fold-const: Fix ICE in fold_read_from_constant_string on invalid code [PR99204]

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.

2021-02-23  Jakub Jelinek  <jakub@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.

diff --git a/gcc/fold-const.c b/gcc/fold-const.c
index c35f987..afc3b7f 100644 (file)
--- a/gcc/fold-const.c
+++ b/gcc/fold-const.c
@@ -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)

diff --git a/gcc/testsuite/gfortran.dg/pr99204.f90 b/gcc/testsuite/gfortran.dg/pr99204.f90
new file mode 100644 (file)
index 0000000..9b6f27f
--- /dev/null
+++ b/gcc/testsuite/gfortran.dg/pr99204.f90
@@ -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