Re: [PATCH V2] A jump threading opportunity for condition branch

[Richard Biener <rguenther@suse.de>]

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On Thu, 30 May 2019, Jeff Law wrote:

> On 5/30/19 9:03 AM, Jiufu Guo wrote:
> > Jeff Law <law@redhat.com> writes:
> > 
> >> On 5/29/19 6:36 AM, Richard Biener wrote:
> >>> On Tue, 28 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.
> >>>>
> >>>> This version is based on the proposal of Richard, Jeff and Andrew, and
> >>>> refined to incorporate comments.  Thanks for the reviews!
> >>>>
> >>>> Bootstrapped and tested on powerpc64le and powerpc64be with no
> >>>> regressions (one case is improved) and new testcases are added. Is this
> >>>> ok for trunk?
> >>>>
> >>>> Example of this opportunity 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.
> >>>>
> >>>> Thanks!
> >>>> Jiufu Guo
> >>>>
> >>>>
> >>>> [gcc]
> >>>> 2019-05-28  Jiufu Guo  <guojiufu@linux.ibm.com>
> >>>> 	    Lijia He  <helijia@linux.ibm.com>
> >>>>
> >>>> 	PR tree-optimization/77820
> >>>> 	* tree-ssa-threadedge.c
> >>>> 	(edge_forwards_cmp_to_conditional_jump_through_empty_bb_p): New
> >>>> 	function.
> >>>> 	(thread_across_edge): Add call to
> >>>> 	edge_forwards_cmp_to_conditional_jump_through_empty_bb_p.
> >>>>
> >>>> [gcc/testsuite]
> >>>> 2019-05-28  Jiufu Guo  <guojiufu@linux.ibm.com>
> >>>> 	    Lijia He  <helijia@linux.ibm.com>
> >>>>
> >>>> 	PR tree-optimization/77820
> >>>> 	* gcc.dg/tree-ssa/phi_on_compare-1.c: New testcase.
> >>>> 	* gcc.dg/tree-ssa/phi_on_compare-2.c: New testcase.
> >>>> 	* gcc.dg/tree-ssa/phi_on_compare-3.c: New testcase.
> >>>> 	* gcc.dg/tree-ssa/phi_on_compare-4.c: New testcase.
> >>>> 	* gcc.dg/tree-ssa/split-path-6.c: Update testcase.
> >>>>
> >>>> ---
> >>>>  gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-1.c | 30 ++++++++++
> >>>>  gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-2.c | 23 +++++++
> >>>>  gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-3.c | 25 ++++++++
> >>>>  gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-4.c | 40 +++++++++++++
> >>>>  gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c     |  2 +-
> >>>>  gcc/tree-ssa-threadedge.c                        | 76 +++++++++++++++++++++++-
> >>>>  6 files changed, 192 insertions(+), 4 deletions(-)
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-1.c
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-2.c
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-3.c
> >>>>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-4.c
> >>>>
> >>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-1.c b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-1.c
> >>>> new file mode 100644
> >>>> index 0000000..5227c87
> >>>> --- /dev/null
> >>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-1.c
> >>>> @@ -0,0 +1,30 @@
> >>>> +/* { dg-do compile } */
> >>>> +/* { dg-options "-Ofast -fdump-tree-vrp1" } */
> >>>> +
> >>>> +void g (int);
> >>>> +void g1 (int);
> >>>> +
> >>>> +void
> >>>> +f (long a, long b, long c, long d, long x)
> >>>> +{
> >>>> +  _Bool t;
> >>>> +  if (x)
> >>>> +    {
> >>>> +      g (a + 1);
> >>>> +      t = a < b;
> >>>> +      c = d + x;
> >>>> +    }
> >>>> +  else
> >>>> +    {
> >>>> +      g (b + 1);
> >>>> +      a = c + d;
> >>>> +      t = c > d;
> >>>> +    }
> >>>> +
> >>>> +  if (t)
> >>>> +    g1 (c);
> >>>> +
> >>>> +  g (a);
> >>>> +}
> >>>> +
> >>>> +/* { dg-final { scan-tree-dump-times "Removing basic block" 1 "vrp1" } } */
> >>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-2.c b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-2.c
> >>>> new file mode 100644
> >>>> index 0000000..eaf89bb
> >>>> --- /dev/null
> >>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-2.c
> >>>> @@ -0,0 +1,23 @@
> >>>> +/* { dg-do compile } */
> >>>> +/* { dg-options "-Ofast -fdump-tree-vrp1" } */
> >>>> +
> >>>> +void g (void);
> >>>> +void g1 (void);
> >>>> +
> >>>> +void
> >>>> +f (long a, long b, long c, long d, int x)
