[PATCH, PR50527] Don't assume alignment of vla-related allocas.

Richard Guenther richard.guenther@gmail.com
Tue Oct 4 13:06:00 GMT 2011


On Tue, Oct 4, 2011 at 9:43 AM, Tom de Vries <Tom_deVries@mentor.com> wrote:
> On 10/01/2011 05:46 PM, Tom de Vries wrote:
>> On 09/30/2011 03:29 PM, Richard Guenther wrote:
>>> On Thu, Sep 29, 2011 at 3:15 PM, Tom de Vries <Tom_deVries@mentor.com> wrote:
>>>> On 09/28/2011 11:53 AM, Richard Guenther wrote:
>>>>> On Wed, Sep 28, 2011 at 11:34 AM, Tom de Vries <Tom_deVries@mentor.com> wrote:
>>>>>> Richard,
>>>>>>
>>>>>> I got a patch for PR50527.
>>>>>>
>>>>>> The patch prevents the alignment of vla-related allocas to be set to
>>>>>> BIGGEST_ALIGNMENT in ccp. The alignment may turn out smaller after folding
>>>>>> the alloca.
>>>>>>
>>>>>> Bootstrapped and regtested on x86_64.
>>>>>>
>>>>>> OK for trunk?
>>>>>
>>>>> Hmm.  As gfortran with -fstack-arrays uses VLAs it's probably bad that
>>>>> the vectorizer then will no longer see that the arrays are properly aligned.
>>>>>
>>>>> I'm not sure what the best thing to do is here, other than trying to record
>>>>> the alignment requirement of the VLA somewhere.
>>>>>
>>>>> Forcing the alignment of the alloca replacement decl to BIGGEST_ALIGNMENT
>>>>> has the issue that it will force stack-realignment which isn't free (and the
>>>>> point was to make the decl cheaper than the alloca).  But that might
>>>>> possibly be the better choice.
>>>>>
>>>>> Any other thoughts?
>>>>
>>>> How about the approach in this (untested) patch? Using the DECL_ALIGN of the vla
>>>> for the new array prevents stack realignment for folded vla-allocas, also for
>>>> large vlas.
>>>>
>>>> This will not help in vectorizing large folded vla-allocas, but I think it's not
>>>> reasonable to expect BIGGEST_ALIGNMENT when writing a vla (although that has
>>>> been the case up until we started to fold). If you want to trigger vectorization
>>>> for a vla, you can still use the aligned attribute on the declaration.
>>>>
>>>> Still, the unfolded vla-allocas will have BIGGEST_ALIGNMENT, also without using
>>>> an attribute on the decl. This patch exploits this by setting it at the end of
>>>> the 3rd pass_ccp, renamed to pass_ccp_last. This is not very effective in
>>>> propagation though, because although the ptr_info of the lhs is propagated via
>>>> copy_prop afterwards, it's not propagated anymore via ccp.
>>>>
>>>> Another way to do this would be to set BIGGEST_ALIGNMENT at the end of ccp2 and
>>>> not fold during ccp3.
>>>
>>> Ugh, somehow I like this the least ;)
>>>
>>> How about lowering VLAs to
>>>
>>>   p = __builtin_alloca (...);
>>>   p = __builtin_assume_aligned (p, DECL_ALIGN (vla));
>>>
>>> and not assume anything for alloca itself if it feeds a
>>> __builtin_assume_aligned?
>>>
>>> Or rather introduce a __builtin_alloca_with_align () and for VLAs do
>>>
>>>  p = __builtin_alloca_with_align (..., DECL_ALIGN (vla));
>>>
>>> that's less awkward to use?
>>>
>>> Sorry for not having a clear plan here ;)
>>>
>>
>> Using assume_aligned is a more orthogonal way to represent this in gimple, but
>> indeed harder to use.
>>
>> Another possibility is to add a 'tree vla_decl' field to struct
>> gimple_statement_call, which is probably the easiest to implement.
>>
>> But I think __builtin_alloca_with_align might have a use beyond vlas, so I
>> decided to try this one. Attached patch implements my first stab at this  (now
>> testing on x86_64).
>>
>> Is this an acceptable approach?
>>
>
> bootstrapped and reg-tested (including ada) on x86_64.
>
> Ok for trunk?

The idea is ok I think.  But

     case BUILT_IN_ALLOCA:
+    case BUILT_IN_ALLOCA_WITH_ALIGN:
       /* If the allocation stems from the declaration of a variable-sized
         object, it cannot accumulate.  */
       target = expand_builtin_alloca (exp, CALL_ALLOCA_FOR_VAR_P (exp));
       if (target)
        return target;
+      if (DECL_FUNCTION_CODE (get_callee_fndecl (exp))
+         == BUILT_IN_ALLOCA_WITH_ALIGN)
+       {
+         tree new_call = build_call_expr_loc (EXPR_LOCATION (exp),
+                                              built_in_decls[BUILT_IN_ALLOCA],
+                                              1, CALL_EXPR_ARG (exp, 0));
+         CALL_ALLOCA_FOR_VAR_P (new_call) = CALL_ALLOCA_FOR_VAR_P (exp);
+         exp = new_call;
+       }

Ick.  Why can't the rest of the compiler not handle
BUILT_IN_ALLOCA_WITH_ALIGN the same as BUILT_IN_ALLOCA?
(thus, arrange things so the assembler name of alloca-with-align is alloca?)

