On 1/18/22 05:17, Jakub Jelinek wrote:
On Thu, Jan 13, 2022 at 04:18:33PM -0500, Jason Merrill wrote:
Note, address_compare has some special cases, e.g. it assumes that
static vars are never adjacent to automatic vars, which is the case
for the usual layout where automatic vars are on the stack and after
.rodata/.data sections there is heap:
/* Assume that automatic variables can't be adjacent to global
variables. */
else if (is_global_var (base0) != is_global_var (base1))
;
Is it ok that during constant evaluation we don't treat those as undefined
behavior, or shall that be with !folding_initializer && too?
I guess that's undefined as well.
Ok, following patch seems to guard that too
Another special case is:
if ((DECL_P (base0) && TREE_CODE (base1) == STRING_CST)
|| (TREE_CODE (base0) == STRING_CST && DECL_P (base1))
|| (TREE_CODE (base0) == STRING_CST
&& TREE_CODE (base1) == STRING_CST
&& ioff0 >= 0 && ioff1 >= 0
&& ioff0 < TREE_STRING_LENGTH (base0)
&& ioff1 < TREE_STRING_LENGTH (base1)
/* This is a too conservative test that the STRING_CSTs
will not end up being string-merged. */
&& strncmp (TREE_STRING_POINTER (base0) + ioff0,
TREE_STRING_POINTER (base1) + ioff1,
MIN (TREE_STRING_LENGTH (base0) - ioff0,
TREE_STRING_LENGTH (base1) - ioff1)) != 0))
;
else if (!DECL_P (base0) || !DECL_P (base1))
return 2;
Here we similarly assume that vars aren't adjacent to string literals
or vice versa. Do we need to stick !folding_initializer && to those
DECL_P vs. STRING_CST cases?
Seems so.
and this too
Though, because of the return 2; for
non-DECL_P that would mean rejecting comparisons like &var == &"foobar"[3]
etc. which ought to be fine, no? So perhaps we need to watch for
decls. vs. STRING_CSTs like for DECLs whether the address is at the start
or at the end of the string literal or somewhere in between (at least
for folding_initializer)?
Agreed.
and this as well.
Furthermore I've fixed constexpr-compare2.C by assuming if
folding_initializer that addresses of non-aliased (visibly to the compiler)
FUNCTION_DECLs are different and that functions are non-zero sized for the
purpose of var vs. function comparisons.
And yet another chapter but probably unsolvable is comparison of
string literal addresses. I think pedantically in C++
&"foo"[0] == &"foo"[0] is undefined behavior, different occurences of
the same string literals might still not be merged in some implementations.
I disagree; it's unspecified whether string literals are merged, but I think
the comparison result is well specified depending on that implementation
behavior.
Can you please comment on https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86369#c1
then?
Done.
About the rest of the patch, I thought I had seen richi comment on IRC
(but can't find the comment now) that these cases where we could give a
constant answer but decide not to because of C++ rules should be handled
in the front end rather than generic code, which makes sense to me; that
is, use folding_initializer only for giving more constant results, not
for giving fewer constant results. Maybe add another flag for limiting
constant results if we think it's significantly easier to recognize
these cases in fold?
Anyway, the following has been successfully bootstrapped/regtested on
x86_64-linux and i686-linux, ok for trunk?
2022-01-18 Jakub Jelinek <ja...@redhat.com>
PR c++/89074
PR c++/104033
* fold-const.c (address_compare): Restrict the decl vs. STRING_CST
or vice versa or STRING_CST vs. STRING_CST or
is_global_var != is_global_var optimizations to !folding_initializer.
Punt for FUNCTION_DECLs with non-zero offsets. If folding_initializer,
assume non-aliased functions have non-zero size and have different
addresses. For folding_initializer, punt on comparisons of start
of some object and end of another one, regardless whether it is a decl
or string literal. Also punt for folding_initializer of
STRING_CST vs. STRING_CST comparisons if the two literals could be
overlapping.
* g++.dg/cpp1y/constexpr-89074-3.C: New test.
