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[PATCH] Object size checking (stage 2 project; take 3)


On Mon, Jun 06, 2005 at 09:44:10AM -0400, Diego Novillo wrote:
> Given that the stage2 deadline is fast approaching and neither
> Jakub nor myself will likely have time to work on this any time
> soon, I withdraw my objection.
> Jakub has agreed to eventually rewrite the propagation using the
> engine.  Since that would be just a cleanup, the behaviour of the
> patch would not be affected.  This cleanup can be delayed until
> the next stage1.
> If the builtin bits are approved, then this patch can go in.

Attached is an updated version of the patch against current CVS
(tests bzip2ped, to make the mail not too large).

Can anyone please review the builtins bits?

BTW, with current GCC CVS 4 of the added tests
(gcc.dg/builtin-object-size-{1,2,3,4}.c) fail with:
/usr/src/gcc/gcc/testsuite/gcc.dg/builtin-object-size-1.c: In function 'test2':
/usr/src/gcc/gcc/testsuite/gcc.dg/builtin-object-size-1.c:159: error:
Pointers with a memory tag, should have points-to sets or r_1, name memory tag: NMT.52, points-to vars: { } r, UID 0, char *
/usr/src/gcc/gcc/testsuite/gcc.dg/builtin-object-size-1.c:159: internal
compiler error: verify_flow_sensitive_alias_info failed.Please submit a full bug report,
with preprocessed source if appropriate.
See <URL:> for instructions.

but a) that looks like a tree-ssa-alias bug to me (see below)
b) it is a checking only failure, when not checking, it works
c) it is not something people usually do with the builtins.
Usually the builtin result is passed to a function that also uses
the pointer passed to the __builtin_object_size (ptr, cst) function
and therefore the pointer use is usually not completely optimized
away when __builtin_object_size (ptr, cst) is optimized into a

In gcc.dg/builtin-object-size{1,2,3,4}.c I'm only testing
the __builtin_object_size builtin, so e.g. in test2 function
__builtin_object_size is the only place that uses the r variable.
At alias4 time, the variable is:
rD.1709_1, name memory tag: NMT.52D.2245, is dereferenced, its value escapes, points-to vars: { buf3D.1710 aD.1708 }
  # rD.1709_1 = PHI <&aD.1708.buf1D.1706[1](1), &aD.1708.buf2D.1707[7](2), &buf3D.1710[5](3), rD.1709_40(4), &aD.1708.buf1D.1706[9](5)>;
  D.1726_12 = __builtin_object_size (rD.1709_1, 0);
Now, objsz pass (but similarly could optimize e.g. a fab pass) optimizes
the last line into:
  D.1726_12 = 20;
and when alias5 starts, in init_alias_info it clears
rD.1709_1's is_dereferenced, clears pi->pt_vars
bitmap etc., but does not clear it's pi->name_mem_tag
(this is I guess desirable, so that in the likely case the mem tag
will be needed again a new mem tag is not created again).
Next collect_points_to_info_for is called for pointer uses
and collect_points_to_info_for leads to populating the processed_ptrs
array (and for pointers in processed_ptrs varray it clears pi->name_mem_tag
in create_name_tags).  The problem here is that rD.1709_1
is not used any longer, so pi->name_mem_tag is not cleared,
while pi->is_dereferenced is 0, pi->pt_vars is empty bitmap
and verify_flow_sensitive_alias_info complains.
PR tree-optimization/21584 sounds like the same problem.
Now, unless we require a DCE pass right before every may_alias
pass, I'm afraid we have to live with pointers that had a name_mem_tag
set, but after some optimization are no longer used anywhere, but not yet
eliminated as dead code.  So, either create_name_tags would need to
walk over all SSA names with pointer type and do:
      if (pi->pt_anything || !pi->is_dereferenced)
          /* No name tags for pointers that have not been
             dereferenced or point to an arbitrary location.  */
          pi->name_mem_tag = NULL_TREE;
on them, not just on those in ai->processed_ptrs,
or verify_flow_sensitive_alias_info should allow non-NULL
name_mem_tag if pt_anything != 0 || pi->is_dereferenced = NULL.


Attachment: gcc41-chk.patch
Description: Text document

Attachment: gcc41-chk-tests.patch.bz2
Description: BZip2 compressed data

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