Runtime pointer disambiguation
Résumé
In order to optimize code effectively, compilers must deal with memory dependences.
However, the state-of-the-art heuristics available in the literature to
track memory dependencies are inherently imprecise and computationally expensive.
Consequently, the most advanced code transformations that compilers have today
are ineffective when applied on real-world programs.
The goal of this paper is to solve this conundrum through the hybrid
disambiguation of pointers.
We provide a static analysis that generates dynamic tests to determine when two
memory locations can overlap.
We then produce two versions of a loop: one that is aliasing-free - hence, easy
to optimize - and another that is not.
Our checks lets us safely branch to the optimizable region.
We have applied these ideas on polly-llvm, a loop optimizer built on top of the llvm compilation infrastructure.
Our experiments indicate that our method is precise, effective and useful: we
can disambiguate every pair of pointer in the loop
intensive polybench benchmark suite.
The result of this precision is code quality: the binaries that we generate
are 10% faster than those that polly-llvm produces without our optimization,
at the -O3 optimization level of llvm.
Given the current technology to statically solve alias analysis, we believe that
our ideas are a necessary step to make modern compiler optimizations useful in
practice.