On 11/29/21 11:31, Jakub Jelinek wrote:
Hi!
P1272R4 has added to the std::byteswap new stuff to me quite unrelated
clarification for std::bit_cast.
std::bit_cast is already in C++20 though and std::byteswap is a new
C++23 feature, it is unclear to me if that bit_cast addition in there
is really meant for C++23 and later only (e.g. related to only
C++23 actually allowing non-literal vars and thus other spots where
indeterminate values can appear during constant evaluation), or if
it was meant as a DR and should apply to C++20 too.
It's a DR. Really, it was intended to be part of C++20; at the Cologne
meeting in 2019 CWG thought byteswap was going to make C++20, so this
bugfix could go in as part of that paper.
Also, allowing indeterminate values that are never read was in C++20
(P1331).
The following so far only lightly tested patch (dg.exp=bit-cast*.C
testing) implements it as applying to C++23 only.
Also, it is unclear to me how bitfields should behave, whether having
indeterminate bits in
struct S { unsigned char a : 8, b : 5, c : 3, d, e; unsigned int f : 8, g : 24;
};
struct T1 { unsigned char a : 1, : 7, b : 5, c : 3, d, e; unsigned int f : 8, g
: 24; };
struct T2 { unsigned char a : 8, b : 1, : 4, c : 3, d, e; unsigned int f : 8, g
: 24; };
struct T3 { unsigned char a : 8, b : 5, c : 1, : 2, d, e; unsigned int f : 8, g
: 24; };
struct T4 { unsigned char a : 8, b : 5, c : 3, d, e; unsigned int f : 1, : 7, g
: 24; };
constexpr bool f1 () { T1 t = { 0, 0, 0, 0, 0, 0, 0 }; S s = __builtin_bit_cast
(S, t); return true; }
constexpr bool f2 () { T2 t = { 0, 0, 0, 0, 0, 0, 0 }; S s = __builtin_bit_cast
(S, t); return true; }
constexpr bool f3 () { T3 t = { 0, 0, 0, 0, 0, 0, 0 }; S s = __builtin_bit_cast
(S, t); return true; }
constexpr bool f4 () { T4 t = { 0, 0, 0, 0, 0, 0, 0 }; S s = __builtin_bit_cast
(S, t); return true; }
constexpr bool a = f1 ();
constexpr bool b = f2 ();
constexpr bool c = f3 ();
constexpr bool d = f4 ();
The patch currently uses TREE_TYPE and doesn't check DECL_BIT_FIELD,
so it accepts f1 and f4 cases where the TREE_TYPE is unsigned char
and doesn't accept f2 and f3 where the type is some 5 or 3 bit precision
unsigned type. But possibly either it should look at the declared
type of the bitfield and don't allow f4 and allow f1-f3, or don't allow
any of the f1-f4 cases.
I think in all of them the result of the cast has (some) indeterminate
value. So f1-3 are OK because the indeterminate value has unsigned char
type and is never used; f4() is non-constant because S::f has
non-byte-access type and so the new wording says it's undefined.
2021-11-29 Jakub Jelinek <ja...@redhat.com>
* constexpr.c (clear_uchar_or_std_byte_in_mask): New function.
(cxx_eval_bit_cast): Don't error about padding bits if target
type is unsigned char or std::byte, instead return no clearing
ctor. Use clear_uchar_or_std_byte_in_mask.
* g++.dg/cpp2a/bit-cast11.C: New test.
* g++.dg/cpp2a/bit-cast12.C: New test.
* g++.dg/cpp2a/bit-cast13.C: New test.
--- gcc/cp/constexpr.c.jj 2021-11-22 10:13:17.156882931 +0100
+++ gcc/cp/constexpr.c 2021-11-29 15:41:54.611361669 +0100
@@ -4268,6 +4268,59 @@ check_bit_cast_type (const constexpr_ctx
return false;
}
+/* Helper function for cxx_eval_bit_cast. For unsigned char or
+ std::byte members of CONSTRUCTOR (recursively) if they contain
+ some indeterminate bits (as set in MASK), remove the ctor elts,
+ mark the CONSTRUCTOR as CONSTRUCTOR_NO_CLEARING and clear the
+ bits in MASK. */
+
+static void
+clear_uchar_or_std_byte_in_mask (tree t, unsigned char *mask)
+{
+ if (TREE_CODE (t) != CONSTRUCTOR)
+ return;
+
+ unsigned i, j = 0;
+ tree index, value;
+ FOR_EACH_CONSTRUCTOR_ELT (CONSTRUCTOR_ELTS (t), i, index, value)
+ {
+ tree type = TREE_TYPE (value);
+ if (is_byte_access_type (type)
+ && TYPE_MAIN_VARIANT (type) != char_type_node)
+ {
+ HOST_WIDE_INT pos;
+ if (TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
+ pos = tree_to_shwi (index);
+ else
+ pos = int_byte_position (index);
+ if (mask[pos])
+ {
+ CONSTRUCTOR_NO_CLEARING (t) = 1;
+ mask[pos] = 0;
+ continue;
+ }
+ }
+ if (TREE_CODE (value) == CONSTRUCTOR)
+ {
+ HOST_WIDE_INT pos;
+ if (TREE_CODE (TREE_TYPE (t)) == ARRAY_TYPE)
+ pos = tree_to_shwi (index)
+ * tree_to_shwi (TYPE_SIZE_UNIT (TREE_TYPE (TREE_TYPE (t))));
+ else
+ pos = int_byte_position (index);
+ clear_uchar_or_std_byte_in_mask (value, mask + pos);
+ }
+ if (i != j)
+ {
+ CONSTRUCTOR_ELT (t, j)->index = index;
+ CONSTRUCTOR_ELT (t, j)->value = value;
+ }
+ ++j;
+ }
+ if (CONSTRUCTOR_NELTS (t) != j)
+ vec_safe_truncate (CONSTRUCTOR_ELTS (t), j);
+}
+
/* Subroutine of cxx_eval_constant_expression.
