The ability to directly reason about binary machine code is desirable, not only because it allows analyzing binaries even when the source code is not available (e.g., legacy code, closed-source software, or malware), but also avoids the need to trust the correctness of compilers. Binary analysis is generally performed by existing decompiler projects by (1) converting raw bytes from the binary into a stream of assembly instructions through disassembly, (2) translating machine code to an intermediate representation (IR) using a binary lifter, and (3) performing various analysis and transformations on the IR pertaining to the specific goals of the decompiler. Many binary analysis frameworks published in academia or as open-source code, use such a lifter as the first step in their pipeline. Validating the correctness of binary lifters is pivotal to gain trust in binary analysis, especially when used in scenarios where correctness is essential. Unfortunately, existing approaches focus on validating the correctness of lifting a single instruction and do not scale to full programs. I believe an effort in the direction would enable both the developers of binary translators, to validate their implementation, and the clients of those translators, to gain trust in their analysis results. The overall goal of my work is to develop formal and informal techniques to achieve high confidence in the correctness of binary lifting by leveraging the semantics of the languages involved (e.g., Intel's x86-64 and LLVM IR). Towards that goal, I made two broad contributions. First, I defined the most complete and thoroughly tested formal semantics of x86-64 to date. The semantics faithfully formalizes all the non-deprecated, sequential user-level instructions of the x86-64 Haswell instruction set architecture. The formal specification covers 3155 instruction variants, corresponding to 774 mnemonics. The semantics is fully executable and has been tested against the GCC C-torture test suite. Moreover, each instruction is individually tested against more than 7,000 instruction-level test cases. This extensive testing paid off, revealing bugs in both the x86-64 reference manual and other existing semantics, which are all acknowledged, and some are fixed. Also, I illustrated potential applications of the semantics in different formal analyses, and discuss how it can be useful for processor verification. Second, I show that formal translation validation of single instructions for a complex ISA like x86-64 is not only practical but can be used as a building block for scalable full-program validation. My work is the first to do translation validation of single instructions on an Coming up to this point is one of the biggest challenges I have ever pursued in my life. This thesis would not have been possible if I did not have the support of the following people. I owe my deepest gratitude to my adviser, Professor Vikram Adve, whose guidance, patience, and encouragement have been pivotal in the successful completion of this thesis. He is one of the ...

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