[titzer]

For Virgil I went through three different compile-time interpreters. The first walked a tree-like IR that predated SSA. Then, after SSA, I designed a linked-list-like representation specifically for interpretation speed. After dozens of little discrepancies between this custom interpreter and compile output, I finally got rid of it and wrote an interpreter that works directly on the SSA intermediate representation. In the worst case, the SSA interpreter is only 2X slower than the custom interpreter. In the best case, it's faster, and saves a translation step. I feel it is worth it because of the maintenance burden and bugs.