Attempt to evaluate a BIT_CAST_EXPR. */
@@ -4344,12 +4397,30 @@ cxx_eval_bit_cast (const constexpr_ctx *
tree r = NULL_TREE;
if (can_native_interpret_type_p (TREE_TYPE (t)))
- r = native_interpret_expr (TREE_TYPE (t), ptr, len);
+ {
+ r = native_interpret_expr (TREE_TYPE (t), ptr, len);
+ if (cxx_dialect >= cxx23
+ && is_byte_access_type (TREE_TYPE (t))
+ && TYPE_MAIN_VARIANT (TREE_TYPE (t)) != char_type_node)
+ {
+ gcc_assert (len == 1);
+ if (mask[0])
+ {
+ memset (mask, 0, len);
+ r = build_constructor (TREE_TYPE (r), NULL);
+ CONSTRUCTOR_NO_CLEARING (r) = 1;
+ }
+ }
+ }
else if (TREE_CODE (TREE_TYPE (t)) == RECORD_TYPE)
{
r = native_interpret_aggregate (TREE_TYPE (t), ptr, 0, len);
if (r != NULL_TREE)
- clear_type_padding_in_mask (TREE_TYPE (t), mask);
+ {
+ clear_type_padding_in_mask (TREE_TYPE (t), mask);
+ if (cxx_dialect >= cxx23)
+ clear_uchar_or_std_byte_in_mask (r, mask);
+ }
}
if (r != NULL_TREE)
--- gcc/testsuite/g++.dg/cpp2a/bit-cast11.C.jj 2021-11-29 16:21:38.470327372
+0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast11.C 2021-11-29 16:23:24.536811602
+0100
@@ -0,0 +1,69 @@
+// P1272R4
+// { dg-do compile { target c++14 } }
+
+struct S { unsigned char a[2]; alignas(sizeof 0) int b; };
+struct T { char a; alignas(sizeof 0) int b; };
+struct U { char a : 1; char : 6; char b : 1; };
+struct V { int a; S b; };
+struct W { unsigned a; T b; };
+
+constexpr bool
+f1 ()
+{
+ T t = { 1, 2 };
+ S s = __builtin_bit_cast (S, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ S u = s;
+ return u.a[0] == 1;
+}
+
+constexpr bool
+f2 ()
+{
+ U u = { 0, 0 };
+ unsigned char a = __builtin_bit_cast (unsigned char, u); // { dg-error "accessing
uninitialized byte" "" { target c++20_down } }
+ unsigned char b = a;
+ return true;
+}
+
+constexpr bool
+f3 ()
+{
+ T t = { 1, 2 };
+ S s = __builtin_bit_cast (S, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ S u = s;
+ return u.a[1] == 0;
+}
+
+constexpr bool
+f4 ()
+{
+ U u = { 0, 0 };
+ unsigned char a = __builtin_bit_cast (unsigned char, u); // { dg-error "accessing
uninitialized byte" "" { target c++20_down } }
+ unsigned char b = a;
+ return b == 0; // { dg-error "is not a constant expression" ""
{ target c++23 } }
+}
+
+constexpr bool
+f5 ()
+{
+ W t = { 1, 2 };
+ V s = __builtin_bit_cast (V, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ V u = s;
+ return u.b.a[0] == 1;
+}
+
+constexpr bool
+f6 ()
+{
+ W t = { 1, 2 };
+ V s = __builtin_bit_cast (V, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ V u = s;
+ return u.b.a[1] == 1;
+}
+
+constexpr bool a = f1 ();
+constexpr bool b = f2 ();
+constexpr bool c = f3 (); // { dg-error "accessing uninitialized array element"
"" { target c++23 } }
+constexpr bool d = f4 ();
+constexpr bool e = f5 ();
+constexpr bool f = f6 (); // { dg-error "accessing uninitialized array element"
"" { target c++23 } }
--- gcc/testsuite/g++.dg/cpp2a/bit-cast12.C.jj 2021-11-29 16:22:25.106660906
+0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast12.C 2021-11-29 16:23:40.012590436
+0100
@@ -0,0 +1,74 @@
+// P1272R4
+// { dg-do compile { target c++14 } }
+
+namespace std
+{
+ enum class byte : unsigned char {};
+}
+
+struct S { unsigned char a[2]; alignas(sizeof 0) int b; };
