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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 ARM, 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.
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 4 asm instructions :

0x0114	     0x1842           ADD	       R2, R0, R1
0x0116	     0xB212           SXTH	       R2, R2
; y end address is: 0 (R0)
; k end address is: 4 (R1)
0x0118	     0xB102           CBZ	       R2, L__SSA_Test2
;Example.mpas, 8 :: 	      nop;
0x011A	     0xBF00           NOP

Without SSA enabled, procedure SSA_Test is consisted of 6 asm instructions :

0x0114        0xF9BD1004       LDRSH        R1, [SP, #4]
0x0118        0xF9BD0000       LDRSH        R0, [SP, #0]
0x011C        0x1840           ADD          R0, R0, R1
0x011E        0xB200           SXTH         R0, R0
0x0120        0xB100           CBZ          R0, L__SSA_Test2
;Example.mpas, 8 ::              nop;
0x0122        0xBF00           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.

Asm code and SSA optimization

If converting code from an earlier version of the compiler, which consists of mixed asm code with the C code, keep in mind that the generated code can substantially differ when SSA optimization option is enabled or disabled.
This is due to the fact that SSA optimization uses certain working registers to store routine parameters (W10-W13), rather than storing them onto the function frame.

Because of this, user must be very careful when writing asm code as existing values in the working registers used by SSA optimization can be overwritten.
To avoid this, it is recommended that user includes desired asm code in a separate routine.

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 :

begin {
  var y : word;

  if (y) then       // Variable y might not have been initialized
    GPIO_PORTD := 0;
end;

Related topics: Code Optimization, ARM Specifics, mikroPascal PRO for ARM specifics, Memory type specifiers