--- gcc/fold-const.c.jj 2022-01-17 14:19:08.817376382 +0100
+++ gcc/fold-const.c 2022-01-17 15:50:16.687211071 +0100
@@ -16608,21 +16608,27 @@ address_compare (tree_code code, tree ty
HOST_WIDE_INT ioff0 = -1, ioff1 = -1;
off0.is_constant (&ioff0);
off1.is_constant (&ioff1);
- if ((DECL_P (base0) && TREE_CODE (base1) == STRING_CST)
- || (TREE_CODE (base0) == STRING_CST && DECL_P (base1))
- || (TREE_CODE (base0) == STRING_CST
- && TREE_CODE (base1) == STRING_CST
- && ioff0 >= 0 && ioff1 >= 0
- && ioff0 < TREE_STRING_LENGTH (base0)
- && ioff1 < TREE_STRING_LENGTH (base1)
- /* This is a too conservative test that the STRING_CSTs
- will not end up being string-merged. */
- && strncmp (TREE_STRING_POINTER (base0) + ioff0,
- TREE_STRING_POINTER (base1) + ioff1,
- MIN (TREE_STRING_LENGTH (base0) - ioff0,
- TREE_STRING_LENGTH (base1) - ioff1)) != 0))
+ if (!folding_initializer
+ && ((DECL_P (base0) && TREE_CODE (base1) == STRING_CST)
+ || (TREE_CODE (base0) == STRING_CST && DECL_P (base1))
+ || (TREE_CODE (base0) == STRING_CST
+ && TREE_CODE (base1) == STRING_CST
+ && ioff0 >= 0 && ioff1 >= 0
+ && ioff0 < TREE_STRING_LENGTH (base0)
+ && ioff1 < TREE_STRING_LENGTH (base1)
+ /* This is a too conservative test that the STRING_CSTs
+ will not end up being string-merged. */
+ && strncmp (TREE_STRING_POINTER (base0) + ioff0,
+ TREE_STRING_POINTER (base1) + ioff1,
+ MIN (TREE_STRING_LENGTH (base0) - ioff0,
+ TREE_STRING_LENGTH (base1) - ioff1)) != 0)))
;
- else if (!DECL_P (base0) || !DECL_P (base1))
+ /* Punt on non-zero offsets from functions. */
+ else if ((TREE_CODE (base0) == FUNCTION_DECL && ioff0)
+ || (TREE_CODE (base1) == FUNCTION_DECL && ioff1))
+ return 2;
+ else if ((!DECL_P (base0) && TREE_CODE (base0) != STRING_CST)
+ || (!DECL_P (base1) && TREE_CODE (base1) != STRING_CST))
return 2;
/* If this is a pointer comparison, ignore for now even
valid equalities where one pointer is the offset zero
@@ -16631,18 +16637,62 @@ address_compare (tree_code code, tree ty
;
/* Assume that automatic variables can't be adjacent to global
variables. */
- else if (is_global_var (base0) != is_global_var (base1))
+ else if (!folding_initializer
+ && is_global_var (base0) != is_global_var (base1))
+ ;
+ /* For initializers, assume addresses of different functions are
+ different. */
+ else if (folding_initializer
+ && TREE_CODE (base0) == FUNCTION_DECL
+ && TREE_CODE (base1) == FUNCTION_DECL)
;
else
{
- tree sz0 = DECL_SIZE_UNIT (base0);
- tree sz1 = DECL_SIZE_UNIT (base1);
- /* If sizes are unknown, e.g. VLA or not representable, punt. */
- if (!tree_fits_poly_int64_p (sz0) || !tree_fits_poly_int64_p (sz1))
- return 2;
+ poly_int64 size0, size1;
+ if (TREE_CODE (base0) == STRING_CST)
+ {
+ if (!folding_initializer
+ || ioff0 < 0
+ || ioff0 > TREE_STRING_LENGTH (base0))
+ return 2;
+ size0 = TREE_STRING_LENGTH (base0);
+ }
+ /* For initializers, assume function decls don't overlap and have
+ non-empty size. */
+ else if (folding_initializer && TREE_CODE (base0) == FUNCTION_DECL)
+ size0 = 1;
+ else
+ {
+ tree sz0 = DECL_SIZE_UNIT (base0);
+ /* If sizes are unknown, e.g. VLA or not representable, punt. */
+ if (!tree_fits_poly_int64_p (sz0))
+ return 2;
+
+ size0 = tree_to_poly_int64 (sz0);
+ }
+
+ if (TREE_CODE (base1) == STRING_CST)
+ {
+ if (!folding_initializer
+ || ioff1 < 0
+ || ioff1 > TREE_STRING_LENGTH (base1))
+ return 2;
+ size1 = TREE_STRING_LENGTH (base1);
+ }
+ /* For initializers, assume function decls don't overlap and have
+ non-empty size. */
+ else if (folding_initializer && TREE_CODE (base1) == FUNCTION_DECL)
+ size1 = 1;
+ else
+ {
+ tree sz1 = DECL_SIZE_UNIT (base1);
+ /* If sizes are unknown, e.g. VLA or not representable, punt. */
+ if (!tree_fits_poly_int64_p (sz1))
+ return 2;
+
+ size1 = tree_to_poly_int64 (sz1);
+ }
- poly_int64 size0 = tree_to_poly_int64 (sz0);
