Technical data

Driver Options

Optimizing

The default optimizing option,–O1, causes the code generator and assembler

phases of compilation to improve the performance of your executable object.

You can prevent optimization by specifying –O0.

Table 1-6 summarizes the optimizing functions available.

The default option, –O1, causes the code generator and the assembler to

perform basic optimizations such as constant folding, common

subexpression elimination within individual statements, and common

subexpression elimination between statements.

The global optimizer, invoked with the –O2 option, is a single program that

improves the performance of an object program by transforming existing

code into more efﬁcient coding sequences. Although the same optimizer

processes all compiler optimizations, it does distinguish between the

various languages supported by the compiler system programs to take

advantage of the different language semantics involved.

Table 1-6 Optimizer Options

Option Result

–O3 Performs all optimizations, including global register allocation. With

this option, a ucode object ﬁle is created for each Fortran source ﬁle and

left in a .u ﬁle. The newly created ucode object ﬁles, the ucode object

ﬁles speciﬁed on the command lines, the run-time startup routine,

and all of the run-time libraries are ucode linked. Optimization is done

globally on the resulting ucode linked ﬁle, and then it is linked as

normal, producing an a.out ﬁle. No .o ﬁle is left from the ucode linked

result.

–c cannot be speciﬁed with –O3.

–O2 The global optimizer (uopt) phase executes. It performs optimization

only within the bounds of individual compilation units.

–O1 Default option. The code generator and the assembler perform basic

optimizations in a more limited scope.

–O0 No optimization.