LLVM’s backends translate its intermediate representation (IR) to assembly or object code. Alongside register allocation and instruction selection, these backends contain many analogues of components traditionally associated with compiler middle ends: dataflow analyses, common subexpression elimination, loop invariant code motion, and a first-class IR—MIR, the “machine IR.” In effect, this kind of compiler backend is a highly optimizing compiler in its own right, with all of the correctness hazards entailed by a million lines of intricate C++. As a step towards gaining confidence in the correctness of work done by LLVM backends, we have created arm-tv, which formally verifies translations between LLVM IR and AArch64 (64-bit ARM) code. Ours is not the first translation validation work for LLVM, but we have advanced the state of the art along multiple fronts: arm-tv is a checking validator that enforces numerous ABI rules; we have extended Alive2 (which we reuse as a verification backend) to deal with unstructured mixes of pointers and integers that are typical of assembly code; we investigate the tradeoffs between hand-written AArch64 semantics and those derived mechanically from ARM’s published formal semantics; and, we have used arm-tv to discover 45 previously unknown miscompilation bugs in this LLVM backend, most of which are now fixed in upstream LLVM.

A superoptimizing compiler—one that performs a meaningful search of the program space as part of the optimization process—can find optimization opportunities that are missed by even the best existing optimizing compilers. We created Minotaur: a superoptimizer for LLVM that uses program synthesis to improve its code generation, focusing on integer and floating-point SIMD code. On an Intel Cascade Lake processor, Minotaur achieves an average speedup of 7.3% on the GNU Multiple Precision library (GMP)’s benchmark suite, with a maximum speedup of 13%. On SPEC CPU 2017, our superoptimizer produces an average speedup of 1.5%, with a maximum speedup of 4.5% for 638.imagick. Every optimization produced by Minotaur has been formally verified, and several optimizations that it has discovered have been implemented in LLVM as a result of our work.