CompCert is a moderately optimizing C compiler with a formal, machine-checked, proof of correctness: after successful compilation, the assembly code has a behavior faithful to the source code. Previously, it only supported target instruction sets with sequential semantics, and did not attempt reordering instructions for optimization.We present here a CompCert backend for a VLIW core (i.e. with explicit parallelism at the instruction level), the first CompCert backend providing scalable and efficient instruction scheduling. Furthermore, its highly modular implementation can be easily adapted to other VLIW or non-VLIW pipelined processors.
On in-order processors, without dynamic instruction scheduling, program running times may be significantly reduced by compile-time instruction scheduling. We present here the first effective certified instruction scheduler that operates over superblocks (it may move instructions across branches), along with its performance evaluation. It is integrated within the CompCert C compiler, providing a complete machinechecked proof of semantic preservation from C to assembly.Our optimizer composes several passes designed by translation validation: program transformations are proposed by untrusted oracles, which are then validated by certified and scalable checkers. Our main checker is an architectureindependent simulation-test over superblocks modulo register liveness, which relies on hash-consed symbolic execution.
We present an approach for implementing a formally certified loop-invariant code motion optimization by composing an unrolling pass and a formally certified yet efficient global subexpression elimination. This approach is lightweight: each pass comes with a simple and independent proof of correctness. Experiments show the approach significantly narrows the performance gap between the CompCert certified compiler and state-of-the-art optimizing compilers. Our static analysis employs an efficient yet verified hashed set structure, resulting in fast compilation.
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