User`s guide

Improving the Performance of Ported Code

4.3 Code Optimization

• REFERENCES

The compiler recognizes that the same data value is referenced multiple

times, and only loads the data once for use by multiple references, subject to

restrictions to ensure that the data being used is not stale.

• PEEPHOLE

The compiler identiﬁes instruction sequences that can be identically

performed by smaller instruction sequences, and replaces the longer

sequences with the shorter ones.

• SCHEDULING

The compiler uses its knowledge of the nature of the multiple instruction

issue ability of the Alpha and Itanium architectures to reschedule the code

for optimum performance.

• VAXREGS (Alpha systems only)

By default, the registers from R13 through R28 may be used as temporary

scratch registers by the compiler if they are not used in the source code.

When VAXREGS is speciﬁed, the compiler may also use any of the VAX

source code. VAX registers used in this way will be restored to their original

values at routine exit unless declared SCRATCH.

Note

Debugging is simpliﬁed if you specify /NOOPTIMIZE, because the

optimizations include relocating and rescheduling code. For more

information, see Section 2.13.1.

4.3.1 Using the VAXREGS Optimization (OpenVMS Alpha only)

To use the VAXREGS optimization, you must ensure that all routines correctly

declare their register usage in their .CALL_ENTRY, .JSB_ENTRY, or .JSB32_

ENTRY routine declarations. In addition, you must identify any VAX registers

that are required or modiﬁed by any routines that are called. By default, the

compiler assumes that no VAX registers are required as input to any called

routine, and that all VAX registers except R0 and R1 are preserved across

the call. To declare this usage, use the READ and WRITTEN qualiﬁers to the

compiler directive .SET_REGISTERS. For example:

.SET_REGISTERS READ=<R3,R4>, WRITTEN=R5

JSB DO_SOMETHING_USEFUL

In this example, the compiler will assume that R3 and R4 are required inputs

to the routine DO_SOMETHING_USEFUL, and that R5 is overwritten by the

routine. The register usage can be determined by using the input mask of DO_

SOMETHING_USEFUL as the READ qualiﬁer, and the combined output and

scratch masks as the WRITE qualiﬁer.

Note

Using the VAXREGS qualiﬁer without correct register declaration for both

routine entry points and routine calls will produce incorrect code.

4–8 Improving the Performance of Ported Code