Re: [PATCH] c++/modules: optimize tree flag streaming

[Patrick Palka <ppalka@redhat.com>]

On Fri, 16 Feb 2024, Patrick Palka wrote:

> On Thu, 15 Feb 2024, Patrick Palka wrote:
> 
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look
> > OK for trunk?
> > 
> > -- >8 --
> > 
> > One would expect consecutive calls to bytes_in/out::b for streaming
> > adjacent bits, as we do for tree flag streaming, to at least be
> > optimized by the compiler into individual bit operations using
> > statically known bit positions (and ideally merged into larger sized
> > reads/writes).
> > 
> > Unfortunately this doesn't happen because the compiler has trouble
> > tracking the values of this->bit_pos and this->bit_val across such
> > calls, likely because the compiler doesn't know 'this' and so it's
> > treated as global memory.  This means for each consecutive bit stream
> > operation, bit_pos and bit_val are loaded from memory, checked if
> > buffering is needed, and finally the bit is extracted from bit_val
> > according to the (unknown) bit_pos, even though relative to the previous
> > operation (if we didn't need to buffer) bit_val is unchanged and bit_pos
> > is just 1 larger.  This ends up being quite slow, with tree_node_bools
> > taking 10% of time when streaming in parts of the std module.
> > 
> > This patch optimizes this by making tracking of bit_pos and bit_val
> > easier for the compiler.  Rather than bit_pos and bit_val being members
> > of the (effectively global) bytes_in/out objects, this patch factors out
> > the bit streaming code/state into separate classes bits_in/out that get
> > constructed locally as needed for bit streaming.  Since these objects
> > are now clearly local, the compiler can more easily track their values.
> > 
> > And since bit streaming is intended to be batched it's natural for these
> > new classes to be RAII-enabled such that the bit stream is flushed upon
> > destruction.
> > 
> > In order to make the most of this improved tracking of bit position,
> > this patch changes parts where we conditionally stream a tree flag
> > to unconditionally stream (the flag or a dummy value).  That way
> > the number of bits streamed and the respective bit positions are as
> > statically known as reasonably possible.  In lang_decl_bools and
> > lang_type_bools we flush the current bit buffer at the start so that
> > subsequent bit positions are statically known.  And in core bools, we
> > can add explicit early exits utilizing invariants that the compiler
> > can't figure out itself (e.g. a tree code can't have both TS_TYPE_COMMON
> > and TS_DECL_COMMON, and if a tree code doesn't have TS_DECL_COMMON then
> > it doesn't have TS_DECL_WITH_VIS).  Finally if we're streaming fewer
> > than 4 bits, it's more space efficient to stream them as individual
> > bytes rather than as packed bits (due to the 32-bit buffer).
> 
> Oops, this last sentence is wrong.  Although the size of the bit buffer
> is 32 bits, upon flushing we rewind unused bytes within the buffer,
> which means streaming 2-8 bits ends up using only one byte not all four.
> So v2 below undoes this pessimization.
> 
> > This patch also moves the definitions of the relevant streaming classes
> > into anonymous namespaces so that the compiler can make more informed
> > decisions about inlining their member functions.
> > 
> > After this patch, compile time for a simple Hello World using the std
> > module is reduced by 7% with a release compiler.  The on-disk size of
> > the std module increases by 0.7% (presumably due to the extra flushing
> > done in lang_decl_bools and lang_type_bools).
> 
> The on-disk std module now only grows 0.4% instead of 0.7%.
> 
> > 
> > The bit stream out performance isn't improved as much as the stream in
> > due to the spans/lengths instrumentation performed on stream out (which
> > probably should be e.g. removed for release builds?)
> 
> -- >8 --
> 
> gcc/cp/ChangeLog:
> 
> 	* module.cc: Update comment about classes defined.
> 	(class data): Enclose in an anonymous namespace.
> 	(data::calc_crc): Moved from bytes::calc_crc.
> 	(class bytes): Remove.  Move bit_flush to namespace scope.
> 	(class bytes_in): Enclose in an anonymous namespace.  Inherit
> 	directly from data and adjust accordingly.  Move b and bflush
> 	members to bits_in.
> 	(class bytes_out): As above.  Remove is_set static data member.
> 	(bit_flush): Moved from class bytes.
> 	(struct bits_in): Define.
> 	(struct bits_out): Define.
> 	(bytes_out::bflush): Moved to bits_out/in.
> 	(bytes_in::bflush): Likewise
> 	(bytes_in::bfill): Removed.
