[NFC][CLANG] Fix static analyzer bugs about unnecessary object copies with auto keyword

[smanna12]

Reported by Static Analyzer Tool:

In ​EmitAssemblyHelper::​RunOptimizationPipeline(): Using the auto keyword without an & causes the copy of an object of type function.

/// List of pass builder callbacks ("CodeGenOptions.h").
std::vector<std::function<void(llvm::PassBuilder &)>> PassBuilderCallbacks;

[llvmbot]

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: None (smanna12)

Changes

… with auto

---

Full diff: https://github.com/llvm/llvm-project/pull/75082.diff

1 Files Affected:

• (modified) clang/lib/CodeGen/BackendUtil.cpp (+1-1)

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 77455c075cab0..1d77fbe016307 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -876,7 +876,7 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
           << PluginFN << toString(PassPlugin.takeError());
     }
   }
-  for (auto PassCallback : CodeGenOpts.PassBuilderCallbacks)
+  for (const auto &PassCallback : CodeGenOpts.PassBuilderCallbacks)
     PassCallback(PB);
 #define HANDLE_EXTENSION(Ext)                                                  \
   get##Ext##PluginInfo().RegisterPassBuilderCallbacks(PB);

[tahonermann]

Looks good to me!

[tahonermann]

PassBuilderCallbacks is defined as:

clang/include/clang/Basic/CodeGenOptions.h:
404   /// List of pass builder callbacks.
405   std::vector<std::function<void(llvm::PassBuilder &)>> PassBuilderCallbacks;

In the case where a std::function element wraps a lambda, copies can be expensive, so such avoidance makes sense. The operator() member of std::function is already const qualified, so the addition of const doesn't effect the behavior, at least not in the current language standards. Such calls are valid even when a mutable lambda is wrapped. That arguably results in a const violation, so there have been some papers that argued for changes (N4348, P0045) but to my knowledge, the standard has not been changed.

This looks like a good change.

[smanna12]

Thank you @tahonermann for reviews and information!