RFC: Segment aware vm addressing #480
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This patch introduces the infrastructure for segment aware virtual memory addressing, enabling support for universal Mach-O binaries where multiple address spaces need to coexist. The relaxations in ComputeRollup are necessary because with segment separated address spaces, secondary maps can have gaps when switching between segments.
correctly analyze universal binaries where each architecture has its own address space.
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Bloaty currently cannot correctly analyze macho universal binaries. When processing a universal binary containing multiple architectures. e.g. arm64 and x86_64, each architecture slice should have its own virtual address space. However Bloaty's current implementation uses a single flat address space, causing these overlapping addresses to conflict and produce incorrect results. As suggested in #153 (comment), this adds a segment id, and then updates the logic to handle multiple address spaces.
This bulk of the changes is introducing a VMAddr structure which contains a segment identifier and an address. The more challenging and complex changes are to ComputeRollup which required relaxing the logic for secondary maps to accommodate things like, padding or gaps (otherwise these maps trigger asserts in the previous code). With the segment infrastructure changes in place, the macho changes were fairly straightforward.
@haberman Does this approach seem reasonable to you? I hope my understanding of the rollup algorithm and the changes are correct. All of the existing tests are passing for me locally but it's possible I've missed something, so any feedback would be appreciated.