> 	(bytes_out::b): Moved to bits_out/in.
> 	(bytes_in::b): Likewise.
> 	(class trees_in): Enclose in an anonymous namespace.
> 	(class trees_out): Enclose in an anonymous namespace.
> 	(trees_out::core_bools): Add bits_out/in parameter and use it.
> 	Unconditionally stream a bit for public_flag.  Add early exits
> 	as appropriate.
> 	(trees_out::core_bools): Likewise.
> 	(trees_out::lang_decl_bools): Add bits_out/in parameter and use
> 	it.  Flush the current bit buffer at the start.  Unconditionally
> 	stream a bit for module_keyed_decls_p.
> 	(trees_in::lang_decl_bools): Likewise.
> 	(trees_out::lang_type_bools): Add bits_out/in parameter and use
> 	it.  Flush the current bit buffer at the start.
> 	(trees_in::lang_type_bools): Likewise.
> 	(trees_out::tree_node_bools): Construct a bits_out object and
> 	use/pass it.
> 	(trees_in::tree_node_bools): Likewise.
> 	(trees_out::decl_value): Likewise.
> 	(trees_in::decl_value): Likewise.
> 	(module_state::write_define): Likewise.
> 	(module_state::read_define): Likewise.

Ping.

> ---
>  gcc/cp/module.cc | 418 ++++++++++++++++++++++++++---------------------
>  1 file changed, 231 insertions(+), 187 deletions(-)
> 
> diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
> index 0291d456ff5..2ecee007e8a 100644
> --- a/gcc/cp/module.cc
> +++ b/gcc/cp/module.cc
> @@ -153,9 +153,11 @@ Classes used:
>  
>     data - buffer
>  
> -   bytes - data streamer
> -   bytes_in : bytes - scalar reader
> -   bytes_out : bytes - scalar writer
> +   bytes_in - scalar reader
> +   bytes_out - scalar writer
> +
> +   bits_in - bit stream reader
> +   bits_out - bit stream writer
>  
>     elf - ELROND format
>     elf_in : elf - ELROND reader
> @@ -346,6 +348,7 @@ typedef hash_map<void *,signed,ptr_int_traits> ptr_int_hash_map;
>  
>  /* Variable length buffer.  */
>  
> +namespace {
>  class data {
>  public:
>    class allocator {
> @@ -393,6 +396,8 @@ protected:
>      return res;
>    }
>  
> +  unsigned calc_crc (unsigned) const;
> +
>  public:
>    void unuse (unsigned count)
>    {
> @@ -402,6 +407,7 @@ public:
>  public:
>    static allocator simple_memory;
>  };
> +} // anon namespace
>  
>  /* The simple data allocator.  */
>  data::allocator data::simple_memory;
> @@ -447,46 +453,11 @@ data::allocator::shrink (char *ptr)
>    XDELETEVEC (ptr);
>  }
>  
> -/* Byte streamer base.   Buffer with read/write position and smarts
> -   for single bits.  */
> -
> -class bytes : public data {
> -public:
> -  typedef data parent;
> -
> -protected:
> -  uint32_t bit_val;	/* Bit buffer.  */
> -  unsigned bit_pos;	/* Next bit in bit buffer.  */
> -
> -public:
> -  bytes ()
> -    :parent (), bit_val (0), bit_pos (0)
> -  {}
> -  ~bytes () 
> -  {
> -  }
> -
> -protected:
> -  unsigned calc_crc (unsigned) const;
> -
> -protected:
> -  /* Finish bit packet.  Rewind the bytes not used.  */
> -  unsigned bit_flush ()
> -  {
> -    gcc_assert (bit_pos);
> -    unsigned bytes = (bit_pos + 7) / 8;
> -    unuse (4 - bytes);
> -    bit_pos = 0;
> -    bit_val = 0;
> -    return bytes;
> -  }
> -};
> -
>  /* Calculate the crc32 of the buffer.  Note the CRC is stored in the
>     first 4 bytes, so don't include them.  */
>  
>  unsigned
> -bytes::calc_crc (unsigned l) const
> +data::calc_crc (unsigned l) const
>  {
>    return crc32 (0, (unsigned char *)buffer + 4, l - 4);
>  }
> @@ -495,8 +466,9 @@ class elf_in;
>  
>  /* Byte stream reader.  */
>  
> -class bytes_in : public bytes {
> -  typedef bytes parent;
> +namespace {
> +class bytes_in : public data {
> +  typedef data parent;
>  
>  protected:
>    bool overrun;  /* Sticky read-too-much flag.  */
> @@ -531,7 +503,6 @@ public:
>      if (offset > size)
>        set_overrun ();
>      pos = offset;
> -    bit_pos = bit_val = 0;
>    }
>  
>  public:
> @@ -573,14 +544,7 @@ public:
