Hi all,
The attached patch avoids the bogus warning. I completed regression testing on
x86_64.
I would appreciate others to take a look.
OK to commit on mainline?
Regards,
Jerry
---
fortran: Avoid bogus do-subscript warnings in skipped inner
loops [PR94978]
Do not warn from an outer-loop bound substitution when a nested inner loop may
be zero-trip after that substitution, because the guarded array reference may
then be unreachable.
gcc/fortran/ChangeLog:
PR fortran/94978
* frontend-passes.cc (evaluate_loop_bound): New helper.
(inner_loop_may_be_skipped): New helper.
(do_subscript): Skip outer-loop bound warnings when nested inner loops
may be zero-trip for the substituted bound.
gcc/testsuite/ChangeLog:
PR fortran/94978
* gfortran.dg/pr94978.f90: New test.
Signed-off-by: Christopher Albert <[email protected]>
From f87d2443e27cafde0de1923cd186be92f9cef2a0 Mon Sep 17 00:00:00 2001
From: Christopher Albert <[email protected]>
Date: Sat, 28 Mar 2026 17:26:05 +0100
Subject: [PATCH] fortran: Avoid bogus do-subscript warnings in skipped inner
loops [PR94978]
Do not warn from an outer-loop bound substitution when a nested inner loop may
be zero-trip after that substitution, because the guarded array reference may
then be unreachable.
gcc/fortran/ChangeLog:
PR fortran/94978
* frontend-passes.cc (evaluate_loop_bound): New helper.
(inner_loop_may_be_skipped): New helper.
(do_subscript): Skip outer-loop bound warnings when nested inner loops
may be zero-trip for the substituted bound.
gcc/testsuite/ChangeLog:
PR fortran/94978
* gfortran.dg/pr94978.f90: New test.
Signed-off-by: Christopher Albert <[email protected]>
---
gcc/fortran/frontend-passes.cc | 69 ++++++++++++++++++++++++++-
gcc/testsuite/gfortran.dg/pr94978.f90 | 24 ++++++++++
2 files changed, 91 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/gfortran.dg/pr94978.f90
diff --git a/gcc/fortran/frontend-passes.cc b/gcc/fortran/frontend-passes.cc
index 56d8e21b9f4..fa508794e20 100644
--- a/gcc/fortran/frontend-passes.cc
+++ b/gcc/fortran/frontend-passes.cc
@@ -2746,6 +2746,66 @@ insert_index (gfc_expr *e, gfc_symbol *sym, mpz_t val, mpz_t ret)
}
+static bool
+evaluate_loop_bound (gfc_expr *e, gfc_symbol *sym, mpz_t val, mpz_t ret)
+{
+ if (e->expr_type == EXPR_CONSTANT)
+ {
+ mpz_init_set (ret, e->value.integer);
+ return true;
+ }
+
+ return insert_index (e, sym, val, ret);
+}
+
+/* Return true if any loop nested inside LOOP_INDEX is not provably entered
+ after substituting OUTER_VAL for OUTER_SYM. In that case the guarded array
+ reference may never be evaluated, so do not warn from the outer loop alone. */
+
+static bool
+inner_loop_may_be_skipped (int loop_index, gfc_symbol *outer_sym, mpz_t outer_val)
+{
+ int k;
+ do_t *lp;
+
+ FOR_EACH_VEC_ELT_FROM (doloop_list, k, lp, loop_index + 1)
+ {
+ gfc_code *loop = lp->c;
+ int sgn, cmp;
+ mpz_t do_start, do_end, do_step;
+
+ if (loop == NULL || loop->ext.iterator == NULL || loop->ext.iterator->var == NULL)
+ return true;
+
+ if (!evaluate_loop_bound (loop->ext.iterator->step, outer_sym, outer_val, do_step))
+ return true;
+
+ sgn = mpz_cmp_ui (do_step, 0);
+ if (sgn == 0)
+ {
+ mpz_clear (do_step);
+ return true;
+ }
+
+ if (!evaluate_loop_bound (loop->ext.iterator->start, outer_sym, outer_val, do_start)
+ || !evaluate_loop_bound (loop->ext.iterator->end, outer_sym, outer_val, do_end))
+ {
+ mpz_clear (do_step);
+ return true;
+ }
+
+ cmp = mpz_cmp (do_end, do_start);
+ mpz_clear (do_start);
+ mpz_clear (do_end);
+ mpz_clear (do_step);
+
+ if ((sgn > 0 && cmp < 0) || (sgn < 0 && cmp > 0))
+ return true;
+ }
+
+ return false;
+}
+
/* Check array subscripts for possible out-of-bounds accesses in DO
loops with constant bounds. */
@@ -2880,10 +2940,15 @@ do_subscript (gfc_expr **e)
mpz_clear (rem);
}
+ bool skip_start = have_do_start
+ && inner_loop_may_be_skipped (j, do_sym, do_start);
+ bool skip_end = have_do_end
+ && inner_loop_may_be_skipped (j, do_sym, do_end);
+
for (i = 0; i< ar->dimen; i++)
{
mpz_t val;
- if (ar->dimen_type[i] == DIMEN_ELEMENT && have_do_start
+ if (ar->dimen_type[i] == DIMEN_ELEMENT && have_do_start && !skip_start
&& insert_index (ar->start[i], do_sym, do_start, val))
{
if (ar->as->lower[i]
@@ -2909,7 +2974,7 @@ do_subscript (gfc_expr **e)
mpz_clear (val);
}
- if (ar->dimen_type[i] == DIMEN_ELEMENT && have_do_end
+ if (ar->dimen_type[i] == DIMEN_ELEMENT && have_do_end && !skip_end
&& insert_index (ar->start[i], do_sym, do_end, val))
{
if (ar->as->lower[i]
diff --git a/gcc/testsuite/gfortran.dg/pr94978.f90 b/gcc/testsuite/gfortran.dg/pr94978.f90
new file mode 100644
index 00000000000..5d64d227fdc
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr94978.f90
@@ -0,0 +1,24 @@
+! { dg-do compile }
+! { dg-options "-Wdo-subscript" }
+
+implicit none
+
+integer(4) :: i, j
+integer(4) :: pascal(0:8, 0:8)
+
+do i = 0, 8
+ pascal(i, 0) = 1
+ do j = 1, i - 1
+ pascal(i, j) = pascal(i - 1, j) + pascal(i - 1, j - 1) ! { dg-bogus "Array reference at \\(1\\) out of bounds" }
+ end do
+ do j = i, 8
+ pascal(i, j) = 0
+ end do
+ pascal(i, i) = 1
+end do
+
+do i = 0, 8
+ print '(9I4)', pascal(i, :)
+end do
+
+end
--
2.53.0
