Characteristics of compiler-generated HLASM source code

Any assembly instructions that you provide need to work with the instructions that are generated by the compiler. Before you provide those instructions, you need to be aware of the characteristics of compiler-generated HLASM source code.

You need to be aware that:

Because the compiler uses relative-branching instructions, it is not necessary to establish code base registers. When the compiler detects user-embedded assembly statements, you can use the IEABRCX DEFINE instruction to assist any branching instructions that might rely on establishment of a code base register. For other instructions (such as LA or EX) that rely on the establishment of a code base register, you might need to add code to establish your own code base register. To disable the effect of IEABRCX, you can add the instruction IEABRCX DISABLE. For more information about the IEABRCX macro, see z/OS MVS Programming: Authorized Assembler Services Reference EDT-IXG.
If the compiler needs to produce literals, GPR 3 will be set up as the base register to address the literals. This addressability is established after the prolog code. The literals are organized by the LTORG instruction placed at the end of the epilog code. With the presence of user-embedded assembly statements, the compiler assumes there will be literals and establishes GPR 3 to address those literals.
If you want code to be naturally reentrant, you must not use writable static or external variables; such variables are part of the code.
There is only one CSECT for each compilation unit. The CSECT name is controlled by the CSECT option.
Due to the flat name space and the case insensitivity required by HLASM, the compiler prepends extra qualifiers to user names to maintain the uniqueness of each name seen by HLASM. This is referred to as name encoding. External symbols are not subject to the name-encoding scheme as they need to be referenced by the exact symbol names.
The external symbols are determined by the compiler LONGNAME option.
- If the NOLONGNAME option is in effect:
  - All external symbols are truncated to eight characters.
  - All external symbols are converted to upper case.
  - All "_" characters are replaced with the "@" character.
- If the LONGNAME option is in effect the compiler emits an ALIAS instruction to make the real C name externally visible. Because the length limit supported by the ALIAS instruction depends on the HLASM release, the C compiler does not enforce any length limit here.
  Note: The HLASM GOFF option is necessary to allow the ALIAS instructions to be recognized. See Figure 3.
GPR 13 is established as the base of the stack space.
GPR 10 and GPR 11 may be used exclusively to address static and constant data. They should not be used in the user-embedded assembly statements.
The compiler will generate code to preserve FPR 8 through FPR 15 if they are altered by the function.
For AMODE 31 functions: The compiler will generate code to preserve the high halves of the 64-bit GPRs if they are altered or if there are user-embedded assembly statements.
The addressing mode is determined by the compiler option. When the compiler option LP64 is in effect, the addressing mode is AMODE 64; otherwise it is AMODE 31.