Architectural support for execution control of prologue and eplogue

Abstract

For certain classes of software pipelined loops, prologue and epilogue control is provided by loop control structures, rather than by predicated execution features of a VLIW architecture. For loops compatible with two simple constraints, code elements are not required for disabling garbage operations during prologue and epilogue loop periods. As a result, resources associated with implementation of the powerful architectural feature of predicated execution need not be squandered to service loop control. In particular, neither increased instruction width nor an increased number of instructions in the loop body is necessary to provide loop control in accordance with the present invention. Fewer service functions are required in the body of a loop. As a result, loop body code can be more efficiently scheduled by a compiler and, in some cases, fewer instructions will be required, resulting in improved loop performance. Loop control logic includes a loop control registers having an epilogue counter field, a shift register, a side-effects enabled flag, a current loop counter field, a loop mode flag, and side-effects manual control and loads manual control flags. Side-effects enabling logic and load enabling logic respectively issue a side-effects enabled predicate and a loads enabled predicate to respective subsets of execution units. Software pipelined simple and inner loops are supported.