Hi!
fold_read_from_constant_string and expand_expr_real_1 have code to optimize
constant reads from string (tree vs. rtl).
If the STRING_CST array type has zero low bound, index is fold converted to
sizetype and so the compare_tree_int works fine, but if it has some other
low bound, it calls size_diffop_loc and that function from 2 sizetype
operands creates a ssizetype difference. expand_expr_real_1 then uses
tree_fits_uhwi_p + compare_tree_int and so works fine, but fold-const.c
only checked if index is INTEGER_CST and calls compare_tree_int, which means
for negative index it will succeed and result in UB in the compiler.
This patch just follows what expand_expr_real_1 is doing.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2021-02-22 Jakub Jelinek <ja...@redhat.com>
PR tree-optimization/99204
* fold-const.c (fold_read_from_constant_string): Check that
tree_fits_uhwi_p (index) rather than just that index is INTEGER_CST.
* gfortran.dg/pr99204.f90: New test.
--- gcc/fold-const.c.jj 2021-01-04 10:25:39.031231582 +0100
+++ gcc/fold-const.c 2021-02-22 19:46:57.167342075 +0100
@@ -15433,7 +15433,7 @@ fold_read_from_constant_string (tree exp
if (string
&& TYPE_MODE (TREE_TYPE (exp)) == TYPE_MODE (TREE_TYPE (TREE_TYPE
(string)))
&& TREE_CODE (string) == STRING_CST
- && TREE_CODE (index) == INTEGER_CST
+ && tree_fits_uhwi_p (index)
&& compare_tree_int (index, TREE_STRING_LENGTH (string)) < 0
&& is_int_mode (TYPE_MODE (TREE_TYPE (TREE_TYPE (string))),
&char_mode)
--- gcc/testsuite/gfortran.dg/pr99204.f90.jj 2021-02-22 19:52:55.749431186
+0100
+++ gcc/testsuite/gfortran.dg/pr99204.f90 2021-02-22 19:51:07.410612678
+0100
@@ -0,0 +1,10 @@
+! PR tree-optimization/99204
+! { dg-do compile }
+! { dg-options "-O2 -w" }
+
+program pr99204
+ character :: c
+ integer :: i = -12345678
+ c = 'abc'(i:i)
+ print *, c
+end
Jakub
