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Do not emit unnecessary NOPs at -O0
- From: Eric Botcazou <ebotcazou at adacore dot com>
- To: gcc-patches at gcc dot gnu dot org
- Date: Thu, 21 Jun 2018 19:04:23 +0200
- Subject: Do not emit unnecessary NOPs at -O0
When code is compiled at -O0, the RTL middle-end makes sure that location
information is preserved as much as possible by generating NOPs with the
location information (goto_locus) present on edges in the CFG, if it thinks
that these edges are the only place where a particular location is mentioned.
The attached patch prevents this from happening in a couple of cases:
1. if the function has the DECL_IGNORED_P flag set,
2. if the NOP is emitted by merge_blocks and the 2nd block is a forwarder
block whose outgoing edge has no location, because in this case the location
of the to-be-elided edge is copied onto the aforementioned outgoing edge.
Tested (GCC and GDB) on x86-64/Linux, applied on the mainline.
2018-06-21 Eric Botcazou <ebotcazou@adacore.com>
* cfgrtl.c (fixup_reorder_chain): Do not emit NOPs in DECL_IGNORED_P
functions.
(rtl_merge_blocks): Likewise. Do not emit a NOP if the location of the
edge can be forwarded.
(cfg_layout_merge_blocks): Likewise.
--
Eric Botcazou
Index: cfgrtl.c
===================================================================
--- cfgrtl.c (revision 261832)
+++ cfgrtl.c (working copy)
@@ -813,10 +813,14 @@ emit_nop_for_unique_locus_between (basic
static void
rtl_merge_blocks (basic_block a, basic_block b)
{
+ /* If B is a forwarder block whose outgoing edge has no location, we'll
+ propagate the locus of the edge between A and B onto it. */
+ const bool forward_edge_locus
+ = (b->flags & BB_FORWARDER_BLOCK) != 0
+ && LOCATION_LOCUS (EDGE_SUCC (b, 0)->goto_locus) == UNKNOWN_LOCATION;
rtx_insn *b_head = BB_HEAD (b), *b_end = BB_END (b), *a_end = BB_END (a);
rtx_insn *del_first = NULL, *del_last = NULL;
rtx_insn *b_debug_start = b_end, *b_debug_end = b_end;
- bool forwarder_p = (b->flags & BB_FORWARDER_BLOCK) != 0;
int b_empty = 0;
if (dump_file)
@@ -887,9 +891,11 @@ rtl_merge_blocks (basic_block a, basic_b
BB_HEAD (b) = b_empty ? NULL : b_head;
delete_insn_chain (del_first, del_last, true);
- /* When not optimizing and the edge is the only place in RTL which holds
- some unique locus, emit a nop with that locus in between. */
- if (!optimize)
+ /* If not optimizing, preserve the locus of the single edge between
+ blocks A and B if necessary by emitting a nop. */
+ if (!optimize
+ && !forward_edge_locus
+ && !DECL_IGNORED_P (current_function_decl))
{
emit_nop_for_unique_locus_between (a, b);
a_end = BB_END (a);
@@ -918,9 +924,7 @@ rtl_merge_blocks (basic_block a, basic_b
df_bb_delete (b->index);
- /* If B was a forwarder block, propagate the locus on the edge. */
- if (forwarder_p
- && LOCATION_LOCUS (EDGE_SUCC (b, 0)->goto_locus) == UNKNOWN_LOCATION)
+ if (forward_edge_locus)
EDGE_SUCC (b, 0)->goto_locus = EDGE_SUCC (a, 0)->goto_locus;
if (dump_file)
@@ -3916,9 +3920,9 @@ fixup_reorder_chain (void)
force_nonfallthru (e);
}
- /* Ensure goto_locus from edges has some instructions with that locus
- in RTL. */
- if (!optimize)
+ /* Ensure goto_locus from edges has some instructions with that locus in RTL
+ when not optimizing. */
+ if (!optimize && !DECL_IGNORED_P (current_function_decl))
FOR_EACH_BB_FN (bb, cfun)
{
edge e;
@@ -4605,7 +4609,11 @@ cfg_layout_can_merge_blocks_p (basic_blo
static void
cfg_layout_merge_blocks (basic_block a, basic_block b)
{
- bool forwarder_p = (b->flags & BB_FORWARDER_BLOCK) != 0;
+ /* If B is a forwarder block whose outgoing edge has no location, we'll
+ propagate the locus of the edge between A and B onto it. */
+ const bool forward_edge_locus
+ = (b->flags & BB_FORWARDER_BLOCK) != 0
+ && LOCATION_LOCUS (EDGE_SUCC (b, 0)->goto_locus) == UNKNOWN_LOCATION;
rtx_insn *insn;
gcc_checking_assert (cfg_layout_can_merge_blocks_p (a, b));
@@ -4626,9 +4634,11 @@ cfg_layout_merge_blocks (basic_block a,
try_redirect_by_replacing_jump (EDGE_SUCC (a, 0), b, true);
gcc_assert (!JUMP_P (BB_END (a)));
- /* When not optimizing and the edge is the only place in RTL which holds
- some unique locus, emit a nop with that locus in between. */
- if (!optimize)
+ /* If not optimizing, preserve the locus of the single edge between
+ blocks A and B if necessary by emitting a nop. */
+ if (!optimize
+ && !forward_edge_locus
+ && !DECL_IGNORED_P (current_function_decl))
emit_nop_for_unique_locus_between (a, b);
/* Move things from b->footer after a->footer. */
@@ -4695,9 +4705,7 @@ cfg_layout_merge_blocks (basic_block a,
df_bb_delete (b->index);
- /* If B was a forwarder block, propagate the locus on the edge. */
- if (forwarder_p
- && LOCATION_LOCUS (EDGE_SUCC (b, 0)->goto_locus) == UNKNOWN_LOCATION)
+ if (forward_edge_locus)
EDGE_SUCC (b, 0)->goto_locus = EDGE_SUCC (a, 0)->goto_locus;
if (dump_file)