Alias Analysis
Alias analysis is the compiler analysis that determines when two pointers or references cannot point to the same memory location. Covered in ECE459 L18.
Why does the compiler care?
Many optimizations (reordering, register caching, parallelization) are only safe if the compiler can prove two pointers disjoint. When it can’t, it must assume the worst and skip the transformation.
Pointer analysis is the finer-grained sibling: it abstractly tracks what regions of the heap each variable points to (a region might be “memory allocated at program point X”).
Once pointers are known not to alias, their effects are independent. Safe to reorder, safe to cache values in registers.
Rust angle
Rust’s borrowing rules are alias control at the language level: &mut T is the sole path to the data for its lifetime. That gives the compiler the equivalent of noalias for free.
Caveat from L18
The LLVM backend doing the work may not always know about Rust’s guarantees. There is a gap between the Rust model and LLVM’s assumptions.
Automatic parallelization is the ambitious follow-on: prove no aliasing, run iterations in parallel. rayon does a bit of this. Rust’s controlled aliasing makes it more tractable than in C.
Shape analysis builds on pointer analysis to determine whether a data structure is actually a tree, versus a list, DAG, or cyclic graph.
Call graphs
Many interprocedural analyses (inlining, devirtualization) require accurate call graphs. Easy for direct C-style calls, hard for C++/Java virtual methods or C function pointers. Building the call graph itself needs pointer analysis.
Rust-specific challenges: indirect function calls (function pointers), dynamic dispatch through traits.
https://doc.rust-lang.org/nomicon/aliasing.html