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  <[email protected]>

	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

@@ -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

@@ -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