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#23 in Hardware support

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0BSD license

105K SLoC


crate documentation

x86 decoders implemented as part of the yaxpeax project, implementing traits provided by yaxpeax-arch.

Rust users of this library will either want to use the quick and dirty APIs, or more generic decode interfaces from yaxpeax-arch - appropriate when mixing yaxpeax-x86 usage with other yaxpeax decoders, such as yaxpeax-arm. examples of both styles are provided in the documentation.

the ffi/ directory provides a repackaging of yaxpeax-x86 suitable for use by non-Rust callers, such as C or C++. see the examples directory for FFI usage of this library.


  • #[no_std]
  • configurable instruction set extensions
  • very fast
  • pretty small?


the decoders provided by yaxpeax-x86 are designed to be usable in a no_std setting, and does so by default. to build yaxpeax_x86 without std, add the parameter default-features = false to your yaxpeax-x86 dependency; the ffi packaging of yaxpeax_x86 does this and builds without the Rust standard library as well. serde can be enabled without std, but json serialization/deserialization need some careful attention in that mode. as well as the colors feature to render instructions with default (eg terminal-friendly) syntax highlighting.

instruction set extensions

yaxpeax-x86 decoders provide the option to specify what instruction set extensions are eligible when decoding, to support decoding x86 instructions as understood by a particular microarchitecture. the default impls of decoders in yaxpeax_x86 take an optimistic approach to decoding and assumes all feature sets are available, as well as accepting both intel-specific and amd-specific quirks around undefined encodings.

yaxpeax-x86 decodes long-mode (amd64/x86_64), protected-mode (x86/x86_32), and real-mode (x86_16) instructions. the most part, ISA extensions decode equivalently across modes; this is the full list of extensions that are supported:

3dnow*, sse*, sse2*, sse3, ssse3, sse4.1, sse4.2, sse4a, avx, avx2, avx512**, syscall, cmpxchg16b, fma3, aesni, popcnt, rdrand, xsave, sgx, monitor, movbe, sgx, bmi1, bmi2, invpcid, mpx, adx, clflushopt, pcommit, sha, gfni, pclmulqdq, rdtscp, abm, xop, skinit, tbm, svm, f16c, fma4, tsx, enqcmd***, uintr***, keylocker***, store_direct***, cet***, sev/snp***

*: 3dnow, sse, and sse2 are non-optional in x86_64, so it is not permitted to construct a decoder that rejects them. x86_32 and x86_16 could have features to reject these instructions for true 8086 and i386 compatibility, but currently do not.

**: avx512 is fully supported, but decoders rejecting subgroups of the avx512 family are not. if you need granular avx512 compatibility controls, please file an issue.

***: i ran out of space for feature bits. InstDecoder is currently a u64 and all 64 bits are used for x86 features mapping to cpuid bits. supporting these as optional instructions would require growing this to a pair of u64. since the typical case is to decode everything, these are decoded regardless of InstDecoder settings. growing InstDecoder to an u128 is likely acceptable, but has not yet been profiled.

very fast

when hooked up to disas-bench, yaxpeax_x86::long_mode has shown roughly 250mb/s decode throughput and on some hardware is the fastest software x86 decoder available. the likely path through the decoder, through `<yaxpeax_x86::amd64::InstDecoder as yaxpeax_arch::Decoder>::decode_into``, is an average of 58 cycles on a zen2 core.

while there is an in-repo benchmark, i've decided it's so unrealistic as to be unuseful, and prefer disas-bench until it can be made more informative.

pretty small?

yaxpeax_x86::long_mode built on its own is around 143kb of code and data. with data for instruction formatting, this grows to 249kb. while code size can be shrunk some, most of the crate is a few lookup tables - the hot path through yaxpeax-x86's decode logic stays in functions coming out to ~5 kilobytes of code, and lots of supporting logic for less likely instructions.

yaxpeax_x86 may be the smallest library for tasks focused entirely on decoding and instruction formatting, but this crate doesn't have extensive testing to that end.


the canonical copy of yaxpeax-x86 is at https://git.iximeow.net/yaxpeax-x86/.

yaxpeax-x86 is also mirrored on GitHub at https://www.github.com/iximeow/yaxpeax-x86.


yaxpeax_x86 makes regular use of unsafe { unreachable_unchecked(); } and occasional use of unsafe { _.get_unchecked() } for purely performance reasons. yaxpeax_x86 is fuzzed via mishegos and has passed multiple days of fuzzing without issue.


a changelog across crate versions is maintained in the CHANGELOG file located in the repo, as well as online.


unfortunately, pushing commits to the canonical repo at git.iximeow.net is impossible. if you'd like to contribute - thank you! - please send patches to emails iximeow has committed under or by opening PRs against the GitHub mirror. both remotes are kept in sync.

see also

iced is another very good x86_64 decoder, also written in rust. it provides additional information about instruction semantics as part of the crate, as well as the ability to re-encode instructions.

disas-bench, a handy benchmark of several x86_64 decoders including yaxpeax-x86.

mishegos, a differential fuzzer that has made testing the correctness of yaxpeax-x86 much easier.


~14K SLoC