+struct T { char a; alignas(sizeof 0) int b; };
+struct U { char a : 1; char : 6; char b : 1; };
+struct V { int a; S b; };
+struct W { unsigned a; T b; };
+
+constexpr bool
+f1 ()
+{
+ T t = { 1, 2 };
+ S s = __builtin_bit_cast (S, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ S u = s;
+ return u.a[0] == 1;
+}
+
+constexpr bool
+f2 ()
+{
+ U u = { 0, 0 };
+ unsigned char a = __builtin_bit_cast (unsigned char, u); // { dg-error "accessing
uninitialized byte" "" { target c++20_down } }
+ unsigned char b = a;
+ return true;
+}
+
+constexpr bool
+f3 ()
+{
+ T t = { 1, 2 };
+ S s = __builtin_bit_cast (S, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ S u = s;
+ return u.a[1] == 0;
+}
+
+constexpr bool
+f4 ()
+{
+ U u = { 0, 0 };
+ unsigned char a = __builtin_bit_cast (unsigned char, u); // { dg-error "accessing
uninitialized byte" "" { target c++20_down } }
+ unsigned char b = a;
+ return b == 0; // { dg-error "is not a constant expression" ""
{ target c++23 } }
+}
+
+constexpr bool
+f5 ()
+{
+ W t = { 1, 2 };
+ V s = __builtin_bit_cast (V, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ V u = s;
+ return u.b.a[0] == 1;
+}
+
+constexpr bool
+f6 ()
+{
+ W t = { 1, 2 };
+ V s = __builtin_bit_cast (V, t); // { dg-error "accessing uninitialized byte"
"" { target c++20_down } }
+ V u = s;
+ return u.b.a[1] == 1;
+}
+
+constexpr bool a = f1 ();
+constexpr bool b = f2 ();
+constexpr bool c = f3 (); // { dg-error "accessing uninitialized array element"
"" { target c++23 } }
+constexpr bool d = f4 ();
+constexpr bool e = f5 ();
+constexpr bool f = f6 (); // { dg-error "accessing uninitialized array element"
"" { target c++23 } }
--- gcc/testsuite/g++.dg/cpp2a/bit-cast13.C.jj 2021-11-29 16:23:52.403413364
+0100
+++ gcc/testsuite/g++.dg/cpp2a/bit-cast13.C 2021-11-29 16:26:14.432383655
+0100
@@ -0,0 +1,69 @@
+// P1272R4
+// { dg-do compile { target c++14 } }
+
+struct S { char a[2]; alignas(sizeof 0) int b; };
+struct T { char a; alignas(sizeof 0) int b; };
+struct U { char a : 1; char : 6; char b : 1; };
+struct V { int a; S b; };
+struct W { unsigned a; T b; };
+
+constexpr bool
+f1 ()
+{
+ T t = { 1, 2 };
+ S s = __builtin_bit_cast (S, t); // { dg-error "accessing uninitialized
byte" }
+ S u = s;
+ return u.a[0] == 1;
+}
+
+constexpr bool
+f2 ()
+{
+ U u = { 0, 0 };
+ char a = __builtin_bit_cast (char, u); // { dg-error "accessing
uninitialized byte" }
+ char b = a;
+ return true;
+}
+
+constexpr bool
+f3 ()
+{
+ T t = { 1, 2 };
+ S s = __builtin_bit_cast (S, t); // { dg-error "accessing uninitialized
byte" }
+ S u = s;
+ return u.a[1] == 0;
+}
+
+constexpr bool
+f4 ()
+{
+ U u = { 0, 0 };
+ char a = __builtin_bit_cast (char, u); // { dg-error "accessing
uninitialized byte" }
+ char b = a;
+ return b == 0;
+}
+
+constexpr bool
+f5 ()
+{
+ W t = { 1, 2 };
+ V s = __builtin_bit_cast (V, t); // { dg-error "accessing uninitialized
byte" }
+ V u = s;
+ return u.b.a[0] == 1;
+}
+
+constexpr bool
+f6 ()
+{
+ W t = { 1, 2 };
+ V s = __builtin_bit_cast (V, t); // { dg-error "accessing uninitialized
byte" }
+ V u = s;
+ return u.b.a[1] == 1;
+}
+
+constexpr bool a = f1 ();
+constexpr bool b = f2 ();
+constexpr bool c = f3 ();
+constexpr bool d = f4 ();
+constexpr bool e = f5 ();
+constexpr bool f = f6 ();
Jakub
