Re: [trunk<-vta] Re: [vtab] Permit coalescing of user variables

[Alexandre Oliva <aoliva at redhat dot com>]

Ping?  (Updated with improved docs; should the options be renamed to
-ftree-copyrename-* to match the option that covers the entire pass?)

On Jun  4, 2011, Alexandre Oliva <aoliva@redhat.com> wrote:

> On Oct 13, 2009, Alexandre Oliva <aoliva@redhat.com> wrote:
>> On Jun  1, 2009, Alexandre Oliva <aoliva@redhat.com> wrote:
>>> A long time ago, when variable tracking at assignments was just a
>>> distant dream, we ran into one of the first contentious points, which
>>> had to do with coalescing SSA names on copyrename.

>>> On the one hand, coalescing unrelated SSA names made for better code (at
>>> least in theory) but poorer debug information; on the other hand,
>>> refraining from coalescing them exploded compile-time memory use.

>>> We currently implement a trade-off by which variables inlined from other
>>> functions can be coalesced, so as to save compile-time memory, reduce
>>> abstraction penalties and retain debug information for out-of-line
>>> functions.

>>> The patch below (ping) implements two other possibilities: refraining
>>> from coalescing even inlined SSA names, which might enable better debug
>>> information to be generated, and enabling coalescing of all related
>>> variables, for better code at the expense of debug information.

>>> VTA doesn't really care which of the 3 possibilities is used, it works
>>> equally well with all of them.

>> On Jun  1, 2009, Alexandre Oliva <aoliva@redhat.com> also wrote:

>>> And the patch below changes the default so that we can optimize more.

>> This patch combines the two patches described above, now that VTA is
>> enabled by default.

> This is an updated version of the patch, adjusting the testcases that
> didn't expect this kind of variable coalescing.

Regstrapped on x86_64-linux-gnu and i686-linux-gnu.  Ok?

for  gcc/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* common.opt (ftree-coalesce-inlined-vars): New.
	(ftree-coalesce-vars): New.
	* doc/invoke.texi: Document them.
	* tree-ssa-copyrename.c (copy_rename_partition_coalesce):
	Implement them.

for  gcc/testsuite/ChangeLog
from  Alexandre Oliva  <aoliva@redhat.com>

	* g++.dg/tree-ssa/ivopts-2.C: Adjust for coalescing.
	* gcc.dg/tree-ssa/forwprop-11.c: Likewise.
	* gcc.dg/tree-ssa/ssa-fre-1.c: Likewise.

Index: gcc/common.opt
===================================================================
--- gcc/common.opt.orig	2012-04-08 01:44:00.731983395 -0300
+++ gcc/common.opt	2012-04-08 01:50:38.387224820 -0300
@@ -1926,6 +1926,14 @@ ftree-ch
 Common Report Var(flag_tree_ch) Optimization
 Enable loop header copying on trees
 
+ftree-coalesce-inlined-vars
+Common Report Var(flag_ssa_coalesce_vars,1) Init(2) RejectNegative Optimization
+Enable coalescing of copy-related user variables that are inlined
+
+ftree-coalesce-vars
+Common Report Var(flag_ssa_coalesce_vars,2) Optimization
+Enable coalescing of all copy-related user variables
+
 ftree-copyrename
 Common Report Var(flag_tree_copyrename) Optimization
 Replace SSA temporaries with better names in copies
Index: gcc/tree-ssa-copyrename.c
===================================================================
--- gcc/tree-ssa-copyrename.c.orig	2012-04-08 01:44:00.858981874 -0300
+++ gcc/tree-ssa-copyrename.c	2012-04-08 01:50:38.474223779 -0300
@@ -194,20 +194,21 @@ copy_rename_partition_coalesce (var_map 
   ign1 = TREE_CODE (root1) == VAR_DECL && DECL_IGNORED_P (root1);
   ign2 = TREE_CODE (root2) == VAR_DECL && DECL_IGNORED_P (root2);
 
-  /* Never attempt to coalesce 2 user variables unless one is an inline
-     variable.  */
+  /* Refrain from coalescing user variables, if requested.  */
   if (!ign1 && !ign2)
     {
-      if (DECL_FROM_INLINE (root2))
-        ign2 = true;
-      else if (DECL_FROM_INLINE (root1))
+      if (flag_ssa_coalesce_vars && DECL_FROM_INLINE (root2))
+	ign2 = true;
+      else if (flag_ssa_coalesce_vars && DECL_FROM_INLINE (root1))
 	ign1 = true;
-      else
+      else if (flag_ssa_coalesce_vars != 2)
 	{
 	  if (debug)
 	    fprintf (debug, " : 2 different USER vars. No coalesce.\n");
 	  return false;
 	}
+      else
+	ign2 = true;
     }
 