> >>>> +{
> >>>> +  _Bool t;
> >>>> +  if (x)
> >>>> +    t = c < d;
> >>>> +  else
> >>>> +    t = a < b;
> >>>> +
> >>>> +  if (t)
> >>>> +    {
> >>>> +      g1 ();
> >>>> +      g ();
> >>>> +    }
> >>>> +}
> >>>> +
> >>>> +/* { dg-final { scan-tree-dump-times "Removing basic block" 1 "vrp1" } } */
> >>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-3.c b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-3.c
> >>>> new file mode 100644
> >>>> index 0000000..d5a1e0b
> >>>> --- /dev/null
> >>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-3.c
> >>>> @@ -0,0 +1,25 @@
> >>>> +/* { dg-do compile } */
> >>>> +/* { dg-options "-Ofast -fdump-tree-vrp1" } */
> >>>> +
> >>>> +void g (void);
> >>>> +void g1 (void);
> >>>> +
> >>>> +void
> >>>> +f (long a, long b, long c, long d, int x)
> >>>> +{
> >>>> +  int t;
> >>>> +  if (x)
> >>>> +    t = a < b;
> >>>> +  else if (d == x)
> >>>> +    t = c < b;
> >>>> +  else
> >>>> +    t = d > c;
> >>>> +
> >>>> +  if (t)
> >>>> +    {
> >>>> +      g1 ();
> >>>> +      g ();
> >>>> +    }
> >>>> +}
> >>>> +
> >>>> +/* { dg-final { scan-tree-dump-times "Removing basic block" 1 "vrp1" } } */
> >>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-4.c b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-4.c
> >>>> new file mode 100644
> >>>> index 0000000..53acabc
> >>>> --- /dev/null
> >>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/phi_on_compare-4.c
> >>>> @@ -0,0 +1,40 @@
> >>>> +/* { dg-do compile } */
> >>>> +/* { dg-options "-Ofast -fdump-tree-vrp1" } */
> >>>> +
> >>>> +void g (int);
> >>>> +void g1 (int);
> >>>> +
> >>>> +void
> >>>> +f (long a, long b, long c, long d, int x)
> >>>> +{
> >>>> +  int t;
> >>>> +  _Bool l1 = 0, l2 = 0;
> >>>> +  if (x)
> >>>> +    {
> >>>> +      g (a);
> >>>> +      c = a + b;
> >>>> +      t = a < b;
> >>>> +      l1 = 1;
> >>>> +    }
> >>>> +  else
> >>>> +    {
> >>>> +      g1 (b);
> >>>> +      t = c > d;
> >>>> +      d = c + b;
> >>>> +      l2 = 1;
> >>>> +    }
> >>>> +
> >>>> +  if (t)
> >>>> +    {
> >>>> +      if (l1 | l2)
> >>>> +	g1 (c);
> >>>> +    }
> >>>> +  else
> >>>> +    {
> >>>> +      g (d);
> >>>> +      g1 (a + b);
> >>>> +    }
> >>>> +  g (c + d);
> >>>> +}
> >>>> +
> >>>> +/* { dg-final { scan-tree-dump-times "Removing basic block" 1 "vrp1" } } */
> >>>> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c b/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c
> >>>> index e9b4f26..1d7b587 100644
> >>>> --- a/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c
> >>>> +++ b/gcc/testsuite/gcc.dg/tree-ssa/split-path-6.c
> >>>> @@ -69,4 +69,4 @@ lookharder (string)
> >>>>      }
> >>>>  }
> >>>>  
> >>>> -/* { dg-final { scan-tree-dump-times "Duplicating join block" 3 "split-paths" } } */
> >>>> +/* { dg-final { scan-tree-dump-times "Duplicating join block" 2 "split-paths" } } */
> >>>> diff --git a/gcc/tree-ssa-threadedge.c b/gcc/tree-ssa-threadedge.c
> >>>> index c3ea2d6..36c413a 100644
> >>>> --- a/gcc/tree-ssa-threadedge.c
> >>>> +++ b/gcc/tree-ssa-threadedge.c
> >>>> @@ -1157,6 +1157,74 @@ thread_through_normal_block (edge e,
> >>>>    return 0;
> >>>>  }
> >>>>  
> >>>> +/* 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, edge (P0,X) or (P1,X) could be marked as EDGE_START_JUMP_THREAD
> >>>> +   And edge (X,Y), (X,Z) is EDGE_COPY_SRC_JOINER_BLOCK
> >>>> +
> >>>> +   Return true if E is (P0,X) or (P1,X)  */
> >>>> +
> >>>> +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;
> >>>      gcond *gs;
> >>>      if (!(gs = safe_dyn_cast <gcond *> (last_and_only_stmt (bb))))
> >>>        return false;
> >>>
> >>> makes the following gimple_cond_ accesses more efficient when
> >>> checking is enabled.
> >>>
> >>>> +
> >>>> +  /* See if gcond's cond is "(phi !=/== 0/1)" in the basic block.  */
> >>>> +  tree cond = gimple_cond_lhs (gs);
> >>>> +  enum tree_code code = gimple_cond_code (gs);
> >>>> +  tree rhs = gimple_cond_rhs (gs);
> >>>> +  if (TREE_CODE (cond) != SSA_NAME
