Dead load store analysis / optimisation #1535
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We need negative integers for `PtrAdd`. Note that other constructs already reject negative integers appropriately, so this change can be rather simple.
This commit adds a basic dead load / store optimisation. It is super conservative: our alias analysis only does something useful when two pointers derive from the same instruction and have a different offset. In other words, we basically track `PtrAdd`s, since these guarantee that our alias analysis is precise. We can clearly be somewhat more liberal, but that will rapidly require deeper thought. In somewhat more detail, the new `HeapValues` struct associates `Address`es with values. Roughly speaking, each load adds more information to this struct; stores will invalidate all previous loads / stores unless we are entirely sure they don't alias; and barriers invalidate all previous loads / stores. Even though this is somewhat basic, it can still do some surprisingly powerful optimisations. For example, give this trace IR (which is a simplification of what we see in big_loop): ``` %0: ptr = param 0 %1: i8 = load %0 %2: i1 = eq %1, 3i8 guard true, %2, [] *%0 = 3i8 ``` we determine that the store at instruction 5 is dead: it is writing back a value to the heap (3i8) we know for sure (via a load and guard) is already stored at that address. It is then turned into a tombstone. The tests in `opt/mod.rs` give a wider view of what this can optimise. The tests in `heapvalues.rs` give an indication of the detailed view of our alias analysis.
ptersilie
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Jan 7, 2025
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This PR adds basic, but somewhat effective, dead load / store analysis optimisation (necessarily containing basic alias analysis). The main commit in this PR is 03c055b: it has, I hope, both adequate motivation in the commit message and sufficient tests and documentation in the code to show what's going on.