   /* If both values have default defs, we can't coalesce.  If only one has a
Index: gcc/doc/invoke.texi
===================================================================
--- gcc/doc/invoke.texi.orig	2012-04-08 01:44:00.989980308 -0300
+++ gcc/doc/invoke.texi	2012-04-08 01:50:38.846219327 -0300
@@ -404,7 +404,8 @@ Objective-C and Objective-C++ Dialects}.
 -fsplit-ivs-in-unroller -fsplit-wide-types -fstack-protector @gol
 -fstack-protector-all -fstrict-aliasing -fstrict-overflow @gol
 -fthread-jumps -ftracer -ftree-bit-ccp @gol
--ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-copy-prop @gol
+-ftree-builtin-call-dce -ftree-ccp -ftree-ch @gol
+-ftree-coalesce-inline-vars -ftree-coalesce-vars -ftree-copy-prop @gol
 -ftree-copyrename -ftree-dce -ftree-dominator-opts -ftree-dse @gol
 -ftree-forwprop -ftree-fre -ftree-loop-if-convert @gol
 -ftree-loop-if-convert-stores -ftree-loop-im @gol
@@ -7434,6 +7435,24 @@ temporaries to other variables at copy l
 variable names which more closely resemble the original variables.  This flag
 is enabled by default at @option{-O} and higher.
 
+@item -ftree-coalesce-inlined-vars
+Tell the copyrename pass (see @option{-ftree-copyrename}) to attempt to
+combine small user-defined variables too, but only if they were inlined
+from other functions.  It is a more limited form of
+@option{-ftree-coalesce-vars}.  This may harm debug information of such
+inlined variables, but it will keep variables of the inlined-into
+function apart from each other, such that they are more likely to
+contain the expected values in a debugging session.  This was the
+default in GCC versions older than 4.7.
+
+@item -ftree-coalesce-vars
+Tell the copyrename pass (see @option{-ftree-copyrename}) to attempt to
+combine small user-defined variables too, instead of just compiler
+temporaries.  This may severely limit the ability to debug an optimized
+program compiled with @option{-fno-var-tracking-assignments}.  In the
+negated form, this flag prevents SSA coalescing of user variables,
+including inlined ones.  This option is enabled by default.
+
 @item -ftree-ter
 @opindex ftree-ter
 Perform temporary expression replacement during the SSA->normal phase.  Single
Index: gcc/testsuite/g++.dg/tree-ssa/ivopts-2.C
===================================================================
--- gcc/testsuite/g++.dg/tree-ssa/ivopts-2.C.orig	2012-04-08 01:44:01.157978296 -0300
+++ gcc/testsuite/g++.dg/tree-ssa/ivopts-2.C	2012-04-08 01:50:38.868219064 -0300
@@ -7,5 +7,5 @@ void test (int *b, int *e, int stride)
       *p = 1;
   }
 
-/* { dg-final { scan-tree-dump-times "PHI <p" 1 "ivopts"} } */
+/* { dg-final { scan-tree-dump-times "PHI <\[pb\]" 1 "ivopts"} } */
 /* { dg-final { cleanup-tree-dump "ivopts" } } */
Index: gcc/testsuite/gcc.dg/tree-ssa/forwprop-11.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/forwprop-11.c.orig	2012-04-08 01:44:01.249977196 -0300
+++ gcc/testsuite/gcc.dg/tree-ssa/forwprop-11.c	2012-04-08 01:50:38.884218873 -0300
@@ -16,5 +16,5 @@ int g(int *p, int n)
   return q[-1];
 }
 
-/* { dg-final { scan-tree-dump-times "= MEM\\\[\\\(int \\\*\\\)a_.. \\\+ 4B\\\];" 2 "forwprop1" } } */
+/* { dg-final { scan-tree-dump-times "= MEM\\\[\\\(int \\\*\\\)\[ap\]_.. \\\+ 4B\\\];" 2 "forwprop1" } } */
 /* { dg-final { cleanup-tree-dump "forwprop1" } } */
Index: gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-1.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-1.c.orig	2012-04-08 01:44:01.429975039 -0300
+++ gcc/testsuite/gcc.dg/tree-ssa/ssa-fre-1.c	2012-04-08 01:50:38.896218729 -0300
@@ -11,5 +11,5 @@ int f(int *a)
   return *c + t;
 }
 
-/* { dg-final { scan-tree-dump "Replaced \\\*c_\[^\n\].*with t_" "fre1" } } */
+/* { dg-final { scan-tree-dump "Replaced \\\*\[ac\]_\[^\n\].*with t_" "fre1" } } */
 /* { dg-final { cleanup-tree-dump "fre1" } } */

-- 
Alexandre Oliva, freedom fighter    http://FSFLA.org/~lxoliva/
You must be the change you wish to see in the world. -- Gandhi
Be Free! -- http://FSFLA.org/   FSF Latin America board member
Free Software Evangelist      Red Hat Brazil Compiler Engineer