On 2/4/22 08:41, Jakub Jelinek wrote:
On Thu, Feb 03, 2022 at 04:34:17PM -0500, Jason Merrill wrote:
On 2/3/22 16:18, Jakub Jelinek wrote:
On Thu, Feb 03, 2022 at 04:04:57PM -0500, Jason Merrill wrote:
I think it would be clearer to leave the !DECL_P case alone and add
/* In C++ it is unspecified, and so non-constant, whether two
equivalent strings have the same address. */
else if (folding_cxx_constexpr
&& (TREE_CODE (base0) == STRING_CST
|| TREE_CODE (base1) == STRING_CST)
The point was to let the first if handle for
!folding_cxx_constexpr the cases with STRING_CST
as one or both operands and if that falls through, return 2.
Ah, I see. And then for folding_cxx_constexpr you have your new code toward
the bottom of the function that can say they're unequal in some cases. Can
you combine the STRING_CST handling for both values of folding_cxx_constexpr
instead of having them so far apart?
Not easily, because for the folding_cxx_constexpr case it primarily reuses
the code from the last else if - computing sizes of objects and checking
if one is at a start of one and another at the end of the other.
And the !folding_cxx_constexpr case shouldn't also use that code?
One further option would be to compute early flags like
enum { OFF_POS_START, OFF_POS_MIDDLE, OFF_POS_END } pos0, pos1;
and then just use them or ignore them in the decisions later.
If that helps to refactor a bit, sure.
Here it is, hopefully it makes the code more readable and understandable.
Much more readable, thanks!
Bootstrapped/regtested on powerpc64le-linux, ok for trunk?
2022-02-04 Jakub Jelinek <ja...@redhat.com>
PR c++/89074
PR c++/104033
* fold-const.h (folding_initializer): Adjust comment.
(folding_cxx_constexpr): Declare.
* fold-const.cc (folding_initializer): Adjust comment.
(folding_cxx_constexpr): New variable.
(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_cxx_constexpr.
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_cxx_constexpr, 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_cxx_constexpr on
STRING_CST vs. STRING_CST comparisons if the two literals could be
overlapping.
* constexpr.cc (cxx_eval_binary_expression): Temporarily set
folding_cxx_constexpr.
* g++.dg/cpp1y/constexpr-89074-3.C: New test.
--- gcc/fold-const.h.jj 2022-02-01 20:10:51.235856007 +0100
+++ gcc/fold-const.h 2022-02-03 15:02:02.700228631 +0100
@@ -20,9 +20,16 @@ along with GCC; see the file COPYING3.
#ifndef GCC_FOLD_CONST_H
#define GCC_FOLD_CONST_H
-/* Non-zero if we are folding constants inside an initializer; zero
- otherwise. */
+/* Nonzero if we are folding constants inside an initializer or a C++
+ manifestly-constant-evaluated context; zero otherwise.
+ Should be used when folding in initializer enables additional
+ optimizations. */
extern int folding_initializer;
+/* Nonzer of we are folding C++ manifestly-constant-evaluated context; zero
Still need to fix this typo.
+ otherwise.
+ Should be used when certain constructs shouldn't be optimized
+ during folding in that context. */
+extern bool folding_cxx_constexpr;
/* Convert between trees and native memory representation. */
extern int native_encode_expr (const_tree, unsigned char *, int, int off =
-1);
--- gcc/fold-const.cc.jj 2022-02-03 14:31:32.243129408 +0100
+++ gcc/fold-const.cc 2022-02-04 10:19:13.812784763 +0100
@@ -86,9 +86,17 @@ along with GCC; see the file COPYING3.
#include "gimple-range.h"
/* Nonzero if we are folding constants inside an initializer or a C++
- manifestly-constant-evaluated context; zero otherwise. */
+ manifestly-constant-evaluated context; zero otherwise.
+ Should be used when folding in initializer enables additional
+ optimizations. */
int folding_initializer = 0;
+/* Nonzer of we are folding C++ manifestly-constant-evaluated context; zero
And here.
+ otherwise.
+ Should be used when certain constructs shouldn't be optimized
+ during folding in that context. */
+bool folding_cxx_constexpr = false;
+
/* The following constants represent a bit based encoding of GCC's
comparison operators. This encoding simplifies transformations
on relational comparison operators, such as AND and OR. */
@@ -16628,41 +16636,55 @@ address_compare (tree_code code, tree ty
Incidentally, the function comment needs to document TYPE.
