Hello,
Post-link optimizers encounter difficulties when trying to reorder data
where the code was compiled with -fsection-anchors. The reordering process
uses relocation information, available when linking with --emit-relocs, to
update references to the relocated objects. However -fsection-anchors
causes inter-object distances, between the anchor and its related objects,
to be embedded in the code without associated relocation information.
The following patch adds {$BLOCK_BEGIN,$BLOCK_END} symbol pair around each
block of anchored objects. The symbols are local, of zero size, so they do
not affect memory allocation. With these symbol pairs a post-link optimizer
can turn the {begin,end} pair into a block object, and the anchored objects
to size-less labels within that block. It can then reorder the blocks
without the risk of violating assumptions about inter-object distances.
Bootstrapped on powerpc64-suse-linux, testing on powerpc64-suse-linux in
progress.
I would like to add it to 4.8. Comments?
- Yaakov
2011-12-05 Yaakov Yaari <ya...@il.ibm.com>
* varasm.c (output_object_block): Add a zero-size $BLOCK_BEGIN
before first object in a block, and a $BLOCK_END after the last
object.
Index: gcc/varasm.c
===================================================================
--- gcc/varasm.c (revision 181801)
+++ gcc/varasm.c (working copy)
@@ -7338,6 +7338,9 @@ output_object_block (struct object_block *block)
HOST_WIDE_INT offset;
tree decl;
rtx symbol;
+ rtx first_symbol = 0, last_symbol;
+ char buffer[100];
+ HOST_WIDE_INT last_size;
if (block->objects == NULL)
return;
@@ -7376,9 +7379,27 @@
{
decl = SYMBOL_REF_DECL (symbol);
assemble_variable_contents (decl, XSTR (symbol, 0), false);
- offset += tree_low_cst (DECL_SIZE_UNIT (decl), 1);
+ /* Add zero-size BLOCK_BEGIN symbol at the beginning of block.
*/
+ if (!first_symbol)
+ {
+ first_symbol = symbol;
+ ASM_OUTPUT_DEF (asm_out_file, "$BLOCK_BEGIN",
+ XSTR (first_symbol, 0));
+ ASM_OUTPUT_SIZE_DIRECTIVE (asm_out_file, "$BLOCK_BEGIN", 0);
+ }
+ last_symbol = symbol;
+ last_size = tree_low_cst (DECL_SIZE_UNIT (decl), 1);
+ offset += last_size;
}
}
+ /* Add a zero-size BLOCK_END symbol at end of block. */
+ if (first_symbol)
+ {
+ sprintf (buffer, "%s +" HOST_WIDE_INT_PRINT_DEC, XSTR (last_symbol,
0),
+ last_size);
+ ASM_OUTPUT_DEF (asm_out_file, "$BLOCK_END", buffer);
+ ASM_OUTPUT_SIZE_DIRECTIVE (asm_out_file, "$BLOCK_END", 0);
+ }
}
/* A htab_traverse callback used to call output_object_block for
