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https://github.com/ziglang/zig/issues/26027#issuecomment-3571227050 tracked some bad performance in `DebugAllocator` on macOS down to a function in dyld which `std.debug.SelfInfo` was calling into. It turns out `dladdr`'s symbol lookup logic is horrendously slow (looking at its source code, it appears to be doing a *linear scan* over all symbols in the image?!). However, we don't actually need the symbol, so we want to try and avoid this logic. Luckily, dyld has more precise APIs for what we need! Unluckily, Apple, in their infinite wisdom, decided they should be deprecated in favour of `dladdr`, despite the latter being several times slower (and by "several times", I have measured a 50x slowdown on repeated calls to `dladdr` compared to the other API). But luckily again, the deprecated APIs are still exposed. So, after a careful analysis of the situation (reading dyld code and cursing Apple engineers), I think it makes sense to just use these deprecated APIs for now. If they ever go away, we can write our own cache for this data to bypass Apple's awfully slow code, but I suspect these functions will stick around for the foreseeable future. Uh, and if `_dyld_get_image_header_containing_address` goes away, there's also `dyld_image_header_containing_address`, which is a seemingly identical function, exported by dyld just the same, but with a separate (functionally identical) implementation, and not documented in the public header file. Apple work in mysterious ways, I guess. |
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