On Thu, 23 May 2019, Jiufu Guo wrote:
> Richard Biener <rguent...@suse.de> writes:
>
> > On Thu, 23 May 2019, Jiufu Guo wrote:
> >
> >> Hi,
> >>
> >> Richard Biener <rguent...@suse.de> writes:
> >>
> >> > On Tue, 21 May 2019, Jiufu Guo wrote:
> >> >
> >> >> Hi,
> >> >>
> >> >> This patch implements a new opportunity of jump threading for PR77820.
> >> >> In this optimization, conditional jumps are merged with unconditional
> >> >> jump.
> >> >> And then moving CMP result to GPR is eliminated.
> >> >>
> >> >> It looks like below:
> >> >>
> >> >> <P0>
> >> >> p0 = a CMP b
> >> >> goto <X>;
> >> >>
> >> >> <P1>
> >> >> p1 = c CMP d
> >> >> goto <X>;
> >> >>
> >> >> <X>
> >> >> # phi = PHI <p0 (P0), p1 (P1)>
> >> >> if (phi != 0) goto <Y>; else goto <Z>;
> >> >>
> >> >> Could be transformed to:
> >> >>
> >> >> <P0>
> >> >> p0 = a CMP b
> >> >> if (p0 != 0) goto <Y>; else goto <Z>;
> >> >>
> >> >> <P1>
> >> >> p1 = c CMP d
> >> >> if (p1 != 0) goto <Y>; else goto <Z>;
> >> >>
> >> >>
> >> >> This optimization eliminates:
> >> >> 1. saving CMP result: p0 = a CMP b.
> >> >> 2. additional CMP on branch: if (phi != 0).
> >> >> 3. converting CMP result if there is phi = (INT_CONV) p0 if there is.
> >> >>
> >> >> Bootstrapped and tested on powerpc64le with no regressions(one case is
> >> >> improved)
> >> >> and new testcases are added. Is this ok for trunk?
> >> >>
> >> >> Thanks!
> >> >> Jiufu Guo
> >> >>
> >> ...
> >> >> diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
> >> >> index c3ea2d6..23000f6 100644
> >> >> --- a/gcc/tree-ssa-threadedge.c
> >> >> +++ b/gcc/tree-ssa-threadedge.c
> >> >> @@ -1157,6 +1157,90 @@ thread_through_normal_block (edge e,
> >> >> return 0;
> >> >> }
> >> >>
> >> >> +/* Return true if PHI's INDEX-th incoming value is a CMP, and the CMP
> >> >> is
> >> >> + defined in the incoming basic block. Otherwise return false. */
> >> >> +static bool
> >> >> +cmp_from_unconditional_block (gphi *phi, int index)
> >> >> +{
> >> >> + tree value = gimple_phi_arg_def (phi, index);
> >> >> + if (!(TREE_CODE (value) == SSA_NAME && has_single_use (value)))
> >> >> + return false;
> >> >
> >> > Not sure why we should reject a constant here but I guess we
> >> > expect it to find a simplified condition anyways ;)
> >> >
> >> Const could be accepted here, like "# t_9 = PHI <5(3), t_17(4)>". I
> >> found this case is already handled by other jump-threading code, like
> >> 'ethread' pass.
> >>
> >> >> +
> >> >> + gimple *def = SSA_NAME_DEF_STMT (value);
> >> >> +
> >> >> + if (!is_gimple_assign (def))
> >> >> + return false;
> >> >> +
> >> >> + if (CONVERT_EXPR_CODE_P (gimple_assign_rhs_code (def)))
> >> >> + {
> >> >> + value = gimple_assign_rhs1 (def);
> >> >> + if (!(TREE_CODE (value) == SSA_NAME && has_single_use (value)))
> >> >> + return false;
> >> >> +
> >> >> + def = SSA_NAME_DEF_STMT (value);
> >> >> +
> >> >> + if (!is_gimple_assign (def))
> >> >> + return false;
> >> >
> >> > too much vertial space.
