FULLDOC: A full reporting debugger for optimized code

. As compilers increasingly rely on optimizations to achieve high performance, the effectiveness of source level debuggers for optimized code continues to falter. Even if values of source variables are computed in the execution of the optimized code, source level debuggers of optimized code are unable to always report the expected values of source variables at breakpoints. In this paper, we present FULLDOC, a debugger that can report all of the expected values of source variables that are computed in the optimized code. FULLDOC uses statically computed information to guide the gathering of dynamic information that enables full reporting. FULLDOC can report expected values at breakpoints when reportability is affected because values have been overwritten early, due to code hoisting or register reuse, or written late, due to code sinking. Our debugger can also report values that are path sensitive in that a value may be computed only along one path or the location of the va..

Register Pressure Sensitive Redundancy Elimination

. Redundancy elimination optimizations avoid repeated computation of the same value by computing the value once, saving it in a temporary, and reusing the value from the temporary when it is needed again. Examples of redundancy elimination optimizations include common subexpression elimination, loop invariant code motion and partial redundancy elimination. We demonstrate that the introduction of temporaries to save computed values can result in a significant increase in register pressure. An increase in register pressure may in turn trigger generation of spill code which can more than offset the gains derived from redundancy elimination. While current techniques minimize increases in register pressure, to avoid spill code generation it is instead necessary to ensure that register pressure does not exceed the number of available registers. In this paper we develop a redundancy elimination algorithm that is sensitive to register pressure: our novel technique first sets upper limits on al..

A Fresh Look at PRE as a Maximum Flow Problem

We show that classic PRE is also a maximum flow problem, thereby revealing the missing link between classic and speculative PRE, and more importantly, establishing a common high-level conceptual basis for this important compiler optimisation. To demonstrate this, we formulate a new, simple unidirectional bit-vector algorithm for classic PRE based only on the well-known concepts of availability and anticipatability. Designed to find a unique minimum cut in a flow network derived from a CFG, which is proved simply but rigorously, our algorithm is simple and intuitive, and its optimality is self-evident. This conceptual simplicity also translates into efficiency, as validated by experiments