Building Metal C programs with IPA

Starting with z/OS® V1R13 XL C compiler, the IPA option can be used with the METAL option. IPA is an optimization option that enables the compiler to find more optimization opportunities to improve your application performance. For more information about IPA, see the Using the IPA option section in z/OS XL C/C++ Programming Guide and the IPA considerations section in z/OS XL C/C++ User's Guide.

Figure 1. The process of building Metal C programs with IPA

The process of building Metal C programs with IPA flow diagram is shown.

You need to be aware of the following adjustments when invoking IPA for METAL.

The LONGNAME option is in effect by default when IPA is specified.
The IPA compile step only produces IPA object in the output file. Only IPA(NOOBJECT) is allowed, which instructs IPA to stop the compile process after the IPA object is produced. It does not produce HLASM source code, so the GENASM option cannot be used.
The output file from IPA link step is one single HLASM source file for the whole program, and the GENASM option is required. There could be multiple structures in the HLASM source program, one for each partition. Under USS, the output HLASM source file resides in the directory where the IPA link took place. The default output file name for USS is a.s. In BATCH mode, the output HLASM source file goes in the dataset allocated to DD SYSLIN in the IPA link step.
At the IPA link phase, all external references must be resolved. For Metal C, IPA does not attempt to convert external object modules or load modules into object code for the inclusion in the IPA produced program. You need to provide the same set of library data sets to both IPA link and the binder for symbol resolution.
If you specify the PROLOG and EPILOG compiler options to supply your own prolog and epilog macros at compile time, the macros will only be applied to the functions defined in the source file.
If you have #pragma insert_asm in your source file, IPA will assume the strong connection between the string provided by the pragma and the functions in the source file. IPA will not move functions defined in that source file to anywhere else.
If you use global register variable or the RESERVE_REGS option during your compile, IPA link will merge the registers specified in the compile steps and apply the merged set of the originated compilation units to a partition.
If you use the DSAUSER option in any of your compile steps, IPA link applies the option to the entire program.

The following compiler options are not supported by METAL with IPA:

DEBUG
REPORT

The following IPA suboptions are not supported with the METAL option:

ATTRIBUTE
GONUM
PDF suboptions

The following IPA control file directives are not supported with the METAL option:

EXPORT
NOEXPORTS

Example

The following example shows how to compile a Metal C program with IPA.

IPA compile phase:

xlc -qmetal -qipa -c x.c

xlc -qmetal -qipa -c y.c

The above commands produce x.o and y.o.

Notes:

The -c option indicates compile.
No HLASM output is generated.
The objects are IPA objects, which can only be used for IPA link.
LONGNAME is implicitly turned on.

IPA link phase:

xlc -qmetal -qipa -S x.o y.o

This command produces a.s.

Note: The structure of the compiler-generated HLASM source program is similar to that described in Structure of a compiler-generated HLASM source program, except that at IPA link there could be multiple structures in the HLASM source program, one for each partition.

The rest of the build process is similar to building Metal C programs without IPA. You need to add the assembly step to produce the object file from the IPA link generated HLASM source file. You also need to supply the object file produced by the assembler along with all other library data sets to the binder for producing the final executable program.

Assembly phase:

as -mgoff a.s

This command produces a.o.

Note: The HLASM GOFF option must be specified because of the LONGNAME compiler option requirement with IPA.

Bind/Link phase:

ld -b case=mixed -e main a.o

This command produces a.out.

Note: Because of the LONGNAME compiler option requirement with IPA, you should use the -b case=mixed ld utility option and the -e option with the entry point in its original case.