1940069 - Replace CompactBufferWriter's buffer allocation to use LifoAlloc

Status: NEW

(Core :: JavaScript Engine: JIT, enhancement, P2)

Description

[Nicolas B. Pierron [:nbp]]

CompactBufferWritter is currently using jemalloc backed allocation which implies that any resize of the vector will not only make a new allocation but also copy the old data. On other vectors we were able to predict ahead the expected sizes of the vectors and pre-allocate them, but the CompactBuffers are compressing the data. Attempts at predicting the sizes of these buffers has so far resulted in a lot of over/under approximations.

Thus, instead of attempting to estimate the sizes, we can simply rely on segmented buffers using either a mozilla::BufferList, implemented as a vector of variable size arrays, or a mozilla::SegmentedVector, implemented as a double linked list of fixed size arrays. (don't ask …)

The CompactBuffer varies in size in functions of their use cases, thus we should either go for a solution with variable segments capacity, or paramettrize with templates for each use case.