Viewing file:  Copy_propagation.cpp (2.96 KB) -rw-rw-r--
Select action/file-type:
 (+) |  (+) |  (+) | Code (+) | Session (+) |  (+) | SDB (+) |  (+) |  (+) |  (+) |  (+) |  (+) |
/*
* 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
*/
#include "Copy_propagation.h"
#include "Use_def_counter.h"
#include "process_ir/General.h"
#include "HIR_visitor.h"
using namespace HIR;
Copy_propagation::Copy_propagation ()
{
}
void Copy_propagation::children_php_script (PHP_script* in)
{
// count uses and defs
in->visit (new Use_def_counter);
Transform::children_php_script (in);
in->visit (new Clear_use_defs);
}
void Copy_propagation::pre_assign_var (Assign_var* in, Statement_list* out)
{
xdebug (in);
debug (in);
if (!isa<HIR::VARIABLE_NAME> (in->rhs))
{
out->push_back (in);
return;
}
VARIABLE_NAME* lhs = in->lhs;
VARIABLE_NAME* rhs = dyc<VARIABLE_NAME> (in->rhs);
xadebug (lhs);
xadebug (rhs);
DEBUG ("is simple assignment");
string slhs = *lhs->value;
string srhs = *rhs->value;
// add to the list of for future consideration
if (lhs->attrs->is_true ("phc.codegen.compiler_generated"))
{
DEBUG ("lhs is compiler generated");
replaceable [slhs] = in;
}
// consider for immediate removal
if (replaceable.has (srhs)
&& rhs->attrs->get_integer ("phc.use_defs.use_count")->value () == 1
&& rhs->attrs->get_integer ("phc.use_defs.def_count")->value () == 1)
{
CTS ("copy propagated");
DEBUG ("rhs is replacable");
replaceable [srhs]->lhs = lhs->clone ();
// note lack of out->push_back (in);
iterate_again = true;
return;
}
out->push_back (in);
}
|