / y"
Then it will walk all uses of top_stmt:
"t2 = x % y" -> dominated by top_stmt
"t3 = x % y" -> dominated by top_stmt
Since all stmts are dominated by top_stmt, we add all three stmts to
vector of stmts and proceed with transform.
For the case where, top_stmt dominates original stmt but not all stmts:
if (cond)
t1 = x / y;
else
{
t2 = x % y;
return;
}
t3 = x % y;
Assuming stmt is "t3 = x % y",
Walking stmt uses will set top_stmt to "t1 = x / y";
Walking immediate uses of top_stmt, we find that "t2 = x % y" is not
dominated by top_stmt,
and hence don't do the transform.
Does this sound reasonable ?
Thanks,
Prathamesh
>
> Richard.
>