>    unsigned u32 ();  	/* Read uncompressed integer.  */
>  
>  public:
> -  bool b ();	    	/* Read a bool.  */
> -  void bflush ();	/* Completed a block of bools.  */
> -
> -private:
> -  void bfill ();	/* Get the next block of bools.  */
> -
> -public:
> -  int c ();		/* Read a char.  */
> +  int c () ATTRIBUTE_UNUSED;		/* Read a char.  */
>    int i ();		/* Read a signed int.  */
>    unsigned u ();	/* Read an unsigned int.  */
>    size_t z ();		/* Read a size_t.  */
> @@ -590,6 +554,7 @@ public:
>    const void *buf (size_t); /* Read a fixed-length buffer.  */
>    cpp_hashnode *cpp_node (); /* Read a cpp node.  */
>  };
> +} // anon namespace
>  
>  /* Verify the buffer's CRC is correct.  */
>  
> @@ -610,8 +575,9 @@ class elf_out;
>  
>  /* Byte stream writer.  */
>  
> -class bytes_out : public bytes {
> -  typedef bytes parent;
> +namespace {
> +class bytes_out : public data {
> +  typedef data parent;
>  
>  public:
>    allocator *memory;	/* Obtainer of memory.  */
> @@ -659,11 +625,7 @@ public:
>    void u32 (unsigned);  /* Write uncompressed integer.  */
>  
>  public:
> -  void b (bool);	/* Write bool.  */
> -  void bflush ();	/* Finish block of bools.  */
> -
> -public:
> -  void c (unsigned char); /* Write unsigned char.  */
> +  void c (unsigned char) ATTRIBUTE_UNUSED; /* Write unsigned char.  */
>    void i (int);		/* Write signed int.  */
>    void u (unsigned);	/* Write unsigned int.  */
>    void z (size_t s);	/* Write size_t.  */
> @@ -694,13 +656,126 @@ protected:
>    /* Instrumentation.  */
>    static unsigned spans[4];
>    static unsigned lengths[4];
> -  static int is_set;
> +  friend struct bits_out;
>  };
> +} // anon namespace
> +
> +/* Finish bit packet.  Rewind the bytes not used.  */
> +static unsigned
> +bit_flush (data& bits, uint32_t& bit_val, unsigned& bit_pos)
> +{
> +  gcc_assert (bit_pos);
> +  unsigned bytes = (bit_pos + 7) / 8;
> +  bits.unuse (4 - bytes);
> +  bit_pos = 0;
> +  bit_val = 0;
> +  return bytes;
> +}
> +
> +/* Bit stream reader (RAII-enabled).  Bools are packed into bytes.  You
> +   cannot mix bools and non-bools.  Use bflush to flush the current stream
> +   of bools on demand.  Upon destruction bflush is called.
> +
> +   When reading, we don't know how many bools we'll read in.  So read
> +   4 bytes-worth, and then rewind when flushing if we didn't need them
> +   all.  You can't have a block of bools closer than 4 bytes to the
> +   end of the buffer.  */
> +
> +namespace {
> +struct bits_in {
> +  bytes_in& in;
> +  uint32_t bit_val = 0;
> +  unsigned bit_pos = 0;
> +
> +  bits_in (bytes_in& in)
> +    : in (in)
> +  { }
> +
> +  ~bits_in ()
> +  {
> +    bflush ();
> +  }
> +
> +  bits_in(const bits_in&) = delete;
> +  bits_in& operator=(const bits_in&) = delete;
> +
> +  /* Completed a block of bools.  */
> +  void bflush ()
> +  {
> +    if (bit_pos)
> +      bit_flush (in, bit_val, bit_pos);
> +  }
> +
> +  /* Read one bit.  */
> +  bool b ()
> +  {
> +    if (!bit_pos)
> +      bit_val = in.u32 ();
> +    bool x = (bit_val >> bit_pos) & 1;
> +    bit_pos = (bit_pos + 1) % 32;
> +    return x;
> +  }
> +};
> +} // anon namespace
> +
> +/* Bit stream writer (RAII-enabled), counterpart to bits_in.  */
> +
> +namespace {
> +struct bits_out {
> +  bytes_out& out;
> +  uint32_t bit_val = 0;
> +  unsigned bit_pos = 0;
> +  char is_set = -1;
> +
> +  bits_out (bytes_out& out)
> +    : out (out)
> +  { }
> +
> +  ~bits_out ()
> +  {
> +    bflush ();
> +  }
> +
> +  bits_out(const bits_out&) = delete;
> +  bits_out& operator=(const bits_out&) = delete;
> +
> +  /* Completed a block of bools.  */
> +  void bflush ()
> +  {
> +    if (bit_pos)
> +      {
> +	out.u32 (bit_val);
> +	out.lengths[2] += bit_flush (out, bit_val, bit_pos);
> +      }
> +    out.spans[2]++;
> +    is_set = -1;
> +  }
> +
> +  /* Write one bit.
