Can You Use Context-Local Labels within GNU Assembler Macros?
Problem: You want to define labels within GNU Assembler (GAS) macros that are local to the macro's invocation, avoiding potential name collisions with other macros or the surrounding code.
Rephrased: Imagine you have a reusable code snippet (a macro) that needs internal labels for branching or referencing instructions. How do you make sure these labels are unique and don't clash with other labels in your code?
The Dilemma: GAS macros don't inherently support true context-local labels. When a macro is expanded, its labels become part of the main code, potentially causing conflicts.
Illustrative Example:
.macro my_macro
.local my_label
my_label:
jmp my_label
.endm
.macro my_other_macro
.local my_label
my_label:
jmp my_label
.endm
my_macro
my_other_macro
In this code, both macros use my_label. If we were to assemble it, we'd get an error due to the conflicting label definition.
Solutions and Workarounds:
While GAS doesn't have built-in context-local labels, we can use these workarounds:
- Prefixing: The most common approach is to prepend a unique identifier to your macro labels. This identifier can be the macro name or a custom prefix.
.macro my_macro
my_macro_label:
jmp my_macro_label
.endm
.macro my_other_macro
my_other_macro_label:
jmp my_other_macro_label
.endm
my_macro
my_other_macro
- Unique Label Generation: If you need a large number of labels within your macro, consider generating unique labels using macro parameters or the
$symbol for automatically generated unique names.
.macro my_macro label_suffix
my_macro_label\label_suffix:
jmp my_macro_label\label_suffix
.endm
my_macro _1
my_macro _2
- Local Symbol Tables: GAS supports local symbol tables. While not directly applicable to individual labels within macros, you can define a local symbol table for your macro and define all its internal labels within this table.
.macro my_macro
.local my_symbol_table
.set my_symbol_table = .
my_symbol_table:
my_label:
jmp my_label
.endm
my_macro
Important Notes:
- Context-local labels are a common request in assembly languages. Several assembly languages offer this feature, but GAS currently doesn't.
- Be mindful of the scope of your labels. In the examples above, the labels are local to the macro's expansion, not to individual calls.
- Use a consistent naming convention to make your code easier to understand and maintain.
References:
- GNU Assembler Manual: https://sourceware.org/binutils/docs-2.36/as/
- Stack Overflow: How to create local labels in a GAS macro?
Conclusion:
While GAS doesn't directly support context-local labels within macros, the techniques discussed provide effective workarounds. Choosing the appropriate approach depends on the complexity of your macros and your coding style. By employing these methods, you can maintain code clarity and prevent label collisions within your assembly programs.
