[RFC,2/3,debug] Add fkeep-vars-live

Message ID 20180724113732.xo4av4fcbgigi7w2@delia
State Superseded
Headers show
Series
  • [RFC,1/3,debug] Add fdebug-nops
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Commit Message

Tom de Vries July 24, 2018, 11:37 a.m.
On Tue, Jul 24, 2018 at 01:30:30PM +0200, Tom de Vries wrote:
> On 07/16/2018 05:10 PM, Tom de Vries wrote:

> > On 07/16/2018 03:50 PM, Richard Biener wrote:

> >> On Mon, 16 Jul 2018, Tom de Vries wrote:

> >>> Any comments?

> >>

> >> Interesting idea.  I wonder if that should be generalized

> >> to other places

> > 

> > I kept the option name general, to allow for that.

> > 

> > And indeed, this is a point-fix patch. I've been playing around with a

> > more generic patch that adds nops such that each is_stmt .loc is

> > associated with a unique insn, but that was embedded in an

> > fkeep-vars-live branch, so this patch is minimally addressing the first

> > problem I managed to reproduce on trunk.

> > 

> >> and how we can avoid compare-debug failures

> >> (var-tracking usually doesn't change code-generation).

> >>

> > 

> 

> I'll post this patch series (the current state of my fkeep-vars-live

> branch) in reply to this email:

> 

>      1  [debug] Add fdebug-nops

>      2  [debug] Add fkeep-vars-live

>      3  [debug] Add fdebug-nops and fkeep-vars-live to Og only

> 

> Bootstrapped and reg-tested on x86_64. ChangeLog entries and function

> header comments missing.

> 

> Comments welcome.

> 


This patch adds fake uses of user variables at the point where they go out of
scope, to keep user variables inspectable throughout the application.

This approach will generate sub-optimal code: in some cases, the executable
code will go through efforts to keep a var alive, while var-tracking can
easily compute the value of the var from something else.

Also, the compiler treats the fake use as any other use, so it may keep an
expensive resource like a register occupied (if we could mark the use as a
cold use or some such, we could tell optimizers that we need the value, but
it's ok if getting the value is expensive, so it could be spilled instead of
occupying a register).

The current implementation is expected to increase register pressure, and
therefore spilling, but we'd still expect less memory accesses then with O0.

Another drawback is that the fake uses confuse the unitialized warning
analysis, so that is switched off for -fkeep-vars-live.

PR debug/78685

[debug] Add fkeep-vars-live

---
 gcc/cfgexpand.c                          |  9 +++++++
 gcc/common.opt                           |  4 +++
 gcc/function.c                           |  5 ++--
 gcc/gimplify.c                           | 44 +++++++++++++++++++++++++++-----
 gcc/testsuite/gcc.dg/guality/pr54200-2.c |  3 +--
 gcc/tree-cfg.c                           |  1 +
 gcc/tree-sra.c                           |  7 +++++
 gcc/tree-ssa-alias.c                     |  4 +++
 gcc/tree-ssa-structalias.c               |  3 ++-
 gcc/tree-ssa-uninit.c                    |  2 +-
 10 files changed, 70 insertions(+), 12 deletions(-)

Comments

Jakub Jelinek July 24, 2018, 11:46 a.m. | #1
On Tue, Jul 24, 2018 at 01:37:32PM +0200, Tom de Vries wrote:
> Another drawback is that the fake uses confuse the unitialized warning

> analysis, so that is switched off for -fkeep-vars-live.


Is that really needed?  I.e. can't you for the purpose of uninitialized
warning analysis ignore the clobber = var uses?

Is the -fkeep-vars-live option -fcompare-debug friendly?

> --- a/gcc/cfgexpand.c

> +++ b/gcc/cfgexpand.c

> @@ -3533,6 +3533,13 @@ expand_clobber (tree lhs)

>      }

>  }

>  

> +static void

> +expand_use (tree rhs)

> +{

> +  rtx target = expand_expr (rhs, NULL_RTX, VOIDmode, EXPAND_NORMAL);

> +  emit_use (target);

> +}


Missing function comment.

> +fkeep-vars-live

> +Common Report Var(flag_keep_vars_live) Optimization

> +Add artificial uses of local vars at end of scope.


at the end of scope?
> --- a/gcc/gimplify.c

> +++ b/gcc/gimplify.c

> @@ -1264,6 +1264,23 @@ asan_poison_variables (hash_set<tree> *variables, bool poison, gimple_seq *seq_p

>      }

>  }

>  

> +static gimple_seq

> +gimple_build_uses (tree vars)


Missing function comment.

