To effectively exploit instruction level parallelism, the compiler must move instructions across branches. Speculative execution refers to moving an instruction above a conditional branch that the instruction is control dependent upon. It is speculative execution since the instruction is executed before it is known whether or not its result is needed. There are potential hazards when speculatively executing instructions. If these hazards can be eliminated, the compiler can more aggressively schedule the code. The hazards of speculative execution are outlined in this paper. Three architectural models: restricted, general and boosting, which have increasing amounts of support for removing these hazards are discussed. The performance gained by each level of additional architecture support is analyzed using the IMPACT C compiler which performs superblock scheduling for superscalar and superpipelined processors.