Article by Ayman Alheraki on July 3 2025 10:39 AM
Label resolution and PC-relative addressing are critical components in the design of an x86-64 assembler, deeply affecting code generation, performance, and relocation. Labels serve as symbolic placeholders for instruction addresses, enabling flexible program flow control through jumps, calls, and data references. PC-relative addressing, a prevalent addressing mode on x86-64, allows instructions to specify targets relative to the current instruction pointer (RIP), optimizing code size and enabling position-independent code (PIC).
This section delves into the mechanisms for resolving labels and generating correct PC-relative encodings, highlighting modern design approaches and challenges in assembler implementation.
Labels in assembly code mark locations in the instruction stream or data sections and are referred to by symbolic names. The assembler must:
Determine the exact byte offset of each label relative to the start of the section or segment.
Replace symbolic references to labels with concrete addresses or offsets during code generation.
Because labels may be referenced before their definition (forward references), the assembler must maintain an internal mapping between labels and their resolved addresses or offsets. This mapping is typically stored in the symbol table with associated metadata.
Backward references to labels are straightforward, as the assembler knows the target address from prior parsing.
Forward references require deferred resolution. The assembler records all instruction locations that reference the label and patches the final addresses once the label definition is processed. This technique is often referred to as backpatching.
Efficient data structures like fixup lists or relocation records are used to track unresolved label references during assembly.
PC-relative addressing on x86-64 is centered around the RIP (instruction pointer) register, introduced in the 64-bit architecture to facilitate position-independent code and reduce instruction size. Instead of encoding absolute 64-bit addresses (which would increase code size), instructions specify the target address as an offset relative to the next instruction’s address.
Key points about PC-relative addressing:
The effective target address is calculated as RIP + offset.
Offsets are typically signed 32-bit values, allowing jumps within ±2GB from the current instruction.
PC-relative addressing is used in control flow instructions (e.g., jmp, call, conditional jumps) and in RIP-relative data addressing (e.g., mov from memory addressed relative to RIP).
During code generation, the assembler must:
Calculate the relative offset between the instruction following the current instruction and the target label address.
Encode this offset in the instruction’s immediate field, usually a 32-bit signed value.
Validate that the offset fits within the allowed range; if not, the assembler may need to generate longer sequences (e.g., absolute jumps via indirect memory operands).
This requires precise tracking of instruction sizes and label positions throughout assembly.
Challenges include:
Out-of-range offsets: Some labels may be too far for a 32-bit relative offset. Modern assemblers must detect this and apply alternative instruction sequences or linker-assisted relaxation techniques.
Instruction size variability: Certain instructions vary in size depending on operands or prefixes, complicating offset calculation during initial passes.
Position-independent code (PIC) generation: PC-relative addressing is essential for PIC, especially in shared libraries. The assembler must ensure all relevant data and control references use RIP-relative addressing.
Solutions include:
Multi-pass assembly or fixup and relaxation phases to iteratively adjust instruction sizes and recalculate offsets.
Use of linker support for resolving and relaxing long jumps.
Careful management of symbol tables and relocation entries to enable proper linkage and runtime address resolution.
Labels may have varying scopes impacting resolution:
Local labels scoped within a function or section are typically resolved relative to their containing scope, reducing symbol table size and improving lookup speed.
Global labels must be uniquely identifiable across translation units, requiring full symbol table entries and linkage metadata.
Assemblers may support naming conventions or symbol attributes to differentiate local from global labels efficiently.
Recent assembler implementations incorporate the following:
Incremental label resolution: Allowing partial assembly and immediate resolution of local labels to speed up incremental builds or interactive development.
On-demand backpatching: Delaying offset calculation until all label addresses are stable, optimizing for code size and enabling advanced instruction relaxation techniques.
Enhanced error detection: Early identification of undefined labels or out-of-range offsets with precise diagnostic messages to improve developer feedback.
Label resolution is essential for converting symbolic references to concrete addresses, especially when forward references occur.
PC-relative addressing in x86-64 relies on 32-bit signed offsets relative to the instruction pointer, supporting efficient and position-independent code.
Handling out-of-range offsets and instruction size variability requires sophisticated assembler logic, often involving multiple passes or linker cooperation.
Proper management of label scope and visibility is necessary to maintain symbol resolution correctness and efficiency.
Modern assemblers adopt flexible, incremental strategies and error detection to meet the demands of contemporary development environments.