> --- a/gcc/tree-ssa-alias.c

> +++ b/gcc/tree-ssa-alias.c

> @@ -1368,6 +1368,10 @@ refs_may_alias_p_1 (ao_ref *ref1, ao_ref *ref2, bool tbaa_p)

>    poly_int64 max_size1 = -1, max_size2 = -1;

>    bool var1_p, var2_p, ind1_p, ind2_p;

>  

> +  if ((ref1->ref && TREE_CLOBBER_P (ref1->ref)) ||


|| should go on the next line.

> +      (ref2->ref && TREE_CLOBBER_P (ref2->ref)))

> +    return false;

> +

>    gcc_checking_assert ((!ref1->ref

>  			|| TREE_CODE (ref1->ref) == SSA_NAME

>  			|| DECL_P (ref1->ref)


> --- a/gcc/tree-ssa-uninit.c

> +++ b/gcc/tree-ssa-uninit.c

> @@ -2628,7 +2628,7 @@ warn_uninitialized_phi (gphi *phi, vec<gphi *> *worklist,

>  static bool

>  gate_warn_uninitialized (void)

>  {

> -  return warn_uninitialized || warn_maybe_uninitialized;

> +  return (warn_uninitialized || warn_maybe_uninitialized) && !flag_keep_vars_live;

>  }


See above.

	Jakub
Tom de Vries July 24, 2018, 12:34 p.m. | #2
On 07/24/2018 01:46 PM, Jakub Jelinek wrote:
> On Tue, Jul 24, 2018 at 01:37:32PM +0200, Tom de Vries wrote:

>> Another drawback is that the fake uses confuse the unitialized warning

>> analysis, so that is switched off for -fkeep-vars-live.

> 

> Is that really needed?  I.e. can't you for the purpose of uninitialized

> warning analysis ignore the clobber = var uses?

> 


This seems to work on the test-case that failed during testing
(g++.dg/uninit-pred-4.C):
...
diff --git a/gcc/tree-ssa-uninit.c b/gcc/tree-ssa-uninit.c
index 77f090bfa80..953db9ed02d 100644
--- a/gcc/tree-ssa-uninit.c
+++ b/gcc/tree-ssa-uninit.c
@@ -132,6 +132,9 @@ warn_uninit (enum opt_code wc, tree t, tree expr,
tree var,
   if (is_gimple_assign (context)
       && gimple_assign_rhs_code (context) == COMPLEX_EXPR)
     return;
+  if (gimple_assign_single_p (context)
+      && TREE_CLOBBER_P (gimple_assign_lhs (context)))
+    return;
   if (!has_undefined_value_p (t))
     return;
...
But I don't know the pass well enough to know whether this is a
sufficient fix.


> Is the -fkeep-vars-live option -fcompare-debug friendly?

> 


I think so, there's no reference to debug flags or instructions.

>> --- a/gcc/cfgexpand.c

>> +++ b/gcc/cfgexpand.c

>> @@ -3533,6 +3533,13 @@ expand_clobber (tree lhs)

>>      }

>>  }

>>  

>> +static void

>> +expand_use (tree rhs)

>> +{

>> +  rtx target = expand_expr (rhs, NULL_RTX, VOIDmode, EXPAND_NORMAL);

>> +  emit_use (target);

>> +}

> 

> Missing function comment.

> 

>> +fkeep-vars-live

>> +Common Report Var(flag_keep_vars_live) Optimization

>> +Add artificial uses of local vars at end of scope.

> 

> at the end of scope?


Is this better?
+Add artificial use for each local variable at the end of the
declaration scope

Thanks,
- Tom
Alexandre Oliva July 24, 2018, 7:11 p.m. | #3
On Jul 24, 2018, Tom de Vries <tdevries@suse.de> wrote:

> This patch adds fake uses of user variables at the point where they go out of

> scope, to keep user variables inspectable throughout the application.


I suggest also adding such uses before sets, so that variables aren't
regarded as dead and get optimized out in ranges between the end of a
live range and a subsequent assignment.

