A computer system includes a processor capable of executing a plurality of N threads of instructions, N being an integer greater than one, with a set of global registers visible to each of the plurality of threads and a plurality of busy bit memory elements used to signal whether or not a register is in use by a thread. The processor includes logic to stall a read from global register if the thread reading the global register is a speculative thread and the busy bits for prior threads are set. The processor might also include a speculative load address memory, into which speculative loads from speculative threads are entered and logic to compare addresses for stores from nonspeculative threads with addressees in the speculative load address memory and invalidate speculative threads corresponding to the speculative load addresses stored in the speculative load address memory. In an efficient implementation, aliasing load instructions can be distinct from nonaliasing load instructions, whereby addresses of aliasing load instructions are selectively stored in the speculative load address memory.