> +
> +     It may be worth optimizing for most bools being zero.  Some kind of
> +     run-length encoding?  */
> +  void b (bool x)
> +  {
> +    if (is_set != x)
> +      {
> +	is_set = x;
> +	out.spans[x]++;
> +      }
> +    out.lengths[x]++;
> +    bit_val |= unsigned (x) << bit_pos++;
> +    if (bit_pos == 32)
> +      {
> +	out.u32 (bit_val);
> +	out.lengths[2] += bit_flush (out, bit_val, bit_pos);
> +      }
> +  }
> +};
> +} // anon namespace
>  
>  /* Instrumentation.  */
>  unsigned bytes_out::spans[4];
>  unsigned bytes_out::lengths[4];
> -int bytes_out::is_set = -1;
>  
>  /* If CRC_PTR non-null, set the CRC of the buffer.  Mix the CRC into
>     that pointed to by CRC_PTR.  */
> @@ -723,73 +798,6 @@ bytes_out::set_crc (unsigned *crc_ptr)
>      }
>  }
>  
> -/* Finish a set of bools.  */
> -
> -void
> -bytes_out::bflush ()
> -{
> -  if (bit_pos)
> -    {
> -      u32 (bit_val);
> -      lengths[2] += bit_flush ();
> -    }
> -  spans[2]++;
> -  is_set = -1;
> -}
> -
> -void
> -bytes_in::bflush ()
> -{
> -  if (bit_pos)
> -    bit_flush ();
> -}
> -
> -/* When reading, we don't know how many bools we'll read in.  So read
> -   4 bytes-worth, and then rewind when flushing if we didn't need them
> -   all.  You can't have a block of bools closer than 4 bytes to the
> -   end of the buffer.  */
> -
> -void
> -bytes_in::bfill ()
> -{
> -  bit_val = u32 ();
> -}
> -
> -/* Bools are packed into bytes.  You cannot mix bools and non-bools.
> -   You must call bflush before emitting another type.  So batch your
> -   bools.
> -
> -   It may be worth optimizing for most bools being zero.  Some kind of
> -   run-length encoding?  */
> -
> -void
> -bytes_out::b (bool x)
> -{
> -  if (is_set != x)
> -    {
> -      is_set = x;
> -      spans[x]++;
> -    }
> -  lengths[x]++;
> -  bit_val |= unsigned (x) << bit_pos++;
> -  if (bit_pos == 32)
> -    {
> -      u32 (bit_val);
> -      lengths[2] += bit_flush ();
> -    }
> -}
> -
> -bool
> -bytes_in::b ()
> -{
> -  if (!bit_pos)
> -    bfill ();
> -  bool v = (bit_val >> bit_pos++) & 1;
> -  if (bit_pos == 32)
> -    bit_flush ();
> -  return v;
> -}
> -
>  /* Exactly 4 bytes.  Used internally for bool packing and a few other
>     places.  We can't simply use uint32_t because (a) alignment and
>     (b) we need little-endian for the bool streaming rewinding to make
> @@ -2846,6 +2854,7 @@ struct post_process_data {
>     read trees with TREE_VISITED.  Thus it's quite safe to have
>     multiple concurrent readers.  Which is good, because lazy
>     loading. */
> +namespace {
>  class trees_in : public bytes_in {
>    typedef bytes_in parent;
>  
> @@ -2870,15 +2879,15 @@ private:
>  
>  public:
>    /* Needed for binfo writing  */
> -  bool core_bools (tree);
> +  bool core_bools (tree, bits_in&);
>  
>  private:
>    /* Stream tree_core, lang_decl_specific and lang_type_specific
>       bits.  */
>    bool core_vals (tree);
> -  bool lang_type_bools (tree);
> +  bool lang_type_bools (tree, bits_in&);
>    bool lang_type_vals (tree);
> -  bool lang_decl_bools (tree);
> +  bool lang_decl_bools (tree, bits_in&);
>    bool lang_decl_vals (tree);
>    bool lang_vals (tree);
>    bool tree_node_bools (tree);
