https://gcc.gnu.org/bugzilla/show_bug.cgi?id=46476
--- Comment #18 from Richard Biener <rguenth at gcc dot gnu.org> ---
We can warn at CFG construction time. Note the ??? though, we'd want to improve
here to avoid duplicate diagnostics. Tricky cases:
/* Unreachable region entry has a predecessor (backedge). */
void foo()
{
return;
for (int i = 0; i < 5; ++i)
;
}
/* Unreachable region not backwards reachable from exit. */
void bar1()
{
return;
__builtin_abort ();
}
void bar2()
{
return ;
for (;;);
}
/* Unreachable code in if (0) block. */
void baz(int *p)
{
if (0)
{
return;
*p = 0;
}
}
bootstrap with the prototype currently fails in libgomp:
/home/rguenther/src/trunk/libgomp/oacc-plugin.c: In function
'GOMP_PLUGIN_acc_default_dim':
/home/rguenther/src/trunk/libgomp/oacc-plugin.c:65:7: error: statement is not
reachable [-Werror]
65 | return -1;
| ^~~~~~
/home/rguenther/src/trunk/libgomp/oacc-profiling.c: In function
'acc_prof_register':
/home/rguenther/src/trunk/libgomp/oacc-profiling.c:354:7: error: statement is
not reachable [-Werror]
354 | __builtin_unreachable ();
| ^~~~~~~~~~~~~~~~~~~~~
/home/rguenther/src/trunk/libgomp/oacc-profiling.c: In function
'acc_prof_unregister':
/home/rguenther/src/trunk/libgomp/oacc-profiling.c:475:7: error: statement is
not reachable [-Werror]
475 | __builtin_unreachable ();
| ^~~~~~~~~~~~~~~~~~~~~
the latter two are an issue with inital CFG construction I think, where
group_case_labels turns
void bar (foo x)
{
<bb 2> :
switch (x) <default: <L2>, case 0: <L0>, case 1: <L1>>
<bb 3> :
<L0>:
goto <L2>;
<bb 4> :
<L1>:
__builtin_unreachable ();
<bb 5> :
<L2>:
return;
into the following with BB 4 now unreachable.
void bar (foo x)
{
<bb 2> :
switch (x) <default: <L2>, case 0: <L0>>
<bb 3> :
<L0>:
goto <L2>;
<bb 4> :
<L1>:
__builtin_unreachable ();
<bb 5> :
<L2>:
return;
The C++ FE also warns about the implicit return in main when there's a
preceeding one (the C frontend "appropriately" assigns an internal
location which supresses the warning).
diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
index b3a27bcd17c..64ab2607c56 100644
--- a/gcc/tree-cfg.c
+++ b/gcc/tree-cfg.c
@@ -242,7 +242,8 @@ build_gimple_cfg (gimple_seq seq)
/* Group case nodes to reduce the number of edges.
We do this after cleaning up dead labels because otherwise we miss
a lot of obvious case merging opportunities. */
- group_case_labels ();
+ /* ??? This interferes with unreachable code diagnostics. */
+ //group_case_labels ();
/* Create the edges of the flowgraph. */
discriminator_per_locus = new hash_table<locus_discrim_hasher> (13);
@@ -374,6 +375,24 @@ execute_build_cfg (void)
fprintf (dump_file, "Scope blocks:\n");
dump_scope_blocks (dump_file, dump_flags);
}
+
+ find_unreachable_blocks ();
+ basic_block bb;
+ FOR_EACH_BB_FN (bb, cfun)
+ if (!(bb->flags & BB_REACHABLE))
+ for (gimple_stmt_iterator gsi = gsi_start_bb (bb); !gsi_end_p (gsi);
+ gsi_next (&gsi))
+ {
+ if ((LOCATION_LOCUS (gimple_location (gsi_stmt (gsi)))
+ > BUILTINS_LOCATION)
+ && !gimple_no_warning_p (gsi_stmt (gsi)))
+ warning_at (gimple_location (gsi_stmt (gsi)), 0,
+ "statement is not reachable");
+ /* ??? Mark blocks reachable from here. And even better make
+ sure to process entries to unreachable regions first. */
+ break;
+ }
+
cleanup_tree_cfg ();
bb_to_omp_idx.release ();
