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Re: [RFA][PATCH] Minor fix to aliasing machinery


On Wed, 6 Nov 2013, Richard Biener wrote:

On Wed, Nov 6, 2013 at 1:19 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
[Discussion started in
http://gcc.gnu.org/ml/gcc-patches/2013-10/msg02472.html ]


On Wed, 30 Oct 2013, Marc Glisse wrote:

On Wed, 30 Oct 2013, Richard Biener wrote:

Btw, get_addr_base_and_unit_offset may also return an offsetted
MEM_REF (from &MEM [p_3, 17] for example).  As we are interested in
pointers this could be handled by not requiring a memory reference
but extracting the base address and offset, covering more cases.


I tried the attached patch, and it almost worked, except for one fortran
testcase (widechar_intrinsics_10.f90):


Now that ao_ref_init_from_ptr_and_size has been fixed, the following patch
passes bootstrap+testsuite (default languages) on x86_64-unknown-linux-gnu.

2013-11-06  Marc Glisse  <marc.glisse@inria.fr>
            Jeff Law  <law@redhat.com>

gcc/
        * tree-ssa-alias.c (stmt_kills_ref_p_1): Use
        ao_ref_init_from_ptr_and_size for builtins.

gcc/testsuite/
        * gcc.dg/tree-ssa/alias-27.c: New testcase.

--
Marc Glisse
Index: gcc/testsuite/gcc.dg/tree-ssa/alias-27.c
===================================================================
--- gcc/testsuite/gcc.dg/tree-ssa/alias-27.c    (revision 0)
+++ gcc/testsuite/gcc.dg/tree-ssa/alias-27.c    (working copy)
@@ -0,0 +1,11 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -fdump-tree-optimized" } */
+
+void f (long *p) {
+  *p = 42;
+  p[4] = 42;
+  __builtin_memset (p, 0, 100);
+}
+
+/* { dg-final { scan-tree-dump-not "= 42" "optimized" } } */
+/* { dg-final { cleanup-tree-dump "optimized" } } */

Property changes on: gcc/testsuite/gcc.dg/tree-ssa/alias-27.c
___________________________________________________________________
Added: svn:keywords
## -0,0 +1 ##
+Author Date Id Revision URL
\ No newline at end of property
Added: svn:eol-style
## -0,0 +1 ##
+native
\ No newline at end of property
Index: gcc/tree-ssa-alias.c
===================================================================
--- gcc/tree-ssa-alias.c        (revision 204453)
+++ gcc/tree-ssa-alias.c        (working copy)
@@ -2001,23 +2001,24 @@ stmt_may_clobber_ref_p (gimple stmt, tre
   return stmt_may_clobber_ref_p_1 (stmt, &r);
 }

 /* If STMT kills the memory reference REF return true, otherwise
    return false.  */

 static bool
 stmt_kills_ref_p_1 (gimple stmt, ao_ref *ref)
 {
   /* For a must-alias check we need to be able to constrain
-     the access properly.  */
-  ao_ref_base (ref);
-  if (ref->max_size == -1)
+     the access properly.
+     FIXME: except for BUILTIN_FREE.  */
+  if (!ao_ref_base (ref)
+      || ref->max_size == -1)
     return false;

   if (gimple_has_lhs (stmt)
       && TREE_CODE (gimple_get_lhs (stmt)) != SSA_NAME
       /* The assignment is not necessarily carried out if it can throw
         and we can catch it in the current function where we could inspect
         the previous value.
         ???  We only need to care about the RHS throwing.  For aggregate
         assignments or similar calls and non-call exceptions the LHS
         might throw as well.  */
@@ -2090,37 +2091,47 @@ stmt_kills_ref_p_1 (gimple stmt, ao_ref
          case BUILT_IN_MEMPCPY:
          case BUILT_IN_MEMMOVE:
          case BUILT_IN_MEMSET:
          case BUILT_IN_MEMCPY_CHK:
          case BUILT_IN_MEMPCPY_CHK:
          case BUILT_IN_MEMMOVE_CHK:
          case BUILT_IN_MEMSET_CHK:
            {
              tree dest = gimple_call_arg (stmt, 0);
              tree len = gimple_call_arg (stmt, 2);
-             tree base = NULL_TREE;
-             HOST_WIDE_INT offset = 0;
+             tree rbase = ref->base;
+             HOST_WIDE_INT roffset = ref->offset;
              if (!host_integerp (len, 0))
                return false;
-             if (TREE_CODE (dest) == ADDR_EXPR)
-               base = get_addr_base_and_unit_offset (TREE_OPERAND (dest,
0),
-                                                     &offset);
-             else if (TREE_CODE (dest) == SSA_NAME)
-               base = dest;
-             if (base
-                 && base == ao_ref_base (ref))
+             ao_ref dref;
+             ao_ref_init_from_ptr_and_size (&dref, dest, len);

What I dislike about this is that it can end up building a new tree node.
Not sure if that should block the patch though as it clearly allows to
simplify the code ...

+             tree base = ao_ref_base (&dref);
+             HOST_WIDE_INT offset = dref.offset;
+             if (!base || dref.size == -1)
+               return false;
+             if (TREE_CODE (base) == MEM_REF)
+               {
+                 if (TREE_CODE (rbase) != MEM_REF)

why's that?  I think that just processing both bases separately
would work as well.

If they differ, there is no point going on, we might as well break early. And this way we maintain the property that either base and rbase are both refs, or they are both pointers, not some weird mix.

+                   return false;
+                 // Compare pointers.
+                 offset += BITS_PER_UNIT
+                           * TREE_INT_CST_LOW (TREE_OPERAND (base, 1));

Use mem_ref_offset (base).

offset += BITS_PER_UNIT * mem_ref_offset(base).to_shwi();

or did you mean the computations should use double_int?

+                 roffset += BITS_PER_UNIT
+                            * TREE_INT_CST_LOW (TREE_OPERAND (rbase, 1));

Likewise.

+                 base = TREE_OPERAND (base, 0);
+                 rbase = TREE_OPERAND (rbase, 0);

Both could be &decl here, so you want

         if (TREE_CODE (base) == ADDR_EXPR)
          base = TREE_OPERAND (base, 0);

I rely on the ao_ref* functions to set base to decl and not MEM_REF[&decl], is that a wrong assumption?

--
Marc Glisse


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