> >> >
> >> Thanks, I will refine it.
> >> >> + }
> >> >> +
> >> >> + if (TREE_CODE_CLASS (gimple_assign_rhs_code (def)) != tcc_comparison)
> >> >> + return false;
> >> >> +
> >> >> + /* Check if phi's incoming value is defined in the incoming
> >> >> basic_block. */
> >> >> + edge e = gimple_phi_arg_edge (phi, index);
> >> >> + if (def->bb != e->src)
> >> >> + return false;
> >> >
> >> > why does this matter?
> >> >
> >> Through preparing pathes and duplicating block, this transform can also
> >> help to combine a cmp in previous block and a gcond in current block.
> >> "if (def->bb != e->src)" make sure the cmp is define in the incoming
> >> block of the current; and then combining "cmp with gcond" is safe. If
> >> the cmp is defined far from the incoming block, it would be hard to
> >> achieve the combining, and the transform may not needed.
> >
> > We're in SSA form so the "combining" doesn't really care where the
> > definition comes from.
> >
> >> >> +
> >> >> + if (!single_succ_p (def->bb))
> >> >> + return false;
> >> >
> >> > Or this? The actual threading will ensure this will hold true.
> >> >
> >> Yes, other thread code check this and ensure it to be true, like
> >> function thread_through_normal_block. Since this new function is invoked
> >> outside thread_through_normal_block, so, checking single_succ_p is also
> >> needed for this case.
> >
> > I mean threading will isolate the path making this trivially true.
> > It's also no requirement for combining, in fact due to the single-use
> > check the definition can be sinked across the edge already (if
> > the edges dest didn't have multiple predecessors which this threading
> > will fix as well).
> >
> I would relax these check and have a test.
>
> And I refactor the code a little as below. Thanks for any comments!
>
> bool
> edge_forwards_cmp_to_conditional_jump_through_empty_bb_p (edge e)
> {
> basic_block bb = e->dest;
>
> /* See if there is only one stmt which is gcond. */
> gimple *gs = last_and_only_stmt (bb);
> if (gs == NULL || gimple_code (gs) != GIMPLE_COND)
> return false;
>
> /* See if gcond's condition is "(phi !=/== 0/1)". */
> tree cond = gimple_cond_lhs (gs);
> if (TREE_CODE (cond) != SSA_NAME
> || gimple_code (SSA_NAME_DEF_STMT (cond)) != GIMPLE_PHI
> || gimple_bb (SSA_NAME_DEF_STMT (cond)) != bb)
> return false;
> enum tree_code code = gimple_cond_code (gs);
> tree rhs = gimple_cond_rhs (gs);
> if (!(code == NE_EXPR || code == EQ_EXPR || integer_onep (rhs)
> || integer_zerop (rhs)))
GCCs coding standard says that if a condition doesn't fit on
a single line you should split after each || or &&
> return false;
>
> gphi *phi = as_a<gphi *> (SSA_NAME_DEF_STMT (cond));
If you had used dyn_cast <gphi *> () above the GIMPLE_PHI
check would have been for phi != NULL and you'd save a line
of code.
> edge_iterator ei;
> edge in_e;
> FOR_EACH_EDGE (in_e, ei, bb->preds)
> {
As said in my first review I'd just check whether for the
edge we want to thread through the definition comes from a CMP.
Suppose you have
# val_1 = PHI <a_2, b_3, c_4>
if (val_1 != 0)
and only one edge has a b_3 = d_5 != 0 condition it's still
worth tail-duplicating the if block.
otherwise it looks ok to me.
Thanks,
Richard.
