[PATCH] D29808: WholeProgramDevirt: Add any unsuccessful llvm.type.checked.load devirtualizations to the list of llvm.type.test users.

[Peter Collingbourne via Phabricator via llvm-commits]

pcc added inline comments.


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Comment at: llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp:1095
+    // devirtualized, we need to add the resulting llvm.type.test intrinsics to
+    // the function summaries so that the LowerTypeTests pass will export them.
+    if (Action == PassSummaryAction::Export && isa<MDString>(S.first.TypeID)) {
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mehdi_amini wrote:
> I'm still not sure what/why this is done. What happens in which phase? 
> What is the LowerTypeTests pass exporting exactly, and for which consumer?
Essentially this is applying the same transformation to the summary as we would make to the function itself with regular LTO.

In the exporting phase we need to figure out how each llvm.type.checked.load intrinsic will be lowered. There are two cases:
1. (not devirtualized) emit a regular virtual call and an llvm.type.test
2. (devirtualized) emit a devirtualized call without an llvm.type.test
This change is necessary to correctly handle case 1. By adding the type ID to the list of type tests, we cause the exporting LowerTypeTests pass to export the summary information for that type ID (i.e. it will fill in the TypeTestResolution field in TypeIdSummary). That information will be consumed by the importing LowerTypeTests pass in the thin backend, after the importing WholeProgramDevirt pass has transformed the llvm.type.checked.load into an llvm.type.test (based on the WholeProgramDevirtResolution's Indir resolution for the type ID).

In case 2 the TypeCheckedLoadUsers vector will be cleared (see D29811) so this code will end up not adding the type test to the summary.


https://reviews.llvm.org/D29808