https://gcc.gnu.org/g:005358093fd8566824b088d9bf7c8512ef5844fa
commit r15-11154-g005358093fd8566824b088d9bf7c8512ef5844fa Author: Christopher Albert <[email protected]> Date: Sat Mar 28 17:26:05 2026 +0100 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]> (cherry picked from commit 790671b708400d1fc6bb1abbf1601f3616e8220d) Diff: --- gcc/fortran/frontend-passes.cc | 69 ++++++++++++++++++++++++++++++++++- gcc/testsuite/gfortran.dg/pr94978.f90 | 24 ++++++++++++ 2 files changed, 91 insertions(+), 2 deletions(-) diff --git a/gcc/fortran/frontend-passes.cc b/gcc/fortran/frontend-passes.cc index ef9c80147cc4..496d17352080 100644 --- a/gcc/fortran/frontend-passes.cc +++ b/gcc/fortran/frontend-passes.cc @@ -2745,6 +2745,66 @@ 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. */ @@ -2879,10 +2939,15 @@ 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] @@ -2908,7 +2973,7 @@ 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 000000000000..5d64d227fdc0 --- /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