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))
- ;
- else if (!DECL_P (base0) || !DECL_P (base1))
+ /* Punt on non-zero offsets from functions. */
+ if ((TREE_CODE (base0) == FUNCTION_DECL && ioff0)
+ || (TREE_CODE (base1) == FUNCTION_DECL && ioff1))
return 2;
- /* If this is a pointer comparison, ignore for now even
- valid equalities where one pointer is the offset zero
- of one object and the other to one past end of another one. */
- else if (!folding_initializer && !INTEGRAL_TYPE_P (type))
- ;
- /* Assume that automatic variables can't be adjacent to global
- variables. */
- else if (is_global_var (base0) != is_global_var (base1))
- ;
+ /* Or if the bases are neither decls nor string literals. */
+ if (!DECL_P (base0) && TREE_CODE (base0) != STRING_CST)
+ return 2;
+ if (!DECL_P (base1) && TREE_CODE (base1) != STRING_CST)
+ return 2;
+
+ /* Compute whether one address points to the start of one
+ object and another one to the end of another one. */
+ poly_int64 size0 = 0, size1 = 0;
+ if (TREE_CODE (base0) == STRING_CST)
+ {
+ if (ioff0 < 0 || ioff0 > TREE_STRING_LENGTH (base0))
+ equal = 2;
+ else
+ size0 = TREE_STRING_LENGTH (base0);
+ }
+ else if (TREE_CODE (base0) == FUNCTION_DECL)
+ size0 = 1;
else
{
tree sz0 = DECL_SIZE_UNIT (base0);
+ if (!tree_fits_poly_int64_p (sz0))
+ equal = 2;
+ else
+ size0 = tree_to_poly_int64 (sz0);
+ }
+ if (TREE_CODE (base1) == STRING_CST)
+ {
+ if (ioff1 < 0 || ioff1 > TREE_STRING_LENGTH (base1))
+ equal = 2;
+ else
+ size1 = TREE_STRING_LENGTH (base1);
+ }
+ else if (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 (sz0) || !tree_fits_poly_int64_p (sz1))
- return 2;
-
- poly_int64 size0 = tree_to_poly_int64 (sz0);
- poly_int64 size1 = tree_to_poly_int64 (sz1);
+ if (!tree_fits_poly_int64_p (sz1))
+ equal = 2;
+ else
+ size1 = tree_to_poly_int64 (sz1);
+ }
+ if (equal == 0)
+ {
/* 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. */
@@ -16678,7 +16700,60 @@ address_compare (tree_code code, tree ty
&& (known_ne (off0, 0)
|| (known_ne (size0, 0) && known_ne (size1, 0))))
equal = 0;
- }
+ }
+
So at this point equal can be 0 or 2, the latter because either the
offset is out of bounds, or because we're comparing offset 0 to
one-past-the-end.
In the out-of-bounds case, we're into undefined behavior, so I'd be
inclined to return 2 immediately rather than continue, so the code below
only needs to worry about possibly overlapping/contiguous objects.
In the code below, in !constexpr mode we decide to return 0 even though
equal == 2 in three cases which need more commentary, either together or
separately:
1) One is a string and the other a decl. Do we know that we can't
layout a string and a global variable next to each other? This overlaps
a lot with...
2) We're comparing as pointers (rather than integers), so we return
unequal even if they could be equal in practice if the objects are
contiguous. The comment says this but still needs a rationale; it
doesn't seem useful to me for the limited cases that could reach here
with equal == 2.
3) We're comparing a local variable and a global, so they really can't
be equal unless the offset is far out of bounds. This is currently
last; we might move it first and treat strings like globals?
I know the above is existing behavior, but I'd like to at least improve
the comments.
+ if (TREE_CODE (base0) == STRING_CST || TREE_CODE (base1) == STRING_CST)
+ {
+ if (TREE_CODE (base0) != TREE_CODE (base1))
+ return folding_cxx_constexpr ? equal : 0;
+
+ /* STRING_CST vs. STRING_CST. */
+ if (folding_cxx_constexpr && equal)
+ return equal;
+
+ if (ioff0 < 0
+ || ioff1 < 0
+ || ioff0 > TREE_STRING_LENGTH (base0)
+ || ioff1 > TREE_STRING_LENGTH (base1))
+ return 2;
+
+ /* 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 0;
+ }
+
+ /* If this is a pointer comparison, ignore for now even
+ valid equalities where one pointer is the offset zero
+ of one object and the other to one past end of another one. */
+ if (!folding_cxx_constexpr && !INTEGRAL_TYPE_P (type))
+ return 0;
+
+ /* Assume that automatic variables can't be adjacent to global
+ variables. */
+ if (!folding_cxx_constexpr && is_global_var (base0) != is_global_var (base1))
+ return 0;
+
+ /* For initializers, assume addresses of different functions are
+ different. */
+ else if (folding_initializer
+ && TREE_CODE (base0) == FUNCTION_DECL
+ && TREE_CODE (base1) == FUNCTION_DECL)
+ return 0;
+
return equal;
}
--- gcc/cp/constexpr.cc.jj 2022-01-26 19:40:22.957796808 +0100
+++ gcc/cp/constexpr.cc 2022-02-03 15:04:00.760558570 +0100
@@ -3413,7 +3413,10 @@ cxx_eval_binary_expression (const conste
if (ctx->manifestly_const_eval
&& (flag_constexpr_fp_except
|| TREE_CODE (type) != REAL_TYPE))
- r = fold_binary_initializer_loc (loc, code, type, lhs, rhs);
+ {
+ auto ofcc = make_temp_override (folding_cxx_constexpr, true);
+ r = fold_binary_initializer_loc (loc, code, type, lhs, rhs);
+ }
else
r = fold_binary_loc (loc, code, type, lhs, rhs);
}
--- gcc/testsuite/g++.dg/cpp1y/constexpr-89074-3.C.jj 2022-02-03
14:58:53.734901694 +0100
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-89074-3.C 2022-02-03
14:58:53.734901694 +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