-- 
Alexandre Oliva, freedom fighter   https://FSFLA.org/blogs/lxo
Be the change, be Free!         FSF Latin America board member
GNU Toolchain Engineer                Free Software Evangelist
Jakub Jelinek July 25, 2018, 11:02 a.m. | #4
On Tue, Jul 24, 2018 at 04:11:11PM -0300, Alexandre Oliva wrote:
> On Jul 24, 2018, Tom de Vries <tdevries@suse.de> wrote:

> 

> > This patch adds fake uses of user variables at the point where they go out of

> > scope, to keep user variables inspectable throughout the application.

> 

> I suggest also adding such uses before sets, so that variables aren't

> regarded as dead and get optimized out in ranges between the end of a

> live range and a subsequent assignment.


But that can be done incrementally, right, and perhaps being controllable
by a level of this option, because such extra uses might make it even more
costly.  Though, if the extra uses and sets aren't in the same stmt, then
the optimizers could still move it appart (in addition of it making the IL
larger).
We need to think about different cases (non-gimple reg vars are probably ok,
they just live in memory unless converted (sra etc.) into gimple reg vars,
then what to do about SSA_NAMEs with underlying user variable if it doesn't
have overlapping ranges, if it does have overlapping ranges).

	Jakub
Alexandre Oliva July 26, 2018, 7:47 a.m. | #5
On Jul 25, 2018, Jakub Jelinek <jakub@redhat.com> wrote:

> On Tue, Jul 24, 2018 at 04:11:11PM -0300, Alexandre Oliva wrote:

>> On Jul 24, 2018, Tom de Vries <tdevries@suse.de> wrote:

>> 

>> > This patch adds fake uses of user variables at the point where they go out of

>> > scope, to keep user variables inspectable throughout the application.

>> 

>> I suggest also adding such uses before sets, so that variables aren't

>> regarded as dead and get optimized out in ranges between the end of a

>> live range and a subsequent assignment.


> But that can be done incrementally, right


Sure

> the optimizers could still move it appart


*nod*

-- 
Alexandre Oliva, freedom fighter   https://FSFLA.org/blogs/lxo
Be the change, be Free!         FSF Latin America board member
GNU Toolchain Engineer                Free Software Evangelist

Patch

diff --git a/gcc/cfgexpand.c b/gcc/cfgexpand.c
index d6e3c382085..eb9e36a8c5b 100644
--- a/gcc/cfgexpand.c
+++ b/gcc/cfgexpand.c
@@ -3533,6 +3533,13 @@  expand_clobber (tree lhs)
     }
 }
 
+static void
+expand_use (tree rhs)
+{
+  rtx target = expand_expr (rhs, NULL_RTX, VOIDmode, EXPAND_NORMAL);
+  emit_use (target);
+}
+
 /* A subroutine of expand_gimple_stmt, expanding one gimple statement
    STMT that doesn't require special handling for outgoing edges.  That
    is no tailcalls and no GIMPLE_COND.  */
@@ -3632,6 +3639,8 @@  expand_gimple_stmt_1 (gimple *stmt)
 	      /* This is a clobber to mark the going out of scope for
 		 this LHS.  */
 	      expand_clobber (lhs);
+	    else if (TREE_CLOBBER_P (lhs))
+	      expand_use (rhs);
 	    else
 	      expand_assignment (lhs, rhs,
 				 gimple_assign_nontemporal_move_p (
diff --git a/gcc/common.opt b/gcc/common.opt
index 984b351cd79..a29e320ba45 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1496,6 +1496,10 @@  fdebug-nops
 Common Report Var(flag_debug_nops) Optimization
 Insert nops if that might improve debugging of optimized code.
 
+fkeep-vars-live
+Common Report Var(flag_keep_vars_live) Optimization
+Add artificial uses of local vars at end of scope.
+
 fkeep-gc-roots-live
 Common Undocumented Report Var(flag_keep_gc_roots_live) Optimization
 ; Always keep a pointer to a live memory block
diff --git a/gcc/function.c b/gcc/function.c
index dee303cdbdd..6367d282db3 100644
--- a/gcc/function.c
+++ b/gcc/function.c
@@ -1964,11 +1964,12 @@  instantiate_virtual_regs (void)
       {
 	/* These patterns in the instruction stream can never be recognized.
 	   Fortunately, they shouldn't contain virtual registers either.  */
-        if (GET_CODE (PATTERN (insn)) == USE
-	    || GET_CODE (PATTERN (insn)) == CLOBBER
+        if (GET_CODE (PATTERN (insn)) == CLOBBER
 	    || GET_CODE (PATTERN (insn)) == ASM_INPUT
 	    || DEBUG_MARKER_INSN_P (insn))
 	  continue;
+        else if (GET_CODE (PATTERN (insn)) == USE)
+	  instantiate_virtual_regs_in_rtx (&PATTERN (insn));
 	else if (DEBUG_BIND_INSN_P (insn))
 	  instantiate_virtual_regs_in_rtx (INSN_VAR_LOCATION_PTR (insn));
 	else
diff --git a/gcc/gimplify.c b/gcc/gimplify.c
index 4a109aee27a..1ce6ef648c6 100644
--- a/gcc/gimplify.c
+++ b/gcc/gimplify.c
@@ -1264,6 +1264,23 @@  asan_poison_variables (hash_set<tree> *variables, bool poison, gimple_seq *seq_p
     }
 }
 