> @@ -2965,6 +2974,7 @@ private:
>  private:
>    void assert_definition (tree, bool installing);
>  };
> +} // anon namespace
>  
>  trees_in::trees_in (module_state *state)
>    :parent (), state (state), unused (0)
> @@ -2982,6 +2992,7 @@ trees_in::~trees_in ()
>  }
>  
>  /* Tree stream writer.  */
> +namespace {
>  class trees_out : public bytes_out {
>    typedef bytes_out parent;
>  
> @@ -3043,11 +3054,11 @@ public:
>    }
>  
>  private:
> -  void core_bools (tree);
> +  void core_bools (tree, bits_out&);
>    void core_vals (tree);
> -  void lang_type_bools (tree);
> +  void lang_type_bools (tree, bits_out&);
>    void lang_type_vals (tree);
> -  void lang_decl_bools (tree);
> +  void lang_decl_bools (tree, bits_out&);
>    void lang_decl_vals (tree);
>    void lang_vals (tree);
>    void tree_node_bools (tree);
> @@ -3126,6 +3137,7 @@ private:
>    static unsigned back_ref_count;
>    static unsigned null_count;
>  };
> +} // anon namespace
>  
>  /* Instrumentation counters.  */
>  unsigned trees_out::tree_val_count;
> @@ -5292,9 +5304,9 @@ trees_in::start (unsigned code)
>  /* Read & write the core boolean flags.  */
>  
>  void
> -trees_out::core_bools (tree t)
> +trees_out::core_bools (tree t, bits_out& bits)
>  {
> -#define WB(X) (b (X))
> +#define WB(X) (bits.b (X))
>    tree_code code = TREE_CODE (t);
>  
>    WB (t->base.side_effects_flag);
> @@ -5314,6 +5326,8 @@ trees_out::core_bools (tree t)
>    if (TREE_CODE_CLASS (code) != tcc_type)
>      /* This is TYPE_CACHED_VALUES_P for types.  */
>      WB (t->base.public_flag);
> +  else
> +    WB (false);
>    WB (t->base.private_flag);
>    WB (t->base.protected_flag);
>    WB (t->base.deprecated_flag);
> @@ -5327,7 +5341,7 @@ trees_out::core_bools (tree t)
>      case TARGET_MEM_REF:
>      case TREE_VEC:
>        /* These use different base.u fields.  */
> -      break;
> +      return;
>  
>      default:
>        WB (t->base.u.bits.lang_flag_0);
> @@ -5359,7 +5373,7 @@ trees_out::core_bools (tree t)
>        break;
>      }
>  
> -  if (CODE_CONTAINS_STRUCT (code, TS_TYPE_COMMON))
> +  if (TREE_CODE_CLASS (code) == tcc_type)
>      {
>        WB (t->type_common.no_force_blk_flag);
>        WB (t->type_common.needs_constructing_flag);
> @@ -5374,7 +5388,10 @@ trees_out::core_bools (tree t)
>        WB (t->type_common.lang_flag_5);
>        WB (t->type_common.lang_flag_6);
>        WB (t->type_common.typeless_storage);
> +      return;
>      }
> +  else if (TREE_CODE_CLASS (code) != tcc_declaration)
> +    return;
>  
>    if (CODE_CONTAINS_STRUCT (code, TS_DECL_COMMON))
>      {
> @@ -5439,6 +5456,8 @@ trees_out::core_bools (tree t)
>        WB (t->decl_common.decl_nonshareable_flag);
>        WB (t->decl_common.decl_not_flexarray);
>      }
> +  else
> +    return;
>  
>    if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS))
>      {
> @@ -5459,6 +5478,8 @@ trees_out::core_bools (tree t)
>        WB (t->decl_with_vis.final);
>        WB (t->decl_with_vis.regdecl_flag);
>      }
> +  else
> +    return;
>  
>    if (CODE_CONTAINS_STRUCT (code, TS_FUNCTION_DECL))
>      {
> @@ -5489,9 +5510,10 @@ trees_out::core_bools (tree t)
>  }
>  
>  bool
> -trees_in::core_bools (tree t)
> +trees_in::core_bools (tree t, bits_in& bits)
