Re: [PATCH] tree: Fix up tree_code_{length,type}

[Patrick Palka <ppalka@redhat.com>]

On Thu, 26 Jan 2023, Jakub Jelinek wrote:

> On Thu, Jan 26, 2023 at 09:45:35AM -0500, Patrick Palka via Gcc-patches wrote:
> > > +#define DEFTREECODE(SYM, NAME, TYPE, LENGTH) TYPE,
> > > +#define END_OF_BASE_TREE_CODES tcc_exceptional,
> > > +
> > > +
> > >  /* Class of tree given its code.  */
> > > -extern const enum tree_code_class tree_code_type[];
> > > +constexpr enum tree_code_class tree_code_type[] = {
> > > +#include "all-tree.def"
> > > +};
> > > +
> > > +#undef DEFTREECODE
> > > +#undef END_OF_BASE_TREE_CODES
> > >  
> > >  /* Each tree code class has an associated string representation.
> > >     These must correspond to the tree_code_class entries.  */
> > >  extern const char *const tree_code_class_strings[];
> > >  
> > >  /* Number of argument-words in each kind of tree-node.  */
> > > -extern const unsigned char tree_code_length[];
> > > +
> > > +#define DEFTREECODE(SYM, NAME, TYPE, LENGTH) LENGTH,
> > > +#define END_OF_BASE_TREE_CODES 0,
> > > +constexpr unsigned char tree_code_length[] = {
> > > +#include "all-tree.def"
> > > +};
> > > +
> > > +#undef DEFTREECODE
> > > +#undef END_OF_BASE_TREE_CODES
> > 
> > IIUC defining these globals as non-inline constexpr gives them internal
> > linkage, and so each TU contains its own unique copy of these globals.
> > This bloats cc1plus by a tiny bit and is technically an ODR violation
> > because some inline functions such as tree_class_check also ODR-use
> > these variables and so each defn of tree_class_check will refer to a
> > "different" tree_code_class.  Since inline variables are a C++17
> > feature, I guess we could fix this by defining the globals the old way
> > before C++17 and as inline constexpr otherwise?
> 
> And I'd argue with the tiny bit.
> In my x86_64-linux cc1plus from today, I see 193 _ZL16tree_code_length vars,
> 374 bytes each, and 324 _ZL14tree_code_type vars, 1496 bytes each.
> So, that means waste of 555016 .rodata bytes, plus being highly non-cache
> friendly.
> 
> The following patch does that.
> 
> So far tested on x86_64-linux in my -O0 working tree (system gcc 12
> compiler) where .rodata shrunk with the patch by 928896 bytes, in last
> stage of a bootstrapped tree (built by today's prev-gcc) where .rodata
> shrunk by 561728 bytes (in neither case .text or most other sections
> changed sizes) and on powerpc64le-linux --disable-bootstrap
> (system gcc 4.8.5) to test also the non-C++17 case.

LGTM FWIW.  On a related note I noticed the function
tree.h:tree_operand_length is declared static and is then used in the
non-static inline functions tree_operand_check etc, which seems to be
also be a (harmless) ODR violation?

We probably should do s/static inline/inline throughout the header files
at some point, which'd hopefully reduce the size of and speed up stage1
cc1plus.

> 
> Ok for trunk if it passes full bootstrap/regtest?
> 
> BTW, wonder if tree_code_type couldn't be an array of unsigned char
> elements rather than enum tree_code_class and we'd then cast it
> to the enum in the macro, that would shrink that array from 1496 bytes
> to 374.  Of course, that sounds like stage1 material.
> 
> 2023-01-26  Patrick Palka  <ppalka@redhat.com>
> 	    Jakub Jelinek  <jakub@redhat.com>
> 
> 	* tree-core.h (tree_code_type, tree_code_length): For
> 	C++17 and later, add inline keyword, otherwise don't define
> 	the arrays, but declare extern arrays.
> 	* tree.cc (tree_code_type, tree_code_length): Define these
> 	arrays for C++14 and older.
> 
> --- gcc/tree-core.h.jj	2023-01-02 09:32:31.188158094 +0100
> +++ gcc/tree-core.h	2023-01-26 16:02:34.212113251 +0100
> @@ -2284,17 +2284,20 @@ struct floatn_type_info {
>  /* Matrix describing the structures contained in a given tree code.  */
>  extern bool tree_contains_struct[MAX_TREE_CODES][64];
>  
> +/* Class of tree given its code.  */
> +#if __cpp_inline_variables >= 201606L
>  #define DEFTREECODE(SYM, NAME, TYPE, LENGTH) TYPE,
>  #define END_OF_BASE_TREE_CODES tcc_exceptional,
>  
> -
> -/* Class of tree given its code.  */
> -constexpr enum tree_code_class tree_code_type[] = {
> +constexpr inline enum tree_code_class tree_code_type[] = {
>  #include "all-tree.def"
>  };
>  
>  #undef DEFTREECODE
>  #undef END_OF_BASE_TREE_CODES
> +#else
> +extern const enum tree_code_class tree_code_type[];
> +#endif
>  
>  /* Each tree code class has an associated string representation.
>     These must correspond to the tree_code_class entries.  */
> @@ -2302,14 +2305,18 @@ extern const char *const tree_code_class
>  
>  /* Number of argument-words in each kind of tree-node.  */
>  
> +#if __cpp_inline_variables >= 201606L
>  #define DEFTREECODE(SYM, NAME, TYPE, LENGTH) LENGTH,
>  #define END_OF_BASE_TREE_CODES 0,
> -constexpr unsigned char tree_code_length[] = {
> +constexpr inline unsigned char tree_code_length[] = {
>  #include "all-tree.def"
>  };
>  
>  #undef DEFTREECODE
>  #undef END_OF_BASE_TREE_CODES
> +#else
> +extern const unsigned char tree_code_length[];
> +#endif
>  
>  /* Vector of all alias pairs for global symbols.  */
>  extern GTY(()) vec<alias_pair, va_gc> *alias_pairs;
> --- gcc/tree.cc.jj	2023-01-13 17:37:45.259482663 +0100
> +++ gcc/tree.cc	2023-01-26 16:03:59.796878082 +0100
> @@ -74,7 +74,33 @@ along with GCC; see the file COPYING3.
>  #include "asan.h"
>  #include "ubsan.h"
>  
> +#if __cpp_inline_variables < 201606L
> +/* Tree code classes.  */
>  
> +#define DEFTREECODE(SYM, NAME, TYPE, LENGTH) TYPE,
> +#define END_OF_BASE_TREE_CODES tcc_exceptional,
> +
> +const enum tree_code_class tree_code_type[] = {
> +#include "all-tree.def"
> +};
> +
> +#undef DEFTREECODE
> +#undef END_OF_BASE_TREE_CODES
> +
> +/* Table indexed by tree code giving number of expression
> +   operands beyond the fixed part of the node structure.
> +   Not used for types or decls.  */
> +
> +#define DEFTREECODE(SYM, NAME, TYPE, LENGTH) LENGTH,
> +#define END_OF_BASE_TREE_CODES 0,
> +
> +const unsigned char tree_code_length[] = {
> +#include "all-tree.def"
> +};
> +
> +#undef DEFTREECODE
> +#undef END_OF_BASE_TREE_CODES
> +#endif
>  
>  /* Names of tree components.
>     Used for printing out the tree and error messages.  */
> 
> 	Jakub
> 
>