opt_deadcode.mx

一个内存数据库的源代码这是服务器端还有客户端

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@f opt_deadcode
@- Dead Code Removal
Dead code fragments are recognized by assignments to variables
whose value is not consumed any more. 
It can be detected  by marking all variables used as 
arguments as being relevant. 
In parallel, we built a list of instructions that should appear 
in the final result.
The new code block is than built in one scan, discarding the
superflous instructions.

Instructions that produce side effects to the environment, 
e.g. printing and BAT updates, should be taken into account. 
Such (possibly recursive) functions should be marked with a property 
(@sc{unsafe}) For now we recognize a few important ones
Likewise instructions marked as control flow instructions should be retained.

An illustrative example is the following MAL snippet:
@verbatim
	V7 := bat.new(:oid,:int);
	V10 := bat.new(:int,:oid);
	V16 := algebra.markH(V7);
	V17 := algebra.join(V16,V7);
	V19 := bat.new(:oid,:int);
	V22 := bat.new(:oid,:int);
	V23 := algebra.join(V16,V22);
	io.print("done");
	optimizer.deadCodeRemoval();
@end verbatim
The dead code removal trims this program to the following short block:
@verbatim
	io.print("done");
@end verbatim

A refinement of the dead code comes from using arguments
that ceased to exist due to actions taken by an optimizer.
For example, in the snippet below the @sc{pushranges} optimizer
may conclude that variable V31 becomes empty and simply
injects a 'dead' variable by dropping the assignment statement.
This makes other code dead as well.
@example
	V30 := algebra.select( V7, 10,100);
	V31 := algebra.select(V30,-1,5);
	V32 := aggr.sum(V31);
	io.print(V32);
@end example
[implementation pending]
@{
@mal
pattern optimizer.deadcode():str
address OPTdeadcode;
pattern optimizer.deadcode(mod:str, fcn:str):str
address OPTdeadcode
comment "Dead code optimizer";
@h
#ifndef _OPT_DEADCODE_
#define _OPT_DEADCODE_
#include "opt_prelude.h"
#include "opt_support.h"
#include "mal_interpreter.h"
#include "mal_instruction.h"
#include "mal_function.h"

/* #define DEBUG_OPT_DEAD      trace its behavior */

#endif
@c

@c
#include "mal_config.h"
#include "opt_deadcode.h"

static int 
OPTdeadcodeImplementation(MalBlkPtr mb, MalStkPtr stk, InstrPtr pci)
{
	int i, k, se,limit;
	InstrPtr p=0, *old= mb->stmt;
	int actions = 0;

	(void) pci;
	(void) stk;		/* to fool compilers */

	setLifespan(mb);
#ifdef DEBUG_OPT_DEAD
	stream_printf(GDKout,"ENTERING DEAD CODE ELIMINATION\n");
	printFunction(GDKout,mb,LIST_MAL_ALL);
#endif

	limit= mb->stop;
	newMalBlkStmt(mb, mb->stop); /* a new statement stack */

	pushInstruction(mb, old[0]);
	for (i = 1; i < limit; i++) {
		p= old[i];

		se = p->token == ENDsymbol;
		if( se){
			pushInstruction(mb,p);
			for(i++; i<limit; i++)
				pushInstruction(mb,old[i]);
			break;
		}
		if( p->token != NOOPsymbol)
		for (k = 0; k < p->retc; k++)
			if( isVarUsed(mb,getArg(p,k)) ){
				se++;
				break;
			} 

		if (se || hasSideEffects(p, FALSE) )
			pushInstruction(mb,p);
		else {
			freeInstruction(p);
			actions++;
		}
	}
	/* we may have uncovered new use-less operations */
	if( actions) 
		i= OPTdeadcodeImplementation(mb, stk, pci);

#ifdef DEBUG_OPT_DEAD
	stream_printf(GDKout,"LEAVING DEAD CODE ELIMINATION\n");
	printFunction(GDKout,mb,LIST_MAL_ALL);
#endif
	return actions+i;
}
@include optimizerWrapper.mx
@h
@:exportOptimizer(deadcode)@
@c
@:wrapOptimizer(deadcode,OPT_CHECK_ALL)@
@}