>  {
> -#define RB(X) ((X) = b ())
> +#define RB(X) ((X) = bits.b ())
> +
>    tree_code code = TREE_CODE (t);
>  
>    RB (t->base.side_effects_flag);
> @@ -5507,6 +5529,8 @@ trees_in::core_bools (tree t)
>    RB (t->base.static_flag);
>    if (TREE_CODE_CLASS (code) != tcc_type)
>      RB (t->base.public_flag);
> +  else
> +    bits.b ();
>    RB (t->base.private_flag);
>    RB (t->base.protected_flag);
>    RB (t->base.deprecated_flag);
> @@ -5520,7 +5544,7 @@ trees_in::core_bools (tree t)
>      case TARGET_MEM_REF:
>      case TREE_VEC:
>        /* These use different base.u fields.  */
> -      break;
> +      goto done;
>  
>      default:
>        RB (t->base.u.bits.lang_flag_0);
> @@ -5539,7 +5563,7 @@ trees_in::core_bools (tree t)
>        break;
>      }
>  
> -  if (CODE_CONTAINS_STRUCT (code, TS_TYPE_COMMON))
> +  if (TREE_CODE_CLASS (code) == tcc_type)
>      {
>        RB (t->type_common.no_force_blk_flag);
>        RB (t->type_common.needs_constructing_flag);
> @@ -5554,7 +5578,10 @@ trees_in::core_bools (tree t)
>        RB (t->type_common.lang_flag_5);
>        RB (t->type_common.lang_flag_6);
>        RB (t->type_common.typeless_storage);
> +      goto done;
>      }
> +  else if (TREE_CODE_CLASS (code) != tcc_declaration)
> +    goto done;
>  
>    if (CODE_CONTAINS_STRUCT (code, TS_DECL_COMMON))
>      {
> @@ -5584,6 +5611,8 @@ trees_in::core_bools (tree t)
>        RB (t->decl_common.decl_nonshareable_flag);
>        RB (t->decl_common.decl_not_flexarray);
>      }
> +  else
> +    goto done;
>  
>    if (CODE_CONTAINS_STRUCT (code, TS_DECL_WITH_VIS))
>      {
> @@ -5604,6 +5633,8 @@ trees_in::core_bools (tree t)
>        RB (t->decl_with_vis.final);
>        RB (t->decl_with_vis.regdecl_flag);
>      }
> +  else
> +    goto done;
>  
>    if (CODE_CONTAINS_STRUCT (code, TS_FUNCTION_DECL))
>      {
> @@ -5627,20 +5658,22 @@ trees_in::core_bools (tree t)
>  
>        /* decl_type is a (misnamed) 2 bit discriminator.	 */
>        unsigned kind = 0;
> -      kind |= unsigned (b ()) << 0;
> -      kind |= unsigned (b ()) << 1;
> +      kind |= unsigned (bits.b ()) << 0;
> +      kind |= unsigned (bits.b ()) << 1;
>        t->function_decl.decl_type = function_decl_type (kind);
>      }
>  #undef RB
> +done:
>    return !get_overrun ();
>  }
>  
>  void
> -trees_out::lang_decl_bools (tree t)
> +trees_out::lang_decl_bools (tree t, bits_out& bits)
>  {
> -#define WB(X) (b (X))
> +#define WB(X) (bits.b (X))
>    const struct lang_decl *lang = DECL_LANG_SPECIFIC (t);
>  
> +  bits.bflush ();
>    WB (lang->u.base.language == lang_cplusplus);
>    WB ((lang->u.base.use_template >> 0) & 1);
>    WB ((lang->u.base.use_template >> 1) & 1);
> @@ -5664,6 +5697,8 @@ trees_out::lang_decl_bools (tree t)
>    WB (lang->u.base.module_attach_p);
>    if (VAR_OR_FUNCTION_DECL_P (t))
>      WB (lang->u.base.module_keyed_decls_p);
> +  else
> +    WB (false);
>    switch (lang->u.base.selector)
>      {
>      default:
> @@ -5714,15 +5749,16 @@ trees_out::lang_decl_bools (tree t)
>  }
>  
>  bool
> -trees_in::lang_decl_bools (tree t)
> +trees_in::lang_decl_bools (tree t, bits_in& bits)
>  {
> -#define RB(X) ((X) = b ())
> +#define RB(X) ((X) = bits.b ())
