[LTO] Use .at instead of .lookup to avoid copies. (NFC)

[amharc]

DenseMap::lookup returns by value (because it default-creates the returned value if the key isn't present in the map), which means that we do a lot of copying here. Since we assert that something is present in the returned value two lines below this call, it's safe to use .at here instead.

Copying and then destroying dense maps here is responsible for 60% of the time spent in LTO indexing in a large internal build.

[llvmbot]

@llvm/pr-subscribers-lto

@llvm/pr-subscribers-llvm-transforms

Author: Krzysztof Pszeniczny (amharc)

Changes

DenseMap::lookup returns by value (because it default-creates the returned value if the key isn't present in the map), which means that we do a lot of copying here. Since we assert that something is present in the returned value two lines below this call, it's safe to use .at here instead.

Copying and then destroying dense maps here is responsible for 60% of the time spent in LTO indexing in a large internal build.

---

Full diff: https://github.com/llvm/llvm-project/pull/117888.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/IPO/FunctionImport.cpp (+1-2)

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index fee27f72f208b0..9cca3cdc761451 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -1540,8 +1540,7 @@ void llvm::gatherImportedSummariesForModule(
     auto &SummariesForIndex =
         LookupOrCreate(ModuleToSummariesForIndex, FromModule);
 
-    const auto &DefinedGVSummaries =
-        ModuleToDefinedGVSummaries.lookup(FromModule);
+    const auto &DefinedGVSummaries = ModuleToDefinedGVSummaries.at(FromModule);
     const auto &DS = DefinedGVSummaries.find(GUID);
     assert(DS != DefinedGVSummaries.end() &&
            "Expected a defined summary for imported global value");