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This is a pretty classic optimization. Instead of processing symbol records and copying them to temporary storage, do a first pass to measure how large the module symbol stream will be, and then copy the data into place in the PDB file. This requires defering relocation until much later, which accounts for most of the complexity in this patch. This patch avoids copying the contents of all live .debug$S sections into heap memory, which is worth about 20% of private memory usage when making PDBs. However, this is not an unmitigated performance win, because it can be faster to read dense, temporary, heap data than it is to iterate symbol records in object file backed memory a second time. Results on release chrome.dll: peak mem: 5164.89MB -> 4072.19MB (-1,092.7MB, -21.2%) wall-j1: 0m30.844s -> 0m32.094s (slightly slower) wall-j3: 0m20.968s -> 0m20.312s (slightly faster) wall-j8: 0m19.062s -> 0m17.672s (meaningfully faster) I gathered similar numbers for a debug, component build of content.dll in Chrome, and the performance impact of this change was in the noise. The memory usage reduction was visible and similar. Because of the new parallelism in the PDB commit phase, more cores makes the new approach faster. I'm assuming that most C++ developer machines these days are at least quad core, so I think this is a win. Differential Revision: https://reviews.llvm.org/D94267 |
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