>    struct lang_decl *lang = DECL_LANG_SPECIFIC (t);
>  
> -  lang->u.base.language = b () ? lang_cplusplus : lang_c;
> +  bits.bflush ();
> +  lang->u.base.language = bits.b () ? lang_cplusplus : lang_c;
>    unsigned v;
> -  v = b () << 0;
> -  v |= b () << 1;
> +  v = bits.b () << 0;
> +  v |= bits.b () << 1;
>    lang->u.base.use_template = v;
>    /* lang->u.base.not_really_extern is not streamed.  */
>    RB (lang->u.base.initialized_in_class);
> @@ -5738,6 +5774,8 @@ trees_in::lang_decl_bools (tree t)
>    RB (lang->u.base.module_attach_p);
>    if (VAR_OR_FUNCTION_DECL_P (t))
>      RB (lang->u.base.module_keyed_decls_p);
> +  else
> +    bits.b ();
>    switch (lang->u.base.selector)
>      {
>      default:
> @@ -5786,11 +5824,12 @@ trees_in::lang_decl_bools (tree t)
>  }
>  
>  void
> -trees_out::lang_type_bools (tree t)
> +trees_out::lang_type_bools (tree t, bits_out& bits)
>  {
> -#define WB(X) (b (X))
> +#define WB(X) (bits.b (X))
>    const struct lang_type *lang = TYPE_LANG_SPECIFIC (t);
>  
> +  bits.bflush ();
>    WB (lang->has_type_conversion);
>    WB (lang->has_copy_ctor);
>    WB (lang->has_default_ctor);
> @@ -5852,11 +5891,12 @@ trees_out::lang_type_bools (tree t)
>  }
>  
>  bool
> -trees_in::lang_type_bools (tree t)
> +trees_in::lang_type_bools (tree t, bits_in& bits)
>  {
> -#define RB(X) ((X) = b ())
> +#define RB(X) ((X) = bits.b ())
>    struct lang_type *lang = TYPE_LANG_SPECIFIC (t);
>  
> +  bits.bflush ();
>    RB (lang->has_type_conversion);
>    RB (lang->has_copy_ctor);
>    RB (lang->has_default_ctor);
> @@ -5864,8 +5904,8 @@ trees_in::lang_type_bools (tree t)
>    RB (lang->ref_needs_init);
>    RB (lang->has_const_copy_assign);
>    unsigned v;
> -  v = b () << 0;
> -  v |= b () << 1;
> +  v = bits.b () << 0;
> +  v |= bits.b () << 1;
>    lang->use_template = v;
>  
>    RB (lang->has_mutable);
> @@ -5877,8 +5917,8 @@ trees_in::lang_type_bools (tree t)
>    RB (lang->has_new);
>    RB (lang->has_array_new);
>  
> -  v = b () << 0;
> -  v |= b () << 1;
> +  v = bits.b () << 0;
> +  v |= bits.b () << 1;
>    lang->gets_delete = v;
>    RB (lang->interface_only);
>    RB (lang->interface_unknown);
> @@ -7106,18 +7146,19 @@ trees_out::tree_node_bools (tree t)
>    gcc_checking_assert (TREE_CODE (t) != NAMESPACE_DECL
>  		       || DECL_NAMESPACE_ALIAS (t));
>  
> -  core_bools (t);
> +  bits_out bits (*this);
> +  core_bools (t, bits);
>  
>    switch (TREE_CODE_CLASS (TREE_CODE (t)))
>      {
>      case tcc_declaration:
>        {
>  	bool specific = DECL_LANG_SPECIFIC (t) != NULL;
> -	b (specific);
> +	bits.b (specific);
>  	if (specific && VAR_P (t))
> -	  b (DECL_DECOMPOSITION_P (t));
> +	  bits.b (DECL_DECOMPOSITION_P (t));
>  	if (specific)
> -	  lang_decl_bools (t);
> +	  lang_decl_bools (t, bits);
>        }
>        break;
>  
> @@ -7128,9 +7169,9 @@ trees_out::tree_node_bools (tree t)
>  	gcc_assert (TYPE_LANG_SPECIFIC (t)
>  		    == TYPE_LANG_SPECIFIC (TYPE_MAIN_VARIANT (t)));
>  
> -	b (specific);
> +	bits.b (specific);
>  	if (specific)
> -	  lang_type_bools (t);
> +	  lang_type_bools (t, bits);
>        }
>        break;
>  
> @@ -7138,34 +7179,35 @@ trees_out::tree_node_bools (tree t)
>        break;
>      }
