Hi!
The following testcase ICEs (on ARM/RISCV with certain options), because niters
analysis
computes number of latch executions for the loop as
(short unsigned int) (a.0_1 + 255) + 1 > 256 ? ~(short unsigned int) (a.0_1 +
255) : 0
where a.0_1 is unsigned char. This is correct, but given that a.0_1 + 255
is done in unsigned char the condition is never true and so it is actually
equivalent to 0, but the folders don't know that.
The vectorizer sets LOOP_VINFO_NITERSM1 to that expression and does on with
computing LOOP_VINFO_NITERS by fold_build2 PLUS_EXPR of that expression
unshared and INTEGER_CST one. In that folding we trigger various
optimizations, first it is correctly simplified into
(short unsigned int) (a.0_1 + 255) + 1 > 256 ? -(short unsigned int) (a.0_1 +
255) : 1
and next using
/* (X + 1) > Y ? -X : 1 simplifies to X >= Y ? -X : 1 when
X is unsigned, as when X + 1 overflows, X is -1, so -X == 1. */
into
(short unsigned int) (a.0_1 + 255) >= 256 ? -(short unsigned int) (a.0_1 + 255)
: 1
and for this the first COND_EXPR argument is folded and figured out to be 0
and so while LOOP_VINFO_NITERSM1 is a complex expression (unknown to be
equivalent to 0), LOOP_VINFO_NITERS is INTEGER_CST 1.
vect_analyze_loop_costing then uses LOOP_VINFO_NITERS_KNOWN_P (which checks
if LOOP_VINFO_NITERS is INTEGER_CST which fits into shwi or something like
that) and from that assumes that LOOP_VINFO_NITERSM1 will be INTEGER_CST.
The following patch fixes that by adding verification for that too.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2024-01-06 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/113210
* tree-vect-loop.cc (vect_analyze_loop_costing): If LOOP_VINFO_NITERSM1
is not INTEGER_CST, don't try to use it.
* gcc.c-torture/compile/pr113210.c: New test.
--- gcc/tree-vect-loop.cc.jj 2024-01-03 11:51:22.787852547 +0100
+++ gcc/tree-vect-loop.cc 2024-01-05 17:12:06.511512557 +0100
@@ -2264,7 +2264,8 @@ vect_analyze_loop_costing (loop_vec_info
epilogue we can also decide whether the main loop leaves us
with enough iterations, prefering a smaller vector epilog then
also possibly used for the case we skip the vector loop. */
- if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo))
+ if (LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
+ && TREE_CODE (LOOP_VINFO_NITERSM1 (loop_vinfo)) == INTEGER_CST)
{
widest_int scalar_niters
= wi::to_widest (LOOP_VINFO_NITERSM1 (loop_vinfo)) + 1;
--- gcc/testsuite/gcc.c-torture/compile/pr113210.c.jj 2024-01-05
17:18:29.792257043 +0100
+++ gcc/testsuite/gcc.c-torture/compile/pr113210.c 2024-01-05
17:17:57.522699521 +0100
@@ -0,0 +1,13 @@
+/* PR tree-optimization/113210 */
+
+unsigned char a, c;
+unsigned short b;
+
+void
+foo (void)
+{
+ c = a + 255;
+ b = c;
+ while (++b > 256)
+ ;
+}
Jakub
