Hi!
The match.pd framework uses op0 == op1 || operand_equal_p (op0, op1, 0)
style comparisons heavily; the reason why we don't optimize away say
volatile int i;
...
int j = i - i;
into int j = (i, 0);
is only because both the C and C++ FE convert the volatile arguments to
non-volatile ones before the arithmetic operation (e.g. c_common_type
returns a non-qualified type). Unfortunately, in C FE this doesn't happen
for POINTER_DIFF_EXPR, where no conversion happens. The following patch
makes sure we convert the pointer arguments to some non-qualified pointers
first.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk/8.2?
2018-06-15 Jakub Jelinek <ja...@redhat.com>
PR c/86093
* c-typeck.c (pointer_diff): Cast both pointers to unqualified types
before doing POINTER_DIFF_EXPR.
* c-c++-common/pr86093.c: New test.
--- gcc/c/c-typeck.c.jj 2018-05-31 20:53:32.003439351 +0200
+++ gcc/c/c-typeck.c 2018-06-15 15:35:44.185308552 +0200
@@ -3840,7 +3840,12 @@ pointer_diff (location_t loc, tree op0,
op0 = build_binary_op (loc, MINUS_EXPR, convert (inttype, op0),
convert (inttype, op1), false);
else
- op0 = build2_loc (loc, POINTER_DIFF_EXPR, inttype, op0, op1);
+ {
+ /* Cast away qualifiers. */
+ op0 = convert (c_common_type (TREE_TYPE (op0), TREE_TYPE (op0)), op0);
+ op1 = convert (c_common_type (TREE_TYPE (op1), TREE_TYPE (op1)), op1);
+ op0 = build2_loc (loc, POINTER_DIFF_EXPR, inttype, op0, op1);
+ }
/* This generates an error if op1 is pointer to incomplete type. */
if (!COMPLETE_OR_VOID_TYPE_P (TREE_TYPE (TREE_TYPE (orig_op1))))
--- gcc/testsuite/c-c++-common/pr86093.c.jj 2018-06-15 15:51:40.344192815
+0200
+++ gcc/testsuite/c-c++-common/pr86093.c 2018-06-15 15:51:23.340177502
+0200
@@ -0,0 +1,12 @@
+/* PR c/86093 */
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+/* { dg-final { scan-tree-dump-not "return 0;" "optimized" } } */
+
+char *volatile p;
+
+__PTRDIFF_TYPE__
+foo (void)
+{
+ return p - p;
+}
Jakub