+static gimple_seq
+gimple_build_uses (tree vars)
+{
+  gimple_seq seq = NULL;
+
+  for (tree var = vars; var; var = DECL_CHAIN (var))
+    {
+      if (DECL_ARTIFICIAL (var))
+	continue;
+
+      gimple *stmt = gimple_build_assign (build_clobber (TREE_TYPE (var)), var);
+      gimple_seq_add_stmt (&seq, stmt);
+    }
+
+  return seq;
+}
+
 /* Gimplify a BIND_EXPR.  Just voidify and recurse.  */
 
 static enum gimplify_status
@@ -1363,7 +1380,7 @@  gimplify_bind_expr (tree *expr_p, gimple_seq *pre_p)
       gimplify_seq_add_stmt (&cleanup, stack_restore);
     }
 
-  /* Add clobbers for all variables that go out of scope.  */
+  /* Add clobbers/uses for all variables that go out of scope.  */
   for (t = BIND_EXPR_VARS (bind_expr); t ; t = DECL_CHAIN (t))
     {
       if (VAR_P (t)
@@ -1376,14 +1393,17 @@  gimplify_bind_expr (tree *expr_p, gimple_seq *pre_p)
 	      /* Only care for variables that have to be in memory.  Others
 		 will be rewritten into SSA names, hence moved to the
 		 top-level.  */
-	      && !is_gimple_reg (t)
+	      && (flag_keep_vars_live || !is_gimple_reg (t))
 	      && flag_stack_reuse != SR_NONE)
 	    {
 	      tree clobber = build_clobber (TREE_TYPE (t));
-	      gimple *clobber_stmt;
-	      clobber_stmt = gimple_build_assign (t, clobber);
-	      gimple_set_location (clobber_stmt, end_locus);
-	      gimplify_seq_add_stmt (&cleanup, clobber_stmt);
+	      gimple *stmt;
+	      if (is_gimple_reg (t))
+		stmt = gimple_build_assign (clobber, t);
+	      else
+		stmt = gimple_build_assign (t, clobber);
+	      gimple_set_location (stmt, end_locus);
+	      gimplify_seq_add_stmt (&cleanup, stmt);
 	    }
 
 	  if (flag_openacc && oacc_declare_returns != NULL)
@@ -12763,6 +12783,10 @@  gimplify_body (tree fndecl, bool do_parms)
   gimple *outer_stmt;
   gbind *outer_bind;
 
+  gimple_seq cleanup = NULL;
+  if (flag_keep_vars_live)
+    cleanup = gimple_build_uses (DECL_ARGUMENTS (fndecl));
+
   timevar_push (TV_TREE_GIMPLIFY);
 
   init_tree_ssa (cfun);
@@ -12798,6 +12822,14 @@  gimplify_body (tree fndecl, bool do_parms)
   /* Gimplify the function's body.  */
   seq = NULL;
   gimplify_stmt (&DECL_SAVED_TREE (fndecl), &seq);
+
+  if (cleanup)
+    {
+      gtry *gs = gimple_build_try (seq, cleanup, GIMPLE_TRY_FINALLY);
+      seq = NULL;
+      gimplify_seq_add_stmt (&seq, gs);
+    }
+
   outer_stmt = gimple_seq_first_stmt (seq);
   if (!outer_stmt)
     {
diff --git a/gcc/testsuite/gcc.dg/guality/pr54200-2.c b/gcc/testsuite/gcc.dg/guality/pr54200-2.c
index e30e3c92b94..646790af65a 100644
--- a/gcc/testsuite/gcc.dg/guality/pr54200-2.c
+++ b/gcc/testsuite/gcc.dg/guality/pr54200-2.c
@@ -1,6 +1,5 @@ 
 /* { dg-do run } */
-/* { dg-skip-if "" { *-*-* }  { "*" } { "-Og" "-Os" "-O0" } } */
-/* { dg-options "-g -fdebug-nops -DMAIN" } */
+/* { dg-options "-g -fdebug-nops -fkeep-vars-live -DMAIN" } */
 
