A gentle introduction to jump threading optimizations

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2024-10-05 15:00:16

As part of the GCC developers' on-demand range work for GCC 10, I've been playing with improving the backward jump threader so it can thread paths that are range-dependent. This, in turn, had me looking at the jump threader, which is a part of the compiler I've been carefully avoiding for years. If, like me, you're curious about compiler optimizations, but are jump-threading-agnostic, perhaps you'll be interested in this short introduction.

At the highest level, jump threading's major goal is to reduce the number of dynamically executed jumps on different paths through the program's control flow graph. Often this results in improved performance due to the reduction of conditionals, which in turn enables further optimizations. Typically, for every runtime branch eliminated by jump threading, two or three other runtime instructions are eliminated.

Simplification of control flow also dramatically reduces the false-positive rates from warnings such as -Wuninitialized. False positives from -Wuninitialized typically occur because there are paths through the control flow graph that cannot occur at runtime, but remain in the internal representation of the code.