LLVM's Optional Rich Disassembly Output

Introduction

LLVM's default disassembly output is raw text. To allow consumers more ability to introspect the instructions' textual representation or to reformat for a more user friendly display there is an optional rich disassembly output.

This optional output is sufficient to reference into individual portions of the instruction text. This is intended for clients like disassemblers, list file generators, and pretty-printers, which need more than the raw instructions and the ability to print them.

To provide this functionality the assembly text is marked up with annotations. The markup is simple enough in syntax to be robust even in the case of version mismatches between consumers and producers. That is, the syntax generally does not carry semantics beyond "this text has an annotation," so consumers can simply ignore annotations they do not understand or do not care about.

After calling LLVMCreateDisasm() to create a disassembler context the optional output is enable with this call:

LLVMSetDisasmOptions(DC, LLVMDisassembler_Option_UseMarkup);

Then subsequent calls to LLVMDisasmInstruction() will return output strings with the marked up annotations.

Instruction Annotations

Contextual markups

Annotated assembly display will supply contextual markup to help clients more efficiently implement things like pretty printers. Most markup will be target independent, so clients can effectively provide good display without any target specific knowledge.

Annotated assembly goes through the normal instruction printer, but optionally includes contextual tags on portions of the instruction string. An annotation is any '<' '>' delimited section of text(1).

annotation: '<' tag-name tag-modifier-list ':' annotated-text '>'
tag-name: identifier
tag-modifier-list: comma delimited identifier list

The tag-name is an identifier which gives the type of the annotation. For the first pass, this will be very simple, with memory references, registers, and immediates having the tag names "mem", "reg", and "imm", respectively.

The tag-modifier-list is typically additional target-specific context, such as register class.

Clients should accept and ignore any tag-names or tag-modifiers they do not understand, allowing the annotations to grow in richness without breaking older clients.

For example, a possible annotation of an ARM load of a stack-relative location might be annotated as:

ldr <reg gpr:r0>, <mem regoffset:[<reg gpr:sp>, <imm:#4>]>

1: For assembly dialects in which '<' and/or '>' are legal tokens, a literal token is escaped by following immediately with a repeat of the character. For example, a literal '<' character is output as '<<' in an annotated assembly string.

C API Details

The intended consumers of this information use the C API, therefore the new C API function for the disassembler will be added to provide an option to produce disassembled instructions with annotations, LLVMSetDisasmOptions() and the LLVMDisassembler_Option_UseMarkup option (see above).