> /* Check if phi's incoming value is CMP */
> gimple *def;
> tree value = PHI_ARG_DEF_FROM_EDGE (phi, in_e);
> if (TREE_CODE (value) == SSA_NAME && has_single_use (value)
> && is_gimple_assign (SSA_NAME_DEF_STMT (value)))
> def = SSA_NAME_DEF_STMT (value);
> else
> return false;
>
> /* Or if it is (INTCONV) (a CMP b). */
> if (CONVERT_EXPR_CODE_P (gimple_assign_rhs_code (def)))
> {
> value = gimple_assign_rhs1 (def);
> if (TREE_CODE (value) == SSA_NAME && has_single_use (value)
> && is_gimple_assign (SSA_NAME_DEF_STMT (value)))
> def = SSA_NAME_DEF_STMT (value);
> else
> return false;
> }
> if (TREE_CODE_CLASS (gimple_assign_rhs_code (def)) != tcc_comparison)
> return false;
> }
>
> return true;
> }
>
> Thanks,
> Jiufu Guo
> >> >> + return true;
> >> >> +}
> >> >> +
> >> >> +/* There are basic blocks look like:
> >> >> + <P0>
> >> >> + p0 = a CMP b ; or p0 = (INT)( a CMP b)
> >> >> + goto <X>;
> >> >> +
> >> >> + <P1>
> >> >> + p1 = c CMP d
> >> >> + goto <X>;
> >> >> +
> >> >> + <X>
> >> >> + # phi = PHI <p0 (P0), p1 (P1)>
> >> >> + if (phi != 0) goto <Y>; else goto <Z>;
> >> >> +
> >> >> + Then, <X>: a trivial join block.
> >> >> +
> >> >> + Check if BB is <X> in like above. */
> >> >> +
> >> >> +bool
> >> >> +is_trivial_join_block (basic_block bb)
> >> >
> >> > I'd make this work on a specific edge.
> >> >
> >> > edge_forwards_conditional_to_conditional_jump_through_empty_bb_p (edge e)
> >> > {
> >> > basic_block b = e->dest;
> >> >
> >> > maybe too elaborate name ;)
> >> >
> >> Thanks for help to name the function! It is very valuable for me ;)
> >> >> +{
> >> >> + gimple *gs = last_and_only_stmt (bb);
> >> >> + if (gs == NULL)
> >> >> + return false;
> >> >> +
> >> >> + if (gimple_code (gs) != GIMPLE_COND)
> >> >> + return false;
> >> >> +
> >> >> + tree cond = gimple_cond_lhs (gs);
> >> >> +
> >> >> + if (TREE_CODE (cond) != SSA_NAME)
> >> >> + return false;
> >> >
> >> > space after if( too much vertical space in this function
> >> > for my taste btw.
> >> Will update this.
> >> >
> >> > For the forwarding to work we want a NE_EXPR or EQ_EXPR
> >> > as gimple_cond_code and integer_one_p or integer_zero_p
> >> > gimple_cond_rhs.
> >> Right, checking those would be more safe. Since no issue found, during
> >> bootstrap and regression tests, so I did not add these checking. I will
> >> add this checking.
> >> >
> >> >> +
> >> >> + if (gimple_code (SSA_NAME_DEF_STMT (cond)) != GIMPLE_PHI)
> >> >> + return false;
> >> >> +
> >> >> + gphi *phi = as_a<gphi *> (SSA_NAME_DEF_STMT (cond));
> >> >
> >> > I think to match your pattern you want to check that
> >> > gimple_bb (phi) == bb as well here.
> >> Right, it should be checked. I will update.
> >> >
> >> >> + for (unsigned int i = 0; i < phi->nargs; i++)
> >> >> + if (!cmp_from_unconditional_block (phi, i))
> >> >
> >> > Just process the incoming edge argument and inline the
> >> > helper. You can use PHI_ARG_DEF_FROM_EDGE here.
> >> I will refine code, and try to use it.
> >> >
> >> > Thanks for integrating this into jump-threading - it does look
> >> > like a good fit.
> >> >
> >> > How often does this trigger during bootstrap?
> >> Thanks for your sugguestion, this could help to evaluate patch. During
> >> bootstrap(stage 2 or 3), in gcc source code, 1300-1500 basic blocks are
> >> fullfile this tranform.
> >
> > Thanks,
> > Richard.
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany;
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah; HRB 21284 (AG Nürnberg)
