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* [gcc r13-3133] Fix wrong code generated by unroll-and-jam pass
@ 2022-10-06 13:17 Eric Botcazou
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From: Eric Botcazou @ 2022-10-06 13:17 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:3ec926d36fbf7cb3ff45759471139f3a71d1c4de
commit r13-3133-g3ec926d36fbf7cb3ff45759471139f3a71d1c4de
Author: Eric Botcazou <ebotcazou@adacore.com>
Date: Thu Oct 6 15:13:50 2022 +0200
Fix wrong code generated by unroll-and-jam pass
There is a loophole in the unroll-and-jam pass that can quickly result in
wrong code generation. The code reads:
if (!compute_data_dependences_for_loop (outer, true, &loop_nest,
&datarefs, &dependences))
{
if (dump_file && (dump_flags & TDF_DETAILS))
fprintf (dump_file, "Cannot analyze data dependencies\n");
free_data_refs (datarefs);
free_dependence_relations (dependences);
continue;
}
but compute_data_dependences_for_loop may return true even if the analysis
is reported as failing by compute_affine_dependence for a dependence pair:
(compute_affine_dependence
ref_a: data[_14], stmt_a: data[_14] = i_59;
ref_b: data[_14], stmt_b: data[_14] = i_59;
Data ref a:
Data ref b:
affine dependence test not usable: access function not affine or constant.
) -> dependence analysis failed
Note that this is a self-dependence pair and the code for them reads:
/* Nothing interesting for the self dependencies. */
if (dra == drb)
continue;
This means that the pass may reorder "complex" accesses to the same memory
location in successive iterations, which is OK for reads but not for writes.
gcc/
* gimple-loop-jam.cc (tree_loop_unroll_and_jam): Bail out for a self
dependency that is a write-after-write if the access function is not
affine or constant.
gcc/testsuite/
* gcc.c-torture/execute/20221006-1.c: New test.
Diff:
---
gcc/gimple-loop-jam.cc | 18 +++++++++++++--
gcc/testsuite/gcc.c-torture/execute/20221006-1.c | 29 ++++++++++++++++++++++++
2 files changed, 45 insertions(+), 2 deletions(-)
diff --git a/gcc/gimple-loop-jam.cc b/gcc/gimple-loop-jam.cc
index a8a57d3d384..4f7a6e5bbae 100644
--- a/gcc/gimple-loop-jam.cc
+++ b/gcc/gimple-loop-jam.cc
@@ -545,11 +545,25 @@ tree_loop_unroll_and_jam (void)
/* If the refs are independend there's nothing to do. */
if (DDR_ARE_DEPENDENT (ddr) == chrec_known)
continue;
+
dra = DDR_A (ddr);
drb = DDR_B (ddr);
- /* Nothing interesting for the self dependencies. */
+
+ /* Nothing interesting for the self dependencies, except for WAW if
+ the access function is not affine or constant because we may end
+ up reordering writes to the same location. */
if (dra == drb)
- continue;
+ {
+ if (DR_IS_WRITE (dra)
+ && !DR_ACCESS_FNS (dra).is_empty ()
+ && DDR_ARE_DEPENDENT (ddr) == chrec_dont_know)
+ {
+ unroll_factor = 0;
+ break;
+ }
+ else
+ continue;
+ }
/* Now check the distance vector, for determining a sensible
outer unroll factor, and for validity of merging the inner
diff --git a/gcc/testsuite/gcc.c-torture/execute/20221006-1.c b/gcc/testsuite/gcc.c-torture/execute/20221006-1.c
new file mode 100644
index 00000000000..80deb3a148f
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/20221006-1.c
@@ -0,0 +1,29 @@
+#include <stdlib.h>
+
+int
+main (int argc, char** argv)
+{
+ const int len = argc == 2 ? atoi(argv[1]) : 4;
+
+ int count;
+ int data[64];
+ int M1[len][len];
+ int M2[len][len];
+
+ for (int i = 0; i < len; i++)
+ for (int j = 0 ; j < len ; j++)
+ M1[i][j] = M2[i][j] = i*len + j;
+
+ M2[1][0] = M2[0][1];
+
+ /* This writes successively 0 and 1 into data[M2[0][1]]. */
+ for (int i = 0; i < len - 1; i++)
+ for (int j = 0 ; j < len ; j++)
+ if (M1[i+1][j] > M1[i][j])
+ data[M2[i][j]] = i;
+
+ if (data [M2[0][1]] != 1)
+ abort ();
+
+ return 0;
+}
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