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

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Jan 27, 2023 at 07:42:39AM +0000, Richard Biener wrote:
> > 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.
> 
> One could argue the same way for this patch (and instead revert),

Well, this patch is in fact a conditional reversion (revert for
C++11/14, add one keyword to 2 declarations otherwise).

> I'd say if we tweak this now then tweak it to the maximum extent?
> Isn't sth like 'enum unsigned char tree_code_class' now possible?
> (and a static assert the enum values all fit, though that would
> be diagnosed anyway?)

C++11 indeed has
enum tree_code_class : unsigned char {
  tcc_exceptional,
  ...
  tcc_expression
};
and one indeed gets an error if some enumerator doesn't fit.
The problem I see with this is that the type is 8-bit everywhere,
which I'd be afraid could cause worse code generation (of course,
one would need to try to see how much; e.g. build the compiler
unmodified, with the unsigned char array plus explicit casts from
the array and finally with unsigned char as underlying type).
When passing around enum tree_code_class etc., it is fine if it
is 32-bit.  And there isn't a way to create an enum with different
underlying type but with the same enumerators as in another enum.
Perhaps for tree_code_class we could away with the underlying type
because it is mostly used in the macros which immediately compare
it, in gcc/*.cc just in the following explicitly:
expr.cc:get_def_for_expr_class (tree name, enum tree_code_class tclass)
fold-const.cc:  enum tree_code_class tclass;
fold-const.cc:  enum tree_code_class tclass = TREE_CODE_CLASS (code);
fold-const.cc:  enum tree_code_class tclass = TREE_CODE_CLASS (code);
fold-const.cc:  enum tree_code_class kind = TREE_CODE_CLASS (code);
fold-const.cc:  enum tree_code_class kind = TREE_CODE_CLASS (code);
fold-const.cc:  enum tree_code_class kind = TREE_CODE_CLASS (code);
fold-const.cc:  enum tree_code_class kind = TREE_CODE_CLASS (code);
gimple-fold.cc:              enum tree_code_class kind = TREE_CODE_CLASS (subcode);
print-tree.cc:  enum tree_code_class tclass;
print-tree.cc:  enum tree_code_class tclass;
tree.cc:   These must correspond to the tree_code_class entries.  */
tree.cc:const char *const tree_code_class_strings[] =
tree.cc:  enum tree_code_class type = TREE_CODE_CLASS (code);
tree.cc:  enum tree_code_class type = TREE_CODE_CLASS (code);
tree.cc:tree_class_check_failed (const_tree node, const enum tree_code_class cl,
tree.cc:tree_not_class_check_failed (const_tree node, const enum tree_code_class cl,
tree.cc:  const enum tree_code_class c = TREE_CODE_CLASS (TREE_CODE (t));
tree.cc:  const enum tree_code_class c = TREE_CODE_CLASS (TREE_CODE (t));
tree-dump.cc:  enum tree_code_class code_class;
tree-inline.cc:  enum tree_code_class cl = TREE_CODE_CLASS (code);
tree-pretty-print.cc:	enum tree_code_class tclass;
tree-ssa-live.cc:  enum tree_code_class c = TREE_CODE_CLASS (TREE_CODE (t));
tree-ssa-operands.cc:  enum tree_code_class codeclass;
But as I said, one would need to watch for code generation at least on
a couple of common hosts, and while x86_64 should be one of them, it might
have bigger effects on others as x86 has byte comparison etc. instructions.

> 
> > 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"
> >  };
> 
> Do we need an explicit external definition somewhere when
> constant folding isn't possible?

> 
> Otherwise looks good to me.
> 
> Thanks,
> Richard.
> 
> >  #undef DEFTREECODE
> >  #undef END_OF_BASE_TREE_CODES
> > +#else
> > +extern const enum tree_code_class tree_code_type[];

There is one here for the C++11 and C++14 cases.
For C++17 and later it isn't needed,
constexpr inline enum tree_code_class tree_code_type[] = {
...
};
means this is a comdat variable in all TUs which need non-ODR
uses of it (tree_code_type[23] evaluates to constant expression,
but tree_code_type[x] or &tree_code_type[23] etc. often don't and then
the comdat var is emitted and all TUs share one copy of the variable.

	Jakub