https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103120
--- Comment #5 from Aldy Hernandez <aldyh at gcc dot gnu.org> ---
This is an ordering issue in the path solver, and it's not even relation
related! How interesting.
./xgcc -B./ a.c -O2 -fdisable-tree-ethread -fdisable-tree-thread1
-fdisable-tree-thread2 -fdisable-tree-threadfull1 -fno-tree-dominator-opts
-fdump-tree-threadfull2-details-threading -fdbg-cnt=back_threadfull2:1-1
The path is 19->3->??:
<bb 3> [local count: 916928331]:
# value_20 = PHI <value_17(19), value_7(D)(17)>
# n_27 = PHI <n_16(19), 1(17)>
n_16 = n_27 + 4;
value_17 = value_20 / 10000;
if (value_20 > 42949672959999)
goto <bb 19>; [89.00%]
else
goto <bb 4>; [11.00%]
The path solver calculates:
*********** path_range_query ******************
path_range_query: compute_ranges for path: BB 19, BB 3
Registering killing_def (path_oracle) value_17
range_defined_in_block (BB3) for value_20 is long unsigned int [4294967296,
1844674407370955]
range_defined_in_block (BB3) for value_17 is long unsigned int [429496,
184467440737]
range_defined_in_block (BB3) for value_20 is long unsigned int [429496,
184467440737]
So, it's calculating the range for value_20 as it tries to resolve value_17.
The first 2 ranges are correct.
Then it tries to calculate the range for value_20 (again) because it's an
import. However, when looking at the PHI, it ignores that there's already a
cache entry, so it tries the following...
# value_20 = PHI <value_17(19), value_7(D)(17)>
...using the new value of value_17 which is incorrect.
It blows my mind that we haven't run into this before, especially with such a
simple test case.
I need to think about this, but my gut feeling says the following should do it:
diff --git a/gcc/gimple-range-path.cc b/gcc/gimple-range-path.cc
index 9175651e896..7b5b2e5e55d 100644
--- a/gcc/gimple-range-path.cc
+++ b/gcc/gimple-range-path.cc
@@ -356,7 +356,8 @@ path_range_query::compute_ranges_in_block (basic_block bb)
{
tree name = ssa_name (i);
- if (range_defined_in_block (r, name, bb))
+ if (!bitmap_bit_p (m_has_cache_entry, SSA_NAME_VERSION (name))
+ && range_defined_in_block (r, name, bb))
set_cache (r, name);
}
