[memprof] Skip MemProfUsePass on the empty module

[kazutakahirata]

This patch teaches the MemProfUsePass to return immediately on
the empty module.

Aside from saving time to deserialize the MemProf profile, this patch
ensures that we can obtain TLI like so:

TargetLibraryInfo &TLI = FAM.getResult(*M.begin());

when we undrift the MemProf profile in near future.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Kazu Hirata (kazutakahirata)

Changes

This patch teaches the MemProfUsePass on the empty module.

Aside from saving time to deserialize the MemProf profile, this patch
ensures that we can obtain TLI like so:

TargetLibraryInfo &TLI = FAM.getResult<TargetLibraryAnalysis>(*M.begin());

when we undrift the MemProf profile in near future.

---

Full diff: https://github.com/llvm/llvm-project/pull/117210.diff

1 Files Affected:

• (modified) llvm/lib/Transforms/Instrumentation/MemProfiler.cpp (+4)

diff --git a/llvm/lib/Transforms/Instrumentation/MemProfiler.cpp b/llvm/lib/Transforms/Instrumentation/MemProfiler.cpp
index 867158e7822214..33a7a37fa28e65 100644
--- a/llvm/lib/Transforms/Instrumentation/MemProfiler.cpp
+++ b/llvm/lib/Transforms/Instrumentation/MemProfiler.cpp
@@ -1121,6 +1121,10 @@ MemProfUsePass::MemProfUsePass(std::string MemoryProfileFile,
 }
 
 PreservedAnalyses MemProfUsePass::run(Module &M, ModuleAnalysisManager &AM) {
+  // Return immediately if the module doesn't contain any function.
+  if (M.empty())
+    return PreservedAnalyses::all();
+
   LLVM_DEBUG(dbgs() << "Read in memory profile:");
   auto &Ctx = M.getContext();
   auto ReaderOrErr = IndexedInstrProfReader::create(MemoryProfileFileName, *FS);

[teresajohnson]

The code change lgtm but the description might need a slight tweak, I feel like there is a missing/wrong word or so in "This patch teaches the MemProfUsePass on the empty module."

[kazutakahirata]

The code change lgtm but the description might need a slight tweak, I feel like there is a missing/wrong word or so in "This patch teaches the MemProfUsePass on the empty module."

Oops! Thank you for pointing this out! Fixed.