Single Static Assignment Optimization   mikroPascal PRO for dsPIC30/33 and PIC24 Specifics >

Single Static Assignment Optimization

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 mikroPascal 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;

procedure SSA_Test(y : integer; k : integer);

begin
  if (y+k) then
    asm
      nop;
    end
end;

begin
  SSA_Test(5,5);
end.

With SSA enabled, procedure SSA_Test is consisted of 3 asm instructions :

;Example.mpas,34 ::    if (y+k) then
0x0100  0x45000B    ADD W10, W11, W0
0x0102  0x320001    BRA Z L__SSA2
L__SSA_Test6:
;Example.mpas,36 ::    nop;
0x0104  0x000000    NOP

Without SSA enabled, procedure SSA_Test is consisted of 5 asm instructions :

;Example.mpas,34 ::    if (y+k) then
0x0102	0x97B8CE    MOV	[W14-8], W1
0x0104	0x57006A    SUB	W14, #10, W0
0x0106	0x408010    ADD	W1, [W0], W0
0x0108	0x320001    BRA Z	L__SSA2
L__SSA_Test6:
;Example.mpas,36 ::    nop;
0x010A	0x000000    NOP

Proper Coding Recommendations

To get the maximum out of the SSA, user should regard the following rules during the coding process :

• Routines should not contain too many parameters (not more than 4 words).
• Don't change the value of the parameter in the function body (it is better to use a new local variable).
• If the function1 parameters are passed as function2 parameters, then parameter order should remain the same :

  procedure f2(a: integer; b: integer;) { }
  
  procedure f1(x: integer; y: integer;) {
    // routine call
    f2(x,y);  // x->a and y->b (1 to 1 and 2 to 2) is far more efficient than :
    f2(y,x);  // y->a and x->b (1 to 2 and 2 to 1)
  }
  

• Large amount of nested loops and complex structures as its members should be avoided.
• When writing a code in assembly, keep in mind that there are registers reserved exclusively for routine parameters.
• Using goto and label statements in nested loops should be avoided.
• Obtaining address of the local variable with the global pointer and using it to alter the variable's address should be avoided.

Debugging Notes

SSA also influences the code debugging in such a way that the local variables will be available in the Watch Window only in those parts of the procedure where they have useful value (eg. on entering the procedure, variable isn't available until its definition).
Variables can be allocated in one part of the procedure in register W4, and in another part of the procedure in register W2, if the optimizer estimates that it is better that way. That means that the local variable has no static address.

Warning Messages Enhancement

Besides the smaller code, SSA also deals with the intensive code analysis, which in turn has the consequence in enhancing the warning messages.
For example, compiler will warn the user that the uninitialized variable is used :

void main() {
  int y;

  if (y)        // Variable y might not have been initialized
    PORTD = 0;
}    

Related topics: Code Optimization, dsPIC Specifics, mikroPascal PRO for dsPIC30/33 and PIC24 specifics, Memory type specifiers