Teresa Johnson dbca0f9a08 Restore "[ThinLTO] Avoid temporaries when loading global decl attachment metadata" ... This restores commit ab1b4810b55279bcf6fdd87be74a403440be3991 which was reverted in 01b9deba76a950f04574b656c7c31ae389104f2d, with a fix for the issue it caused. We should use a temporary BitstreamCursor when loading the global decl attachment records so that the abbrev ids held in the lazy loading IndexCursor are not clobbered. Enhanced the test so that the issue is exposed there. Original description: When performing ThinLTO importing, the metadata loader attempts to lazy load, by building an index. However, module level global decl attachment metadata was being parsed early while building the index, since the associated (module level) global values aren't materialized on demand. This results in the creation of forward reference temporary metadatas, which are expensive. Normally, these module level global values don't have much attached metadata. However, in the case of -fwhole-program-vtables (e.g. for whole program devirtualization), the vtables may have many attached type metadatas. This was resulting in very slow performance when performing ThinLTO importing with the default lazy loading. This patch restructures the handling of these global decl attachment records, delaying their parsing until after the lazy loading index has been built. Then the parser can use the interface that loads from the index, which resolves forward references immediately instead of creating expensive temporaries. For one ThinLTO backend that imports from modules containing huge numbers of vtables and associated types, I measured the following compile times for the metadata materialization during function importing, rounded to nearest second: No -fwhole-program-vtables: Lazy loading on (head): 1s Lazy loading off (head): 3s Lazy loading on (patch): 1s With -fwhole-program-vtables: Lazy loading on (head): 440s Lazy loading off (head): 4s Lazy loading on (patch): 2s Differential Revision: https://reviews.llvm.org/D87970		2020-10-12 10:11:56 -07:00
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X86	Restore "[ThinLTO] Avoid temporaries when loading global decl attachment metadata"	2020-10-12 10:11:56 -07:00