 #include "prevent-optimization.h"
 
diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
index 14d66b7a728..d3a4465fe25 100644
--- a/gcc/tree-cfg.c
+++ b/gcc/tree-cfg.c
@@ -4484,6 +4484,7 @@  verify_gimple_assign_single (gassign *stmt)
     case PARM_DECL:
       if (!is_gimple_reg (lhs)
 	  && !is_gimple_reg (rhs1)
+	  && !TREE_CLOBBER_P (lhs)
 	  && is_gimple_reg_type (TREE_TYPE (lhs)))
 	{
 	  error ("invalid rhs for gimple memory store");
diff --git a/gcc/tree-sra.c b/gcc/tree-sra.c
index 3e30f6bc3d4..f6488b90378 100644
--- a/gcc/tree-sra.c
+++ b/gcc/tree-sra.c
@@ -3536,6 +3536,13 @@  sra_modify_assign (gimple *stmt, gimple_stmt_iterator *gsi)
       sra_stats.exprs++;
     }
 
+  if (modify_this_stmt && TREE_CLOBBER_P (gimple_assign_lhs (stmt)))
+    {
+      gimple_assign_set_rhs1 (stmt, rhs);
+      gimple_assign_set_lhs (stmt, build_clobber (TREE_TYPE (rhs)));
+      return SRA_AM_MODIFIED;
+    }
+
   if (modify_this_stmt)
     {
       if (!useless_type_conversion_p (TREE_TYPE (lhs), TREE_TYPE (rhs)))
diff --git a/gcc/tree-ssa-alias.c b/gcc/tree-ssa-alias.c
index 7b25778307f..007e6f84c2b 100644
--- a/gcc/tree-ssa-alias.c
+++ b/gcc/tree-ssa-alias.c
@@ -1368,6 +1368,10 @@  refs_may_alias_p_1 (ao_ref *ref1, ao_ref *ref2, bool tbaa_p)
   poly_int64 max_size1 = -1, max_size2 = -1;
   bool var1_p, var2_p, ind1_p, ind2_p;
 
+  if ((ref1->ref && TREE_CLOBBER_P (ref1->ref)) ||
+      (ref2->ref && TREE_CLOBBER_P (ref2->ref)))
+    return false;
+
   gcc_checking_assert ((!ref1->ref
 			|| TREE_CODE (ref1->ref) == SSA_NAME
 			|| DECL_P (ref1->ref)
diff --git a/gcc/tree-ssa-structalias.c b/gcc/tree-ssa-structalias.c
index fd24f84fb14..d83579a09c5 100644
--- a/gcc/tree-ssa-structalias.c
+++ b/gcc/tree-ssa-structalias.c
@@ -4883,7 +4883,8 @@  find_func_aliases (struct function *fn, gimple *origt)
       tree lhsop = gimple_assign_lhs (t);
       tree rhsop = (gimple_num_ops (t) == 2) ? gimple_assign_rhs1 (t) : NULL;
 
-      if (rhsop && TREE_CLOBBER_P (rhsop))
+      if ((rhsop && TREE_CLOBBER_P (rhsop))
+	  || (lhsop && TREE_CLOBBER_P (lhsop)))
 	/* Ignore clobbers, they don't actually store anything into
 	   the LHS.  */
 	;
diff --git a/gcc/tree-ssa-uninit.c b/gcc/tree-ssa-uninit.c
index 8ccbc85970a..77f090bfa80 100644
--- a/gcc/tree-ssa-uninit.c
+++ b/gcc/tree-ssa-uninit.c
@@ -2628,7 +2628,7 @@  warn_uninitialized_phi (gphi *phi, vec<gphi *> *worklist,
 static bool
 gate_warn_uninitialized (void)
 {
-  return warn_uninitialized || warn_maybe_uninitialized;
+  return (warn_uninitialized || warn_maybe_uninitialized) && !flag_keep_vars_live;
 }
 
 namespace {