This PR is an odd case in which, due to the low optimisation level,
we enter vectorisation with:
outer1:
x_1 = PHI <x_3(outer2), ...>;
...
inner:
x_2 = 0;
...
outer2:
x_3 = PHI <x_2(inner)>;
These statements are tentatively treated as a double reduction by
vect_force_simple_reduction, but in the end only x_3 and x_2 are marked
as relevant. vect_analyze_loop_operations skips over x_3, leaving the
vectorizable_reduction check to a presumed future test of x_1, which
in this case never happens. We therefore end up vectorising x_2 only
(complete with peeling for niters!) and leave the scalar x_3 in place.
This caused a segfault in the support for fully-masked loops,
since there were no statements that needed masking. Fixed by
checking for that.
But I think this is also a flaw in vect_analyze_loop_operations.
Outer loop vectorisation reduces the number of times that the
inner loop is executed, so it wouldn't necessarily be valid
to leave the scalar x_3 in place for all vectorisable x_2.
There's already code to forbid that when x_1 isn't present:
/* FORNOW: we currently don't support the case that these phis
are not used in the outerloop (unless it is double reduction,
i.e., this phi is vect_reduction_def), cause this case
requires to actually do something here. */
I think we need to do the same if x_1 is present but not relevant.
Tested on aarch64-linux-gnu, x86_64-linux-gnu and powerpc64le-linux-gnu.
OK to install?
Richard
2018-01-19 Richard Sandiford <richard.sandif...@linaro.org>
gcc/
PR tree-optimization/83922
* tree-vect-loop.c (vect_verify_full_masking): Return false if
there are no statements that need masking.
(vect_active_double_reduction_p): New function.
(vect_analyze_loop_operations): Use it when handling phis that
are not in the loop header.
gcc/testsuite/
PR tree-optimization/83922
* gcc.dg/pr83922.c: New test.
Index: gcc/tree-vect-loop.c
===================================================================
--- gcc/tree-vect-loop.c 2018-01-19 09:36:33.409191362 +0000
+++ gcc/tree-vect-loop.c 2018-01-19 09:52:00.681330865 +0000
@@ -1294,6 +1294,12 @@ vect_verify_full_masking (loop_vec_info
struct loop *loop = LOOP_VINFO_LOOP (loop_vinfo);
unsigned int min_ni_width;
+ /* Use a normal loop if there are no statements that need masking.
+ This only happens in rare degenerate cases: it means that the loop
+ has no loads, no stores, and no live-out values. */
+ if (LOOP_VINFO_MASKS (loop_vinfo).is_empty ())
+ return false;
+
/* Get the maximum number of iterations that is representable
in the counter type. */
tree ni_type = TREE_TYPE (LOOP_VINFO_NITERSM1 (loop_vinfo));
@@ -1739,6 +1745,33 @@ vect_update_vf_for_slp (loop_vec_info lo
}
}
+/* Return true if STMT_INFO describes a double reduction phi and if
+ the other phi in the reduction is also relevant for vectorization.
+ This rejects cases such as:
+
+ outer1:
+ x_1 = PHI <x_3(outer2), ...>;
+ ...
+
+ inner:
+ x_2 = ...;
+ ...
+
+ outer2:
+ x_3 = PHI <x_2(inner)>;
+
+ if nothing in x_2 or elsewhere makes x_1 relevant. */
+
+static bool
+vect_active_double_reduction_p (stmt_vec_info stmt_info)
+{
+ if (STMT_VINFO_DEF_TYPE (stmt_info) != vect_double_reduction_def)
+ return false;
+
+ gimple *other_phi = STMT_VINFO_REDUC_DEF (stmt_info);
+ return STMT_VINFO_RELEVANT_P (vinfo_for_stmt (other_phi));
+}
+
/* Function vect_analyze_loop_operations.
Scan the loop stmts and make sure they are all vectorizable. */
@@ -1786,8 +1819,7 @@ vect_analyze_loop_operations (loop_vec_i
i.e., this phi is vect_reduction_def), cause this case
requires to actually do something here. */
if (STMT_VINFO_LIVE_P (stmt_info)
- && STMT_VINFO_DEF_TYPE (stmt_info)
- != vect_double_reduction_def)
+ && !vect_active_double_reduction_p (stmt_info))
{
if (dump_enabled_p ())
dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
Index: gcc/testsuite/gcc.dg/pr83922.c
===================================================================
--- /dev/null 2018-01-19 09:30:49.543814408 +0000
+++ gcc/testsuite/gcc.dg/pr83922.c 2018-01-19 09:52:00.680331041 +0000
@@ -0,0 +1,21 @@
+/* { dg-options "-O -ftree-vectorize" } */
+
+int j4;
+
+void
+k1 (int ak)
+{
+ while (ak < 1)
+ {
+ int ur;
+
+ for (ur = 0; ur < 2; ++ur)
+ {
+ ++j4;
+ if (j4 != 0)
+ j4 = 0;
+ }
+
+ ++ak;
+ }
+}
