On Tue, Jan 18, 2022 at 05:40:48PM +0100, Jakub Jelinek via Gcc-patches wrote:
> > 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?
>
> I'm afraid avoiding the match.pd & fold-const code here would be a lot of
> work.
Sorry for dropping the ball on this.
Here is a patch that introduces folding_cxx_constexpr flag and documents
that folding_initializer should be used for enabling more folding
optimizations, while folding_cxx_constexpr should be used to prevent
optimizations C++ doesn't allow in the constant expression evaluation.
Ok for trunk if it passes bootstrap/regtest?
2022-02-03 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
+ 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 2022-02-03 16:25:27.541467112 +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
+ 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 +16636,91 @@ 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_cxx_constexpr
+ && ((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)
+ && (!folding_cxx_constexpr || TREE_CODE (base0) != STRING_CST))
+ || (!DECL_P (base1)
+ && (!folding_cxx_constexpr || 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
of one object and the other to one past end of another one. */
- else if (!folding_initializer && !INTEGRAL_TYPE_P (type))
+ else if (!folding_cxx_constexpr && !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))
+ else if (!folding_cxx_constexpr
+ && 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 (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 (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. */
@@ -16678,6 +16736,27 @@ 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/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