I don't see why you still need the special late CCP pass.

-static tree
-fold_builtin_alloca_for_var (gimple stmt)
+static bool
+builtin_alloca_with_align_p (gimple stmt)
 {
-  unsigned HOST_WIDE_INT size, threshold, n_elem;
-  tree lhs, arg, block, var, elem_type, array_type;
-  unsigned int align;
+  tree fndecl;
+  if (!is_gimple_call (stmt))
+    return false;
+
+  fndecl = gimple_call_fndecl (stmt);
+
+  return (fndecl != NULL_TREE
+         && DECL_FUNCTION_CODE (fndecl) == BUILT_IN_ALLOCA_WITH_ALIGN);
+}

equivalent to gimple_call_builtin_p (stmt, BUILT_IN_ALLOCA_WITH_ALIGN).

+  DECL_ALIGN (var) = TREE_INT_CST_LOW (gimple_call_arg (stmt, 1));

Now, users might call __builtin_alloca_with_align (8, align), thus use
a non-constant alignment argument.  You either need to reject this
in c-family/c-common.c:check_builtin_function_arguments with an
error or fall back to BIGGEST_ALIGNMENT (similarly during propagation
I suppose).

+DEF_BUILTIN_STUB       (BUILT_IN_ALLOCA_WITH_ALIGN, "alloca_with_align")

Oh, I see you are not exposing this to users, so ignore the comments
about non-constant align.  Can you move this
DEF down, near the other DEF_BUILTIN_STUBs and add a comment
that this is used for VLAs?

+                                      build_int_cstu (size_type_node,
+                                                      DECL_ALIGN (parm)));

size_int (DECL_ALIGN (parm)), similar in other places.

Let's see if there are comments from others on this - which I like.

Thanks,
Richard.

> Thanks,
> - Tom
>
> 2011-10-03  Tom de Vries  <tom@codesourcery.com>
>
>        PR middle-end/50527
>        * tree.c (build_common_builtin_nodes): Add local_define_builtin for
>        BUILT_IN_ALLOCA_WITH_ALIGN.
>        * builtins.c (expand_builtin_alloca): Handle BUILT_IN_ALLOCA_WITH_ALIGN
>        arglist.
>        (expand_builtin): Handle BUILT_IN_ALLOCA_WITH_ALIGN.  Expand
>        BUILT_IN_ALLOCA_WITH_ALIGN as BUILT_IN_ALLOCA.
>        (is_inexpensive_builtin): Handle BUILT_IN_ALLOCA_WITH_ALIGN.
>        * tree-pass.h (pass_ccp_last): Declare.
>        * passes.c (init_optimization_passes): Replace 3rd pass_ccp with
>        pass_ccp_last.
>        * gimplify.c (gimplify_vla_decl): Lower vla to
>        BUILT_IN_ALLOCA_WITH_ALIGN.
>        * tree-ssa-ccp.c (cpp_last): Define.
>        (ccp_finalize): Improve align ptr_info for BUILT_IN_ALLOCA_WITH_ALIGN.
>        (evaluate_stmt): Set align for BUILT_IN_ALLOCA_WITH_ALIGN.
>        (builtin_alloca_with_align_p): New function.
>        (fold_builtin_alloca_with_align_p): New function, factored out of ...
>        (fold_builtin_alloca_for_var): Renamed to ...
>        (fold_builtin_alloca_with_align): Use fold_builtin_alloca_with_align_p.
>        Set DECL_ALIGN from BUILT_IN_ALLOCA_WITH_ALIGN argument.
>        (ccp_fold_stmt): Try folding using fold_builtin_alloca_with_align if
>        builtin_alloca_with_align_p.
>        (do_ssa_ccp_last): New function.
>        (pass_ccp_last): Define.
>        (optimize_stack_restore): Handle BUILT_IN_ALLOCA_WITH_ALIGN.
>        * builtins.def (BUILT_IN_ALLOCA_WITH_ALIGN): Declare using
>        DEF_BUILTIN_STUB.
>        * ipa-pure-const.c (special_builtin_state): Handle
>        BUILT_IN_ALLOCA_WITH_ALIGN.
>        * tree-ssa-alias.c (ref_maybe_used_by_call_p_1
>        (call_may_clobber_ref_p_1): Same.
>        function.c (gimplify_parameters): Lower vla to
>        BUILT_IN_ALLOCA_WITH_ALIGN.
>        cfgexpand.c (expand_call_stmt): Handle BUILT_IN_ALLOCA_WITH_ALIGN.
>        tree-mudflap.c (mf_xform_statements): Same.
>        tree-ssa-dce.c (mark_stmt_if_obviously_necessary)
>        (mark_all_reaching_defs_necessary_1, propagate_necessity): Same.
>        varasm.c (incorporeal_function_p): Same.
>        tree-object-size.c (alloc_object_size): Same.
>        gimple.c (gimple_build_call_from_tree): Same.
>
>        * gcc.dg/pr50527.c: New test.
>
>
>



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