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/*
* phc -- the open source PHP compiler
* See doc/license/README.license for licensing information
*
* The lowering passes introduce a lot of unnecessary copies, which
* add extra reference counts, resulting in extra copying and poor
* performance.
*
* HACK TODO: Remove.
* This is exactly the sort of hacky thing we wanted
* to remove. This would be much better done with a data-flow
* framework, but is much easier than cleaning up the passes, which
* would otherwise be necessary.
*
* Since we do not have proper data-flow, we are going to be very
* conservative, using the following assumptions:
*
* - Only do this for compiler generated temporaries, this is not
* for optimization of user-code, just as a clean-up.
*
* - Simple example: :
* 1.) $L1 = $R1; // replaceable
* 2.) $L2 = $L1; // removable
* becomes:
* 1.) $L2 = $R1; // replaced
*
* - This is not the same as:
* 1.) $L1 = $R1;
* 2.) $L2 = $L1;
* becoming:
* 2.) $L2 = $R1; // $R1 might have been assigned to in-between
*
* - $L1 must be a simple variable, and must be annotated with
* phc.codegen.compiler_generated
*
* - Statement 1. must occur after statement 2.
*
* - Strange cases which break it, shouldn't happen because we only
* use this for the restricted set of compiler-generated
* temporaries.
*
* The following restrictions may be temporary:
*
* - Both the assignments involved must be of the form
* $R = $L
* and must not be by reference, or involve array_indices.
* - $L1 must be assigned exactly once.
* - $L1 must be used exactly once - in the assignment to $L2.
* - $L2 is assigned exactly once, by $R1
* - We only work in methods, not in the global statement list
*/
#ifndef PHC_COPY_PROPAGATION
#define PHC_COPY_PROPAGATION
// TODO assignments by reference
// TODO function calls by reference
/*
foreach function as f:
// count uses and defs
pre_var { var_count [var] += 1; }
pre_assignment { def_count [var] += 1}
// make a use count from assignments
foreach var_count, def_count:
use_count [var] = var_count [var] - def_count [var]
// find potential statements
stmts = f->stmts
pre_stmt
{
if s.is_simple_assignment
l1 = s.lhs
if (l1.is_compiler_var):
assert replaceable [l1] == NULL
replaceable [l1] = s
// we can remove this if there is a matching replaceable
// statement
if replaceable [l1]
&& use_count [l1] == 1
&& def_count [l1] == 1
&& def_count [l2] == 1
replaceable [l1].rhs = l2
erase s
}
*/
#include "Fix_point.h"
#include "lib/Map.h"
#include "Use_def_counter.h"
class Copy_propagation : public Fix_point
{
private:
Map<string, HIR::Assign_var*> replaceable;
public:
Copy_propagation ();
void children_php_script (HIR::PHP_script* in);
void pre_assign_var (HIR::Assign_var* in, HIR::Statement_list* out);
};
#endif // PHC_COPY_PROPAGATION
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