c++: Allow indeterminate unsigned char or std::byte in bit_cast - P1272R4

  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.

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.

2021-11-29  Jakub Jelinek <jakub@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.


	Jakub
  

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 <jakub@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
>