[ThinLTO] Make inline assembly handling more efficient in summary

Summary:
The change in r285513 to prevent exporting of locals used in
inline asm added all locals in the llvm.used set to the reference
set of functions containing inline asm. Since these locals were marked
NoRename, this automatically prevented importing of the function.

Unfortunately, this caused an explosion in the summary reference lists
in some cases. In my particular example, it happened for a large protocol
buffer generated C++ file, where many of the generated functions
contained an inline asm call. It was exacerbated when doing a ThinLTO
PGO instrumentation build, where the PGO instrumentation included
thousands of private __profd_* values that were added to llvm.used.

We really only need to include a single llvm.used local (NoRename) value
in the reference list of a function containing inline asm to block it
being imported. However, it seems cleaner to add a flag to the summary
that explicitly describes this situation, which is what this patch does.

Reviewers: mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26402

llvm-svn: 286840

1 files changed, 5 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index 3bebc609bea..68626c02038 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -153,7 +153,11 @@ static bool eligibleForImport(const ModuleSummaryIndex &Index,
 
   // Check references (and potential calls) in the same module. If the current
   // value references a global that can't be externally referenced it is not
-  // eligible for import.
+  // eligible for import. First check the flag set when we have possible
+  // opaque references (e.g. inline asm calls), then check the call and
+  // reference sets.
+  if (Summary.hasInlineAsmMaybeReferencingInternal())
+    return false;
   bool AllRefsCanBeExternallyReferenced =
       llvm::all_of(Summary.refs(), [&](const ValueInfo &VI) {
         return canBeExternallyReferenced(Index, VI.getGUID());