Re: cutting the size of CONST_DECLs

[Robert Bowdidge <bowdidge@apple.com>]

On Feb 27, 2004, at 4:20 PM, Dan Nicolaescu wrote:
> You might also want to look at doing something about FUNCTION_DECLs. 
> There are
> a few  fields in struct tree_decl that are only used for
> FUNCTION_DECLs: inlined_fns, saved_tree.

Yep, separating FUNCTION_DECLs from everything else would definitely 
help. One problem is that function declarations aren't the most common 
node we see when parsing our headers; in my examples, FUNCTION_DECLs 
were only 14-20% of declarations.  PARM_DECLs were usually 33-36% of 
declarations, so separating them out makes twice the difference.

Here's the distributions of declaration kinds I saw on some example 
compiles:

		PARAM	FN		CONST	TYPE	FIELD	VAR
FinderFE	30%		29%		24%		16%		?		?
gcc		30%		34%		10%		3%		21%		3%
rogue	27%		59%		0%		5%		9%		5%
AppearanceSample
		36%		20%		18%		16%		7%		0%
SimpleText
		33%		14%		28%		13%		11%		4%

Each represents declaration kinds found during compile of a 
representative file.  FinderFE is the C++ front end to the Mac OS 
Finder.  Appearance Sample and SimpleText are two GUI applications from 
Apple's sample code; AppearanceSample is C++, and SimpleText C.

One possibility I've strongly considered is to have three levels of 
declarations:

small: CONST_DECL
medium: PARM_DECL, TYPE_DECL,FIELD_DECL
large: FN_DECL, VAR_DECL

On 3.3, small declarations to 72 bytes, medium declarations 100 bytes, 
and large declarations 116 bytes.    I've done a few measurements with 
this set of objects; memory use and compile times improve again from 
the CONST_DECL/everything else scheme, though the memory improvements 
are still in the <10% ballpark, and compile time improvements are much 
smaller.  I still need to do more measurements on non-Apple source 
code, and double-check that the changes aren't slowing the compiler.

Adding different kinds of declarations adds a bit of overhead, so we 
don't want to take this too far.  At a minimum, the DECL_P() macro 
needs to do more work to decide if a given node is a declaration.  On a 
stock system, this is a single comparison: is the tree code class 'd'?  
With three kinds of declarations, I could make the tree code classes 
'd', 'D', and '$'.  With these characters, the test in DECL_P() can use 
a bitmask to see if (tree_code & 0x1f) == 0x04 rather than doing three 
comparisons.

Deciding which kinds of declarations behave similarly can be difficult 
from just the table of access patterns.  I ended up using concept 
analysis to identify likely hierarchies.  (Snelting and Tip, 
"Understanding Class Hierarchies using Concept Analysis", ACM 
Transactions on Programming Languages and Systems 22(3), May 2000).  
There's a tool available on Sourceforge ("conexp") to draw an optimized 
class hierarchy (well, a concept lattice) given a set of objects and 
properties.  The three-level scheme pretty much fell out once I looked 
at the lattice.

Robert