Vax performance V-008 SP User's Guide download pdf (Page 93)

Improving the Performance of Ported Code

4.2 Code Flow and Branch Prediction

4.2.5 Forward Jumps into Loops

Because of the way that the compiler follows the code ﬂow, a particular case that

may not compile well is a forward unconditional branch into a loop. The code

generated for this case usually splits the loop into two widely separated pieces.

For example, consider the following macro coding construct:

(Allocate a data block and set up initial pointer)

BRB 20$

10$: (Move block pointer to next section to be moved)

20$: (Move block of data)

(Test - is there more to move?)

(Yes, branch to 10$)

(Remainder of routine)

The MACRO compiler will follow the BRB instruction when generating the

code ﬂow and will then fall through the subsequent conditional branch to 10$.

However, because the code at 10$ was skipped over by the BRB instruction, it

will not be generated until after the end of the routine. This will convert the

conditional branch into a forward branch instead of a backward branch. The

generated code layout will look like the following:

(Allocate a data block and set up initial pointer)

20$: (Move block of data)

(Test - is there more to move?)

(Yes, branch to 10$)

(Remainder of routine)

(Routine exit)

10$: (Move block pointer to next section to be moved)

BRB 20$

This results in the loop being very slow because the branch to 10$ is always

predicted not taken, and the code ﬂow has to keep going back and forth between

the two locations. This situation can be ﬁxed by inserting a .BRANCH_LIKELY

directive before the conditional branch back to 10$. This will result in the

following code ﬂow:

(Allocate a data block and set up initial pointer)

20$: (Move block of data)

(Test - is there more to move?)

(No, branch to $L1)

10$: (Move block pointer to next section to be moved)

BRB 20$

$L1:

(Remainder of routine)

4.3 Code Optimization

The MACRO compiler performs several optimizations on the generated code. It

performs all of them by default except VAXREGS. (VAXREGS is OpenVMS Alpha

systems only.) You can change these default values with the /OPTIMIZE switch

on the command line. The valid options are:

• ADDRESSES

The compiler recognizes that the same address is referenced multiple times,

and only loads the address once for use by multiple references.

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Vax performance V-008 SP User's Guide Page 93

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