On Tue, 23 Feb 2021, Jakub Jelinek wrote:
> 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?
OK.
> 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
>
>
--
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