SCEV has a long-standing issue in that it negates the addend of
a subtraction within a SSA cycle without considering UB from
negating of the most negative value. The following tries to
rectify this.
Bootstrapped and tested on x86_64-unknown-linux-gnu, surprisingly
with no fallout.
We do not really have a better way to address these wrong-code
issues, thus, pushed to trunk (and very unlikely to be backported).
Richard.
PR tree-optimization/124528
* tree-scalar-evolution.cc (scev_dfs::add_to_evolution):
Perform the negation in an unsigned type if we cannot make
sure it will not cause UB.
* gcc.dg/tree-ssa/scev-17.c: New testcase.
---
gcc/testsuite/gcc.dg/tree-ssa/scev-17.c | 15 +++++++++++++++
gcc/tree-scalar-evolution.cc | 20 +++++++++++++++++---
2 files changed, 32 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/scev-17.c
diff --git a/gcc/testsuite/gcc.dg/tree-ssa/scev-17.c
b/gcc/testsuite/gcc.dg/tree-ssa/scev-17.c
new file mode 100644
index 00000000000..4751a2b91d4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/scev-17.c
@@ -0,0 +1,15 @@
+/* PR124528 */
+/* { dg-options "-Os -fdump-tree-sccp" } */
+
+int baz (int n, int m)
+{
+ int r = 100;
+ while (n != 0) {
+ n = n - 1;
+ r = r - m;
+ }
+ return r;
+}
+
+/* Make sure we negate m in unsigned. */
+/* { dg-final { scan-tree-dump "\\\(unsigned int\\\) m_.\\\(D\\\)" "sccp" } }
*/
diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
index 223015c4a8d..e05a171c1b1 100644
--- a/gcc/tree-scalar-evolution.cc
+++ b/gcc/tree-scalar-evolution.cc
@@ -880,9 +880,23 @@ scev_dfs::add_to_evolution (tree chrec_before, enum
tree_code code,
}
if (code == MINUS_EXPR)
- to_add = chrec_fold_multiply (type, to_add, SCALAR_FLOAT_TYPE_P (type)
- ? build_real (type, dconstm1)
- : build_int_cst_type (type, -1));
+ {
+ if (INTEGRAL_TYPE_P (type)
+ && TYPE_OVERFLOW_UNDEFINED (type)
+ && !expr_not_equal_to (to_add,
+ wi::to_wide (TYPE_MIN_VALUE (type))))
+ {
+ tree utype = unsigned_type_for (type);
+ to_add = chrec_convert_rhs (utype, to_add);
+ to_add = chrec_fold_multiply (utype, to_add,
+ build_int_cst_type (utype, -1));
+ to_add = chrec_convert_rhs (type, to_add);
+ }
+ else
+ to_add = chrec_fold_multiply (type, to_add, SCALAR_FLOAT_TYPE_P (type)
+ ? build_real (type, dconstm1)
+ : build_int_cst_type (type, -1));
+ }
res = add_to_evolution_1 (chrec_before, to_add, at_stmt);
--
2.51.0
