llvm-project

25a915a9 - [ThinLTO] Add HasLocal flag to GlobalValueSummaryInfo (#164647)

[ThinLTO] Add HasLocal flag to GlobalValueSummaryInfo (#164647) Add a flag to the GlobalValueSummaryInfo indicating whether the associated SummaryList (all summaries with the same GUID) contains any summaries with local linkage. This flag is set when building the index, so it is associated with the original linkage type before internalization and promotion. Consumers should check the withInternalizeAndPromote() flag on the index before using it. In most cases we expect a 1-1 mapping between a GUID and a summary with local linkage, because for locals the GUID is computed from the hash of "modulepath;name". However, there can be multiple locals with the same GUID if translation units are not compiled with enough path. And in rare but theoretically possible cases, there can be hash collisions on the underlying MD5 computation. So to be safe when looking for local summaries, analyses currently look through all summaries in the list. These lists can be extremely long in the case of large binaries with template function defs in widely used headers (i.e. linkonce_odr). A follow on change will use this flag to reduce ThinLTO analysis time in WPD by 5-6% for a large target (details in PR164046 which will be reworked to use this flag). Note that in the past we have tried to keep bits related to the GUID in the ValueInfo (which has a pointer to the associated GlobalValueSummaryInfo), via its PointerIntPair. However, we are out of bits there. This change does add a byte to every GlobalValueSummaryInfo instance, which I measured as a little under 0.90% overhead in a large target. However, it enables adding 7 bits of other per-GUID flags in the future without adding more overhead. Note that it was lower overhead to add this to the GlobalValueSummaryInfo than the ValueInfo, which tends to be copied into other maps.