Yao Qi
80e68c79c6
`MemoryCache::Read` fetches a whole L2 cache line for any read that fits in one, so reading a few bytes caches the line around them. `ReadRanges` probed only L1, and re-fetched ranges that line already held. Callers hit this whenever they read an array's header and then batch the elements that follow it in the same line, as `AppleObjCRuntimeV2::SharedCacheImageHeaders` and `ClassDescriptorV2::method_list_t` both do. #201166 uses MemoryCache in `Process::ReadRangesFromMemory`, but I didn't see why is L1 used only. Add `FindL2CacheEntry`, a lookup that never reads from the inferior, and consult it after L1. When it serves every range in a batch, `ReadRanges` returns without calling `Process::DoReadMemoryRanges`, so no packet is sent. As in the L1 lookup a range spanning two lines is a miss, and a partially read line is used only up to what it holds. Over the region `TestObjCMethodsNSError.test_runtime_types_efficient_memreads` brackets, `MultiMemRead` drops from 190 packets to 107 and the ranges they carry from 7004 to 6411, with the `m`/`x` count unchanged at 856. That test now also requires no read range to be contained in one an earlier packet already read, which counted 593 ranges before this change and none after. `TestReadMemoryRangesUsesL2Cache` covers the lookup directly. (cherry picked from commit 797a057bccd4f0e96f2775a55d2e6401e46e0275)
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