LLVM integrated assembler: Improving MCExpr and MCValue

Table of Contents

In my previous post, Relocation Generation in Assemblers, I explored some key concepts behind LLVM’s integrated assemblers. This post dives into recent improvements I’ve made to refine that system.

The LLVM integrated assembler handles fixups and relocatable expressions as distinct entities. Relocatable expressions, in particular, are encoded using the MCValue class, which originally looked like this:

class MCValue {
  const MCSymbolRefExpr *SymA = nullptr, *SymB = nullptr;
  int64_t Cst = 0;
  uint32_t RefKind = 0;
};

In this structure:

  • RefKind acts as an optional relocation specifier, though only a handful of targets actually use it.
  • SymA represents an optional symbol reference (the addend).
  • SymB represents another optional symbol reference (the subtrahend).
  • Cst holds a constant value.

While functional, this design had its flaws. For one, the way relocation specifiers were encoded varied across architectures:

  • Targets like COFF, Mach-O, and ELF's PowerPC, SystemZ, and X86 embed the relocation specifier within MCSymbolRefExpr *SymA as part of SubclassData.
  • Conversely, ELF targets such as AArch64, MIPS, and RISC-V store it as a target-specific subclass of MCTargetExpr, and convert it to MCValue::RefKind during MCValue::evaluateAsRelocatable.

Another issue was with SymB. Despite being typed as const MCSymbolRefExpr *, its MCSymbolRefExpr::VariantKind field went unused. This is because expressions like add - sub@got are not relocatable.

Over the weekend, I tackled these inconsistencies and reworked the representation into something cleaner:

class MCValue {
  const MCSymbol *SymA = nullptr, *SymB = nullptr;
  int64_t Cst = 0;
  uint32_t Specifier = 0;
};

This updated design not only aligns more closely with the concept of relocatable expressions but also shaves off some compiler time in LLVM. The ambiguous RefKind has been renamed to Specifier for clarity. Additionally, targets that previously encoded the relocation specifier within MCSymbolRefExpr (rather than using MCTargetExpr) can now access it directly via MCValue::Specifier.

To support this change, I made a few adjustments:

Streamlining Mach-O support

Mach-O assembler support in LLVM has accumulated significant technical debt, impacting both target-specific and generic code. One particularly nagging issue was the const SectionAddrMap *Addrs parameter in MCExpr::evaluateAs* functions. This parameter existed to handle cross-section label differences, primarily for generating (compact) unwind information in Mach-O. A typical example of this can be seen in assembly like:

        .section        __TEXT,__text,regular,pure_instructions
Leh_func_begin0:
        .section        __TEXT,__eh_frame,coalesced,no_toc+strip_static_syms+live_support
Ltmp3:
Ltmp4 = Leh_func_begin0-Ltmp3
        .long   Ltmp4

The SectionAddrMap *Addrs parameter always felt like a clunky workaround to me. It wasn’t until I dug into the Mach-O AArch64 object writer that I realized this hack wasn't necessary for that writer. This discovery prompted a cleanup effort to remove the dependency on SectionAddrMap for ARM and X86 and eliminate the parameter:

While I was at it, I also tidied up MCSymbolRefExpr by removing the clunky HasSubsectionsViaSymbolsBit, further simplifying the codebase.

Stremlining InstPrinter

The MCExpr code also determines how expression operands in assembly instructions are printed. I have made improvements in this area as well: