9.2.1 Dead-Code Elimination

KAP can delete unused lines or sections of code from the transformed code. This can speed up a program by eliminating unnecessary operations and by not using valuable cache space for unused instructions or data. The dead-code eliminations are as follows:

• Unreachable code is removed.

• Empty loops and IF blocks are deleted.

• IFs with constant conditions are simplified.

• Unused definitions are removed.

Examples of dead-code eliminations are as follows:

1. Unreachable code removal:

   GO TO 10
   X = 2    <-- unreachable assignment removed
   10    Y = 13
   

2. Removal of zero trip and empty DO loops:

   DO 20 I = 10, 2 <-- zero trip DO
   ...
   20    CONTINUE
   

3. Elimination of resolved conditionals:

   PARAMETER( N = 12 )
   ...
   IF (N .GT. 10) THEN
   X = 1.
   ELSE
   X = 2.
   ENDIF
   

   Is transformed to:

   PARAMETER( N = 12 )
   ...
   X = 1.
   

   The true branch will always be taken.

4. The removal of unprofitable code.

   KAP performs lifetime analysis to determine the reaching definitions of variables and removes unused definitions, subject to overriding factors such as variables involved in SAVE, EXTERNAL, and COMMON statements, and so on. The /scalaropt > 2 qualifier is required.

   Y = 5  <-- no subsequent use of Y
   X = 3  <-- redefinition of X
   X = 4
   PRINT*, X
   END
   

The full effect of dead-code elimination is realized when combined with other optimizations such as subprogram inlining and forward substitution, which help in exposing useless or unreachable code.

Copyright © Digital Equipment Corporation. 1999. All Rights Reserved.