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Re: [RFC] Should SRA stop producing COMPONENT_REF for non-bit-fields (again)?


On Wed, 4 Apr 2012, Martin Jambor wrote:

> Hi everyone, especially Richi and Eric,
> 
> I'd like to know what is your attitude to changing SRA's
> build_ref_for_model to what it once looked like, so that it produces
> COMPONENT_REFs only for bit-fields.  The non-bit field handling was
> added in order to work-around problems when expanding non-aligned
> MEM_REFs on strict-alignment targets but that should not be a problem
> now and my experiments confirm that.  Last week I successfully
> bootstrapped and tested this patch on sparc64-linux (with the
> temporary MEM_EXPR patch, not including Java), ia64-linux (without
> Ada), x86_64-linux, i686-linux and tested it on hppa-linux (only C and
> C++).
> 
> The main downside of this change I see is that dumps would be a bit
> more difficult to read and understand when the fields disappear from
> them.
> 
> The upsides are:
> 
>   - the expr field of SRA access was originally intended only for
>     debugging (meaning both for compiler-produced debug info and
>     debugging SRA).  It was never intended to influence the memory
>     accesses produced by SRA and when we create them artificially, the
>     effects of the particular form are hard to reason about.  If we
>     ever lower bit-field accesses on gimple level, build_ref_for_model
>     could go away completely (yeah, I know I'm getting carried way
>     here).
> 
>   - If something like PR 51528 creeps up again and we need to create
>     replacements of type returned by lang_hooks.types.type_for_mode,
>     the produced MEM_REFs could simply have this type.  OTOH, the
>     current COMPONENT_REFs would require to be encapsulated in V_C_Es
>     and that is quite a nightmare.  I tried it in December, even made
>     it work, but it was particularly ugly and needed some quite
>     questionable uses of V_C_Es.
> 
>   - Well, it does the same thing and is much simpler, is it not?
> 
> The patch fulfills the criteria to be committed and I can do it soon.
> OTOH, keeping it so on a number of platforms takes quite a lot of time
> (and has uncovered some non-related bugs) so I'd like to know whether
> it's worth it.
> 
> Thanks,
> 
> Martin
> 
> 
> 
> 2012-03-20 Martin Jambor <mjambor@suse.cz>
> 
> 	* tree-sra.c (build_ref_for_model): Create COMPONENT_REFs only for
> 	bit-fields.
> 
> Index: src/gcc/tree-sra.c
> ===================================================================
> *** src.orig/gcc/tree-sra.c
> --- src/gcc/tree-sra.c
> *************** build_ref_for_offset (location_t loc, tr
> *** 1489,1558 ****
>     return fold_build2_loc (loc, MEM_REF, exp_type, base, off);
>   }
>   
> - DEF_VEC_ALLOC_P_STACK (tree);
> - #define VEC_tree_stack_alloc(alloc) VEC_stack_alloc (tree, alloc)
> - 
>   /* Construct a memory reference to a part of an aggregate BASE at the given
> !    OFFSET and of the type of MODEL.  In case this is a chain of references
> !    to component, the function will replicate the chain of COMPONENT_REFs of
> !    the expression of MODEL to access it.  GSI and INSERT_AFTER have the same
> !    meaning as in build_ref_for_offset.  */
>   
>   static tree
>   build_ref_for_model (location_t loc, tree base, HOST_WIDE_INT offset,
>   		     struct access *model, gimple_stmt_iterator *gsi,
>   		     bool insert_after)
>   {
> !   tree type = model->type, t;
> !   VEC(tree,stack) *cr_stack = NULL;
> ! 
> !   if (TREE_CODE (model->expr) == COMPONENT_REF)
>       {
> !       tree expr = model->expr;
> ! 
> !       /* Create a stack of the COMPONENT_REFs so later we can walk them in
> ! 	 order from inner to outer.  */
> !       cr_stack = VEC_alloc (tree, stack, 6);
> ! 
> !       do {
> ! 	tree field = TREE_OPERAND (expr, 1);
> ! 	tree cr_offset = component_ref_field_offset (expr);
> ! 	HOST_WIDE_INT bit_pos
> ! 	  = tree_low_cst (cr_offset, 1) * BITS_PER_UNIT
> ! 	      + TREE_INT_CST_LOW (DECL_FIELD_BIT_OFFSET (field));
>   
> ! 	/* We can be called with a model different from the one associated
> ! 	   with BASE so we need to avoid going up the chain too far.  */
> ! 	if (offset - bit_pos < 0)
> ! 	  break;
> ! 
> ! 	offset -= bit_pos;
> ! 	VEC_safe_push (tree, stack, cr_stack, expr);
> ! 
> ! 	expr = TREE_OPERAND (expr, 0);
> ! 	type = TREE_TYPE (expr);
> !       } while (TREE_CODE (expr) == COMPONENT_REF);
>       }
> ! 
> !   t = build_ref_for_offset (loc, base, offset, type, gsi, insert_after);
> ! 
> !   if (TREE_CODE (model->expr) == COMPONENT_REF)
> !     {
> !       unsigned i;
> !       tree expr;
> ! 
> !       /* Now replicate the chain of COMPONENT_REFs from inner to outer.  */
> !       FOR_EACH_VEC_ELT_REVERSE (tree, cr_stack, i, expr)
> ! 	{
> ! 	  tree field = TREE_OPERAND (expr, 1);
> ! 	  t = fold_build3_loc (loc, COMPONENT_REF, TREE_TYPE (field), t, field,
> ! 			       TREE_OPERAND (expr, 2));
> ! 	}
> ! 
> !       VEC_free (tree, stack, cr_stack);
> !     }
> ! 
> !   return t;
>   }
>   
>   /* Construct a memory reference consisting of component_refs and array_refs to
> --- 1489,1520 ----
>     return fold_build2_loc (loc, MEM_REF, exp_type, base, off);
>   }
>   
>   /* Construct a memory reference to a part of an aggregate BASE at the given
> !    OFFSET and of the same type as MODEL.  In case this is a reference to a
> !    bit-field, the function will replicate the last component_ref of model's
> !    expr to access it.  GSI and INSERT_AFTER have the same meaning as in
> !    build_ref_for_offset.  */
>   
>   static tree
>   build_ref_for_model (location_t loc, tree base, HOST_WIDE_INT offset,
>   		     struct access *model, gimple_stmt_iterator *gsi,
>   		     bool insert_after)
>   {
> !   if (TREE_CODE (model->expr) == COMPONENT_REF
> !       && DECL_BIT_FIELD (TREE_OPERAND (model->expr, 1)))

