[PATCH, PR 50444] Decrease MEM_REF TYPE_ALIGN in build_ref_for_offset

[Martin Jambor <mjambor at suse dot cz>]

Hi,

I tend to believe that this is the SRA part of a fix for PR 50444 (and
possibly some strict alignment SRA-related bugs too.  What it does is
that it decreases the alignment of the built MEM_REF type if it seems
necessary.  That is in cases when the alignment of the type as it
arrived in the parameter is larger than the alignment of the base we
are building the MEM_REF on top of or the MEM_REF we are replacing or
when it is larger than the biggest power of two that divides the
offset we are adding, if it is non zero).

This patch alone fixes the testcase on i686, on x86_64 the testcase
stops segfaulting but the final comparison does not go well and the
testcase returns non-zero exit code.  However, I belive that is most
probably an expansion problem, rather than SRA's.  Perhaps it should
be also noted that if I revert SRA's build_ref_for_model to what it
was in revision 164280 and SRA creates only a MEM_REF rather than a
COMPONENT_REF on top of a MEM_REF which it does now, the assignment
expansion takes the "optab_handler (movmisalign_optab, mode)" route
and the testcase is compiled well, with both the segfault gone and
comparison yielding the correct result.  I plan to have a look at what
goes on in expansion with the current build_ref_for_model shortly but
thought I'd post this for review and comments before that.

A similar patch has passed bootstrap and testsuite on x86_64-linux,
this exact one is undergoing the same as I'm writing this.  Is it the
good direction, should I commit the patch (to trunk and the 4.6
branch) if it passes?

Thanks,

Martin


2012-01-12  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/50444
	* tree-sra.c (build_ref_for_offset): Decrease the alignment of the
	type of the created MEM_REF if necessary.

Index: src/gcc/tree-sra.c
===================================================================
--- src.orig/gcc/tree-sra.c
+++ src/gcc/tree-sra.c
@@ -1458,9 +1458,10 @@ build_ref_for_offset (location_t loc, tr
 		      tree exp_type, gimple_stmt_iterator *gsi,
 		      bool insert_after)
 {
+  HOST_WIDE_INT base_offset, final_offset;
+  unsigned int align;
   tree prev_base = base;
   tree off;
-  HOST_WIDE_INT base_offset;
 
   gcc_checking_assert (offset % BITS_PER_UNIT == 0);
 
@@ -1488,24 +1489,34 @@ build_ref_for_offset (location_t loc, tr
 	gsi_insert_before (gsi, stmt, GSI_SAME_STMT);
       update_stmt (stmt);
 
+      final_offset = offset / BITS_PER_UNIT;
       off = build_int_cst (reference_alias_ptr_type (prev_base),
-			   offset / BITS_PER_UNIT);
+			   final_offset);
       base = tmp;
+      align = get_object_alignment (prev_base);
     }
   else if (TREE_CODE (base) == MEM_REF)
     {
-      off = build_int_cst (TREE_TYPE (TREE_OPERAND (base, 1)),
-			   base_offset + offset / BITS_PER_UNIT);
+      final_offset = base_offset + offset / BITS_PER_UNIT;
+      off = build_int_cst (TREE_TYPE (TREE_OPERAND (base, 1)), final_offset);
       off = int_const_binop (PLUS_EXPR, TREE_OPERAND (base, 1), off);
+      align = get_object_or_type_alignment (base);
       base = unshare_expr (TREE_OPERAND (base, 0));
     }
   else
     {
-      off = build_int_cst (reference_alias_ptr_type (base),
-			   base_offset + offset / BITS_PER_UNIT);
+      final_offset = base_offset + offset / BITS_PER_UNIT;
+      off = build_int_cst (reference_alias_ptr_type (base), final_offset);
+      align = get_object_alignment (base);
       base = build_fold_addr_expr (unshare_expr (base));
     }
 
+  if (final_offset != 0)
+    align = MIN (align,
+                 (1u << ctz_hwi (absu_hwi (final_offset))) * BITS_PER_UNIT);
+  if (align < TYPE_ALIGN (exp_type))
+    exp_type = build_aligned_type (exp_type, align);
+
   return fold_build2_loc (loc, MEM_REF, exp_type, base, off);
 }