On Fri, 5 Aug 2016, Richard Biener wrote:
>
> This avoids regressing gcc.dg/tree-ssa/pr21417.c with the fix for
> PR72772 where after it a forwarder block no longer is present.
> It's easy to simply create it when FSM threading faces the situation
> that the edge ending the path enters a loop.
>
> I also fixed the costs for obviously related anon SSA names (when
> they are the same).
>
> Bootstrap and regtest running on x86_64-unknown-linux-gnu, I'll apply
> if that finishes successfully.
Ok, so that re-introduces the c-c++-common/ubsan/pr71403-2.c miscompile
where we mash two loops retaining a bogus loop->nb_iterations_upper_bound.
I think those two testcases show that it should be the dominance
relation between the entered loop header and the threaded path
that decides whether we can thread or not -- the path may not
become another latch of this loop. I can't seem to figure out
a correct condition for this based on the threading path but the
following works (maybe by accident as I only have those two
testcases):
Index: gcc/tree-ssa-threadbackward.c
===================================================================
--- gcc/tree-ssa-threadbackward.c (revision 239164)
+++ gcc/tree-ssa-threadbackward.c (working copy)
@@ -35,6 +35,7 @@ along with GCC; see the file COPYING3.
#include "tree-pass.h"
#include "gimple-ssa.h"
#include "tree-phinodes.h"
+#include "cfghooks.h"
static int max_threaded_paths;
@@ -560,6 +562,14 @@ fsm_find_control_statement_thread_paths
edge taken_edge = profitable_jump_thread_path (path, bbi, name,
arg);
if (taken_edge)
{
+ /* If the taken edge is a loop entry avoid mashing two
+ loops into one with multiple latches by splitting
+ the edge. This only works if that block won't become
+ a latch of this loop. */
+ if ((bb_loop_depth (taken_edge->src)
+ < bb_loop_depth (taken_edge->dest))
+ && ! single_succ_p (bbi))
+ split_edge (taken_edge);
if (bb_loop_depth (taken_edge->src)
>= bb_loop_depth (taken_edge->dest))
convert_and_register_jump_thread_path (path, taken_edge);
note you need the PR72772 fix to trigger all this.
Any idea?
Thanks,
Richard.
> Richard.
>
> 2016-08-05 Richard Biener <[email protected]>
>
> * tree-ssa-threadbackward.c: Include cfghooks.h.
> (profitable_jump_thread_path): Treat same SSA names related.
> (fsm_find_control_statement_thread_paths): When the taken edge
> enters a loop split it instead of giving up.
>
> Index: gcc/tree-ssa-threadbackward.c
> ===================================================================
> --- gcc/tree-ssa-threadbackward.c (revision 239164)
> +++ gcc/tree-ssa-threadbackward.c (working copy)
> @@ -35,6 +35,7 @@ along with GCC; see the file COPYING3.
> #include "tree-pass.h"
> #include "gimple-ssa.h"
> #include "tree-phinodes.h"
> +#include "cfghooks.h"
>
> static int max_threaded_paths;
>
> @@ -206,8 +207,9 @@ profitable_jump_thread_path (vec<basic_b
> /* Note that if both NAME and DST are anonymous
> SSA_NAMEs, then we do not have enough information
> to consider them associated. */
> - if ((SSA_NAME_VAR (dst) != SSA_NAME_VAR (name)
> - || !SSA_NAME_VAR (dst))
> + if (dst != name
> + && (SSA_NAME_VAR (dst) != SSA_NAME_VAR (name)
> + || !SSA_NAME_VAR (dst))
> && !virtual_operand_p (dst))
> ++n_insns;
> }
> @@ -560,9 +562,13 @@ fsm_find_control_statement_thread_paths
> edge taken_edge = profitable_jump_thread_path (path, bbi, name, arg);
> if (taken_edge)
> {
> + /* If the taken edge is a loop entry avoid mashing two
> + loops into one with multiple latches by splitting
> + the edge. */
> if (bb_loop_depth (taken_edge->src)
> - >= bb_loop_depth (taken_edge->dest))
> - convert_and_register_jump_thread_path (path, taken_edge);
> + < bb_loop_depth (taken_edge->dest))
> + split_edge (taken_edge);
> + convert_and_register_jump_thread_path (path, taken_edge);
> path->pop ();
> }
> }
> @@ -586,9 +592,13 @@ fsm_find_control_statement_thread_paths
> name, arg);
> if (taken_edge)
> {
> + /* If the taken edge is a loop entry avoid mashing two
> + loops into one with multiple latches by splitting
> + the edge. */
> if (bb_loop_depth (taken_edge->src)
> - >= bb_loop_depth (taken_edge->dest))
> - convert_and_register_jump_thread_path (path, taken_edge);
> + < bb_loop_depth (taken_edge->dest))
> + split_edge (taken_edge);
> + convert_and_register_jump_thread_path (path, taken_edge);
> path->pop ();
> }
>
>
--
Richard Biener <[email protected]>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
