Mikroelektronika MIKROE-724 データシート

mikroBasic PRO for dsPIC30/33 and PIC24

MikroElektronika

161

Introduction

In  compiler  design,  static  single  assignment  form  (often  abbreviated  as  SSA  form  or  SSA)  is  an  intermediate 

representation (IR) in which every variable is assigned exactly once.

An SSA-based compiler modifies the program representation so that every time a variable is assigned in the original 

program, a new version of the variable is created. 

A new version of the variable is distinguished (renamed) by subscripting the variable name with its version number or 

an index, so that every definition of each variable in a program becomes unique. 

At a joining point of the control flow graph where two or more different definitions of a variable meet, a hypothetical 

function called a phi-function is inserted so that these multiple definitions are merged.

In mikroBasic PRO for dsPIC, SSA’s main goal is in allocating local variables into the RX space (instead onto the frame). 

To do that, SSA has to make an alias and data flow analysis of the Control Flow Graph.

Besides these savings, there are a number of compiler optimization algorithms enhanced by the use of SSA, like:

 

- Constant Propagation 

 

- Dead Code Elimination 

 

- Global Value Numbering 

 

- Register Allocation 

Changes  that  SSA  brings  is  also  in  the  way  in  which  routine  parameters  are  passed.  When  the  SSA  is  enabled, 

parameters are passed through a part of the RX space which is reserved exclusively for this purpose (W10-W13 for 

dsPIC). 

Allocating local variables and parameters in RX space has its true meaning for those architectures with hardware frame.

Enabling SSA optimization in compiler is done by checking                                     box from the Output Settings Menu.

Lets consider a trivial case:

program Example

sub procedure SSA_Test(dim y as integer, dim k as integer)

  if (y+k) then

      nop

main:

  SSA_Test(5,5)

  

With SSA enabled, sub procedure 

SSA_Test

 this example is consisted of 3 asm instructions:

;Example.mbas,29 ::                 if (y+k) then

0x0100        0x45000B          ADD        W10, W11, W0