On 05/07/12 15:30, Michael Matz wrote:
> Hi,
>
> On Thu, 5 Jul 2012, Tom de Vries wrote:
>
>> The asserts allow the return result to be optimized, but not the cfg
>> conditions.
>>
>> AFAIU, we can insert the asserts earlier. F.i., we can insert
>> aD.1711_6 = ASSERT_EXPR <aD.1711_1(D), aD.1711_1(D) > 0>
>> before the GIMPLE_COND in bb2.
>
> Nope. That would require some more checks, in particular that the BB
> containing builtin_unreachable doesn't contain any other side-effects.
> Given this:
>
> if (i < 0)
> { do_something_interesting();
> __builtin_unreachable();
> }
>
> moving the assert before the if would remove the if condition, hence
> the call to do_something_interesting. You need to retain side-effects if
> there are any.
>
Michael,
Thanks for pointing that out.
I tried a first stab at your suggestion of implementing the optimization in
pass_fold_builtins, it works for the test-case.
Thanks,
- Tom
>
> Ciao,
> Michael.
>
Index: gcc/tree-ssa-ccp.c
===================================================================
--- gcc/tree-ssa-ccp.c (revision 189007)
+++ gcc/tree-ssa-ccp.c (working copy)
@@ -2318,6 +2318,44 @@ optimize_stdarg_builtin (gimple call)
}
}
+/* Return false if there are staments with side-effects before I. Otherwise,
+ return true and make the block of I unreachable. */
+
+static bool
+optimize_unreachable (gimple_stmt_iterator i)
+{
+ gimple stmt;
+ basic_block bb;
+ edge_iterator ei;
+ edge e;
+
+ for (gsi_prev (&i); !gsi_end_p (i); gsi_prev (&i))
+ {
+ stmt = gsi_stmt (i);
+ if (gimple_has_side_effects (stmt))
+ return false;
+ }
+
+ bb = gsi_bb (i);
+
+ FOR_EACH_EDGE (e, ei, bb->preds)
+ {
+ basic_block src = e->src;
+ gimple stmt;
+ i = gsi_last_bb (src);
+ stmt = gsi_stmt (i);
+ gcc_assert (gimple_code (stmt) == GIMPLE_COND);
+ if (e->flags & EDGE_TRUE_VALUE)
+ gimple_cond_make_false (stmt);
+ else if (e->flags & EDGE_FALSE_VALUE)
+ gimple_cond_make_true (stmt);
+ else
+ gcc_unreachable ();
+ }
+
+ return true;
+}
+
/* A simple pass that attempts to fold all builtin functions. This pass
is run after we've propagated as many constants as we can. */
@@ -2379,6 +2417,11 @@ execute_fold_all_builtins (void)
gsi_next (&i);
continue;
+ case BUILT_IN_UNREACHABLE:
+ if (optimize_unreachable (i))
+ cfg_changed = true;
+ break;
+
case BUILT_IN_VA_START:
case BUILT_IN_VA_END:
case BUILT_IN_VA_COPY:
@@ -2393,6 +2436,9 @@ execute_fold_all_builtins (void)
continue;
}
+ if (result == NULL_TREE)
+ break;
+
if (dump_file && (dump_flags & TDF_DETAILS))
{
fprintf (dump_file, "Simplified\n ");