I think you need to test DECL_BIT_FIELD_TYPE here

>       {
> !       /* This access represents a bit-field.  */
> !       tree t, exp_type;
>   
> !       offset -= int_bit_position (TREE_OPERAND (model->expr, 1));

I'm not sure that offset is now byte-aligned for all the funny Ada
bit-layouted records.  But maybe we don't even try to SRA those?

> !       exp_type = TREE_TYPE (TREE_OPERAND (model->expr, 0));
> !       t = build_ref_for_offset (loc, base, offset, exp_type, gsi, insert_after);
> !       return fold_build3_loc (loc, COMPONENT_REF, model->type, t,
> ! 			      TREE_OPERAND (model->expr, 1), NULL_TREE);
>       }
> !   else
> !     return build_ref_for_offset (loc, base, offset, model->type,
> ! 				 gsi, insert_after);
>   }
>   
>   /* Construct a memory reference consisting of component_refs and array_refs to

Otherwise I'm all for doing this change.  I'd even go one step further
and lower bitfield accesses here in SRA - if the FIELD_DECL of the
bitfield COMPONENT_REF has a DECL_BIT_FIELD_REPRESENTATIVE access
that and extract the requested bits via a BIT_FIELD_REF on the RHS
(or do a RMW cycle for stores).

That would have been my first place to "lower" bitfield accesses anyway,
and maybe get rid of some restrictions of SRA that way.

Thanks,
Richard.


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