- poly_int64 size1 = tree_to_poly_int64 (sz1);
/* If one offset is pointing (or could be) to the beginning of one
object and the other is pointing to one past the last byte of the
other object, punt. */
@@ -16658,6 +16708,27 @@ address_compare (tree_code code, tree ty
&& (known_ne (off0, 0)
|| (known_ne (size0, 0) && known_ne (size1, 0))))
equal = 0;
+ if (equal == 0
+ && TREE_CODE (base0) == STRING_CST
+ && TREE_CODE (base1) == STRING_CST)
+ {
+ /* If the bytes in the string literals starting at the pointers
+ differ, the pointers need to be different. */
+ if (memcmp (TREE_STRING_POINTER (base0) + ioff0,
+ TREE_STRING_POINTER (base1) + ioff1,
+ MIN (TREE_STRING_LENGTH (base0) - ioff0,
+ TREE_STRING_LENGTH (base1) - ioff1)) == 0)
+ {
+ HOST_WIDE_INT ioffmin = MIN (ioff0, ioff1);
+ if (memcmp (TREE_STRING_POINTER (base0) + ioff0 - ioffmin,
+ TREE_STRING_POINTER (base1) + ioff1 - ioffmin,
+ ioffmin) == 0)
+ /* If even the bytes in the string literal before the
+ pointers are the same, the string literals could be
+ tail merged. */
+ return 2;
+ }
+ }
}
return equal;
}
--- gcc/testsuite/g++.dg/cpp1y/constexpr-89074-3.C.jj 2022-01-17
15:22:43.743566175 +0100
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-89074-3.C 2022-01-17
16:10:19.182230570 +0100
@@ -0,0 +1,132 @@
+// PR c++/89074
+// { dg-do compile { target c++14 } }
+
+int fn1 (void) { return 0; }
+int fn2 (void) { return 1; }
+
+constexpr bool
+f1 ()
+{
+ char a[] = { 1, 2, 3, 4 };
+
+ if (&a[1] == "foo")
+ return false;
+
+ if (&a[1] == &"foo"[4])
+ return false;
+
+ if (&"foo"[1] == &a[0])
+ return false;
+
+ if (&"foo"[3] == &a[4])
+ return false;
+
+ if (&a[0] == "foo")
+ return false;
+
+ // Pointer to start of one object (var) and end of another one (literal)
+ if (&a[0] == &"foo"[4]) // { dg-error "is not a constant expression" }
+ return false;
+
+ return true;
+}
+
+constexpr bool
+f2 ()
+{
+ char a[] = { 1, 2, 3, 4 };
+
+ // Pointer to end of one object (var) and start of another one (literal)
+ if (&a[4] == "foo") // { dg-error "is not a constant expression" }
+ return false;
+
+ return true;
+}
+
+char v[] = { 1, 2, 3, 4 };
+
+constexpr bool
+f3 ()
+{
+ char a[] = { 1, 2, 3, 4 };
+
+ if (&a[1] == &v[1])
+ return false;
+
+ if (&a[0] == &v[3])
+ return false;
+
+ if (&a[2] == &v[4])
+ return false;
+
+ // Pointer to start of one object (automatic var) and end of another one
(non-automagic var)
+ if (&a[0] == &v[4]) // { dg-error "is not a constant expression" }
+ return false;
+
+ return true;
+}
+
+constexpr bool
+f4 ()
+{
+ char a[] = { 1, 2, 3, 4, 5 };
+
+ // Pointer to end of one object (automatic var) and start of another one
(non-automagic var)
+ if (&a[5] == &v[0]) // { dg-error "is not a constant expression" }
+ return false;
+
+ return true;
+}
+
+constexpr bool
+f5 ()
+{
+ if (fn1 != fn1)
+ return false;
+
+ if (fn1 == fn2)
+ return false;
+
+ if (&"abcde"[0] == &"edcba"[1])
+ return false;
+
+ if (&"abcde"[1] == &"edcba"[6])
+ return false;
+
+ // Pointer to start of one object (literal) and end of another one (literal)
+ if (&"abcde"[0] == &"edcba"[6]) // { dg-error "is not a constant
expression" }
+ return false;
+
+ return true;
+}
+
+constexpr bool
+f6 ()
+{
+ // Pointer to start of one object (literal) and end of another one (literal)
+ if (&"abcde"[6] == &"edcba"[0]) // { dg-error "is not a constant
expression" }
+ return false;
+
+ return true;
+}
+
+constexpr bool
+f7 ()
+{
+ if (&"abcde"[3] == &"fabcde"[3])
+ return false;
+
+ // These could be suffix merged, with &"abcde"[0] == &"fabcde"[1].
+ if (&"abcde"[3] == &"fabcde"[4]) // { dg-error "is not a constant
expression" }
+ return false;
+
+ return true;
+}
+
+constexpr bool a = f1 ();
+constexpr bool b = f2 ();
+constexpr bool c = f3 ();
+constexpr bool d = f4 ();
+constexpr bool e = f5 ();
+constexpr bool f = f6 ();
+constexpr bool g = f7 ();
Jakub
