[PATCH] Add -funconstrained-commons to work around PR/69368 (and others) in SPEC2006 (was: Re: [PATCH] Add -funknown-commons ...)

[Alan Lawrence <alan dot lawrence at arm dot com>]

On 04/03/16 13:27, Richard Biener wrote:
> I think to make it work with LTO you need to mark it 'Optimization'.
> Also it's about
> arrays so maybe
>
> 'Assume common declarations may be overridden with ones with a larger
> trailing array'
>
> also if we document it here we should eventually document it in invoke.texi.
>
> Not sure if "unknown commons" is a good term, maybe "unconstrained
> commons" instead?

All done; I doubt there is really a good word, unconstrained seems as good as
any. I've reused much the same wording in invoke.texi, unless you think there
is more to add.

On 04/03/16 13:33, Jakub Jelinek wrote:
> Also, isn't the *.opt description line supposed to end with a full stop?

Ah, yes, thanks.

Is this version OK for trunk?

gcc/ChangeLog:

DATE  Alan Lawrence  <alan.lawrence@arm.com>
      Jakub Jelinek  <jakub@redhat.com>

        * common.opt (funconstrained-commons, flag_unconstrained_commons): New.
        * tree.c (array_at_struct_end_p): Do not limit to size of decl for
        DECL_COMMONS if flag_unconstrained_commons is set.
        * tree-dfa.c (get_ref_base_and_extent): Likewise.

gcc/testsuite/ChangeLog:

        * gfortran.dg/unconstrained_commons.f: New.
---
 gcc/common.opt                                    |  5 +++++
 gcc/doc/invoke.texi                               |  8 +++++++-
 gcc/testsuite/gfortran.dg/unconstrained_commons.f | 20 ++++++++++++++++++++
 gcc/tree-dfa.c                                    | 15 ++++++++++++++-
 gcc/tree.c                                        |  6 ++++--
 5 files changed, 50 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/gfortran.dg/unconstrained_commons.f

diff --git a/gcc/common.opt b/gcc/common.opt
index 520fa9c..bbf79ef 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2451,6 +2451,11 @@ fsplit-paths
 Common Report Var(flag_split_paths) Init(0) Optimization
 Split paths leading to loop backedges.
 
+funconstrained-commons
+Common Var(flag_unconstrained_commons) Optimization
+Assume common declarations may be overridden with ones with a larger
+trailing array.
+
 funit-at-a-time
 Common Report Var(flag_unit_at_a_time) Init(1)
 Compile whole compilation unit at a time.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 0a2a6f4..68933a1 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -407,7 +407,7 @@ Objective-C and Objective-C++ Dialects}.
 -ftree-parallelize-loops=@var{n} -ftree-pre -ftree-partial-pre -ftree-pta @gol
 -ftree-reassoc -ftree-sink -ftree-slsr -ftree-sra @gol
 -ftree-switch-conversion -ftree-tail-merge -ftree-ter @gol
--ftree-vectorize -ftree-vrp @gol
+-ftree-vectorize -ftree-vrp -funconstrained-commons @gol
 -funit-at-a-time -funroll-all-loops -funroll-loops @gol
 -funsafe-loop-optimizations -funsafe-math-optimizations -funswitch-loops @gol
 -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model -fvpt @gol
@@ -6659,6 +6659,12 @@ the loop optimizer itself cannot prove that these assumptions are valid.
 If you use @option{-Wunsafe-loop-optimizations}, the compiler warns you
 if it finds this kind of loop.
 
+@item -funconstrained-commons
+@opindex funconstrained-commons
+This option tells the compiler that variables declared in common blocks
+(e.g. Fortran) may later be overridden with longer trailing arrays. This
+prevents certain optimizations that depend on knowing the array bounds.
+
 @item -fcrossjumping
 @opindex fcrossjumping
 Perform cross-jumping transformation.
diff --git a/gcc/testsuite/gfortran.dg/unconstrained_commons.f b/gcc/testsuite/gfortran.dg/unconstrained_commons.f
new file mode 100644
index 0000000..f9fc471
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/unconstrained_commons.f
@@ -0,0 +1,20 @@
+! { dg-do compile }
+! { dg-options "-O3 -funconstrained-commons -fdump-tree-dom2-details" }
+
+! Test for PR69368: a single-element array in a common block, which will be
+! overridden with a larger size at link time (contrary to language spec).
+! Dominator opts considers accesses to differently-computed elements of X as
+! equivalent, unless -funconstrained-commons is passed in.
+      SUBROUTINE FOO
+      IMPLICIT DOUBLE PRECISION (X)
+      INTEGER J
+      COMMON /MYCOMMON / X(1)
+      DO 10 J=1,1024
+         X(J+1)=X(J+7)
+  10  CONTINUE
+      RETURN
+      END
+! { dg-final { scan-tree-dump-not "FIND" "dom2" } }
+! We should retain both a read and write of mycommon.x.
+! { dg-final { scan-tree-dump-times "  _\[0-9\]+ = mycommon\\.x\\\[_\[0-9\]+\\\];" 1 "dom2" } }
+! { dg-final { scan-tree-dump-times "  mycommon\\.x\\\[_\[0-9\]+\\\] = _\[0-9\]+;" 1 "dom2" } }
diff --git a/gcc/tree-dfa.c b/gcc/tree-dfa.c
index 0e98056..f133abc 100644
--- a/gcc/tree-dfa.c
+++ b/gcc/tree-dfa.c
@@ -612,9 +612,22 @@ get_ref_base_and_extent (tree exp, HOST_WIDE_INT *poffset,
 
   if (DECL_P (exp))
     {
+      if (flag_unconstrained_commons
+	  && TREE_CODE (exp) == VAR_DECL && DECL_COMMON (exp))
+	{
+	  tree sz_tree = TYPE_SIZE (TREE_TYPE (exp));
+	  /* If size is unknown, or we have read to the end, assume there
+	     may be more to the structure than we are told.  */
+	  if (TREE_CODE (TREE_TYPE (exp)) == ARRAY_TYPE
+	      || (seen_variable_array_ref
+		  && (sz_tree == NULL_TREE
+		      || TREE_CODE (sz_tree) != INTEGER_CST
+		      || (bit_offset + maxsize == wi::to_offset (sz_tree)))))
+	    maxsize = -1;
+	}
       /* If maxsize is unknown adjust it according to the size of the
          base decl.  */
-      if (maxsize == -1
+      else if (maxsize == -1
 	  && DECL_SIZE (exp)
 	  && TREE_CODE (DECL_SIZE (exp)) == INTEGER_CST)
 	maxsize = wi::to_offset (DECL_SIZE (exp)) - bit_offset;
diff --git a/gcc/tree.c b/gcc/tree.c
index 07cb9d9..9179b37 100644
--- a/gcc/tree.c
+++ b/gcc/tree.c
@@ -12957,8 +12957,10 @@ array_at_struct_end_p (tree ref)
     }
 
   /* If the reference is based on a declared entity, the size of the array
-     is constrained by its given domain.  */
-  if (DECL_P (ref))
+     is constrained by its given domain.  (Do not trust commons PR/69368).  */
+  if (DECL_P (ref)
+      && !(flag_unconstrained_commons
+	   && TREE_CODE (ref) == VAR_DECL && DECL_COMMON (ref)))
     return false;
 
   return true;
-- 
1.9.1