> >>>> +      || (code != NE_EXPR && code != EQ_EXPR)
> >>>> +      || (!integer_onep (rhs) && !integer_zerop (rhs)))
> >>>> +    return false;
> >>>> +  gphi *phi = dyn_cast <gphi *> (SSA_NAME_DEF_STMT (cond));
> >>>> +  if (phi == NULL || gimple_bb (phi) != bb)
> >>>> +    return false;
> >>>> +
> >>>> +  /* Check if phi's incoming value is CMP.  */
> >>>> +  gimple *def;
> >>>> +  tree value = PHI_ARG_DEF_FROM_EDGE (phi, 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);
> >>> Same is true here and below if you rewrite to
> >>>
> >>>      gassign *def;
> >>>      tree value = PHI_ARG_DEF_FROM_EDGE (phi, e);
> >>>      if (TREE_CODE (value) != SSA_NAME
> >>>          || !has_single_use (value)
> >>>          || !(def = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (value))))
> >>>        return false;
> >>>
> >>> Otherwise it looks good.  I'd like to have Jeffs opinion and
> >>> final ACK here because we touch jump-threading and he's most
> >>> familiar with that detail and the place you hook into.
> >> I've got the full thread to look over.  At a high level I wouldn't have
> >> guessed it'd be this easy to get the threader handle this, but
> >> occasionally we are surprised in a good way.  Anyway, I'll be looking
> >> through the full discussion.
> >>
> >> Jeff
> > 
> > Hi Jeff, Richard and all,
> > 
> > Thanks a lot for your great comments in all threads. Based on those
> > comments, I refined the code as below:
> > 
> > bool
> > edge_forwards_cmp_to_conditional_jump_through_empty_bb_p (edge e)
> > {
> >   /* See if there is only one stmt which is gcond.  */
> >   gcond *gs;
> >   if (!(gs = safe_dyn_cast<gcond *> (last_and_only_stmt (e->dest))))
> >     return false;
> >   
> >   /* See if gcond's cond is "(phi !=/== 0/1)" in the basic block.  */
> >   tree cond = gimple_cond_lhs (gs);
> >   enum tree_code code = gimple_cond_code (gs);
> >   tree rhs = gimple_cond_rhs (gs);
> >   if (TREE_CODE (cond) != SSA_NAME
> >       || (code != NE_EXPR && code != EQ_EXPR)
> >       || (!integer_onep (rhs) && !integer_zerop (rhs)))
> >     return false;
> >   gphi *phi = dyn_cast <gphi *> (SSA_NAME_DEF_STMT (cond));
> >   if (phi == NULL || gimple_bb (phi) != e->dest)
> >     return false;
> > 
> >   /* Check if phi's incoming value is CMP.  */
> >   gassign *def;
> >   tree value = PHI_ARG_DEF_FROM_EDGE (phi, e);
> >   if (TREE_CODE (value) != SSA_NAME
> >       || !has_single_use (value)
> >       || !(def = dyn_cast <gassign *> (SSA_NAME_DEF_STMT (value))))
> >     return false;
> > 
> >   /* Or if it is (INT) (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)
> > 	  || !(def = dyn_cast<gassign *> (SSA_NAME_DEF_STMT (value))))
> > 	return false;
> >     }
> > 
> >   if (TREE_CODE_CLASS (gimple_assign_rhs_code (def)) != tcc_comparison)
> >     return false;
> > 
> >   if (!single_succ_p (e->src))
> >     return false;

This new check warrants a comment.  As said I don't necessarily see
why this should prevent tail duplicating the block, the CMP can
still be forwarded into the tail duplicated condition.  Again
we should see sinking to do its job, creating a forwarder between
the CMP and the condition block (because sinking splits critical
edges).  But no need to "wait" for that here.

Richard.

> >   return true;
> > }
> > 
> > 
> > In this code, I put "if (!single_succ_p (e->src))" there, which may be
> > helpful for reducing the copy block.
> Sounds good.  My testing did show a regression on the sh port.
> 
> For pr51244-20 on the SH port your changes make the ultimate resulting
> code better, but compromise the test.  We can restore the shape of the
> CFG and get the testing coverage for sh_treg_combine by disabling VRP
> and DOM.
> 
> Can you please include the attached patch in your next update?
> 
> Jeff
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Linux GmbH, Maxfeldstrasse 5, 90409 Nuernberg, Germany;
GF: Felix Imendörffer, Mary Higgins, Sri Rasiah; HRB 21284 (AG Nürnberg)