>  
> -  bflush ();
> +  bits.bflush ();
>  }
>  
>  bool
>  trees_in::tree_node_bools (tree t)
>  {
> -  bool ok = core_bools (t);
> +  bits_in bits (*this);
> +  bool ok = core_bools (t, bits);
>  
>    if (ok)
>      switch (TREE_CODE_CLASS (TREE_CODE (t)))
>        {
>        case tcc_declaration:
> -	if (b ())
> +	if (bits.b ())
>  	  {
> -	    bool decomp = VAR_P (t) && b ();
> +	    bool decomp = VAR_P (t) && bits.b ();
>  
>  	    ok = maybe_add_lang_decl_raw (t, decomp);
>  	    if (ok)
> -	      ok = lang_decl_bools (t);
> -	}
> +	      ok = lang_decl_bools (t, bits);
> +	  }
>  	break;
>  
>        case tcc_type:
> -	if (b ())
> +	if (bits.b ())
>  	  {
>  	    ok = maybe_add_lang_type_raw (t);
>  	    if (ok)
> -	      ok = lang_type_bools (t);
> +	      ok = lang_type_bools (t, bits);
>  	  }
>  	break;
>  
> @@ -7173,7 +7215,7 @@ trees_in::tree_node_bools (tree t)
>  	break;
>        }
>  
> -  bflush ();
> +  bits.bflush ();
>    if (!ok || get_overrun ())
>      return false;
>  
> @@ -7696,11 +7738,11 @@ trees_out::decl_value (tree decl, depset *dep)
>  
>        if (mk != MK_unique)
>  	{
> +	  bits_out bits (*this);
>  	  if (!(mk & MK_template_mask) && !state->is_header ())
>  	    {
>  	      /* Tell the importer whether this is a global module entity,
> -		 or a module entity.  This bool merges into the next block
> -		 of bools.  Sneaky.  */
> +		 or a module entity.  */
>  	      tree o = get_originating_module_decl (decl);
>  	      bool is_attached = false;
>  
> @@ -7709,9 +7751,9 @@ trees_out::decl_value (tree decl, depset *dep)
>  		  && DECL_MODULE_ATTACH_P (not_tmpl))
>  		is_attached = true;
>  
> -	      b (is_attached);
> +	      bits.b (is_attached);
>  	    }
> -	  b (dep && dep->has_defn ());
> +	  bits.b (dep && dep->has_defn ());
>  	}
>        tree_node_bools (decl);
>      }
> @@ -7981,11 +8023,11 @@ trees_in::decl_value ()
>      {
>        if (mk != MK_unique)
>  	{
> +	  bits_in bits (*this);
>  	  if (!(mk & MK_template_mask) && !state->is_header ())
> -	    /* See note in trees_out about where this bool is sequenced.  */
> -	    is_attached = b ();
> +	    is_attached = bits.b ();
>  
> -	  has_defn = b ();
> +	  has_defn = bits.b ();
>  	}
>  
>        if (!tree_node_bools (decl))
> @@ -16682,10 +16724,11 @@ module_state::write_define (bytes_out &sec, const cpp_macro *macro)
>  {
>    sec.u (macro->count);
>  
> -  sec.b (macro->fun_like);
> -  sec.b (macro->variadic);
> -  sec.b (macro->syshdr);
> -  sec.bflush ();
> +  bits_out bits (sec);
> +  bits.b (macro->fun_like);
> +  bits.b (macro->variadic);
> +  bits.b (macro->syshdr);
> +  bits.bflush ();
>  
>    write_location (sec, macro->line);
>    if (macro->fun_like)
> @@ -16780,10 +16823,11 @@ module_state::read_define (bytes_in &sec, cpp_reader *reader) const
>    macro->kind = cmk_macro;
>    macro->imported_p = true;
>  
> -  macro->fun_like = sec.b ();
> -  macro->variadic = sec.b ();
> -  macro->syshdr = sec.b ();
> -  sec.bflush ();
> +  bits_in bits (sec);
> +  macro->fun_like = bits.b ();
> +  macro->variadic = bits.b ();
> +  macro->syshdr = bits.b ();
> +  bits.bflush ();
>  
>    macro->line = read_location (sec);
>  
> -- 
> 2.44.0.rc1.15.g4fc51f00ef
> 
>