[Clang] Map .gcda paths according to -fcoverage-prefix-map

[vit9696]

Summary

https://github.com/llvm/llvm-project/issues/54670

[keith]

Based on the issue it sounds like this should be gated behind a new -fprofile-prefix-map flag? I assume we'd also want -ffile-prefix-map to apply to it as well, similar to the others. And we'll definitely want some tests here!

[vit9696]

In D124767#3486286, @keith wrote:

Based on the issue it sounds like this should be gated behind a new -fprofile-prefix-map flag? I assume we'd also want -ffile-prefix-map to apply to it as well, similar to the others. And we'll definitely want some tests here!

For tests, sure. I will add some once there is general agreement on the option.

As for the new flag, I am confused. To me the timeline is as follows:

• LLVM implemented `-fprofile-prefix-map.
• GCC implemented -fprofile-prefix-map.
• LLVM renamed -fprofile-prefix-map to -fcoverage-prefix-map.
• GCC preliminary agreed to extend -fprofile-prefix-map with gcda path mapping.
• This patch implements this extension to LLVM -fcoverage-prefix-map, which remains being renamed -fprofile-prefix-map.

Do I misunderstand something? These flags are rather confusing in their semantics.

[keith]

I took this comment from the issue:

Since the feature you're proposing is specific to gcov, using a separate flag that matches the name used by GCC would be preferable to me.

To mean that we should introduce a new flag matching gcc's name. I think in general fewer flags would be preferred though, so I think it depends on if it would ever make sense to have separate values for these in the same build?

[vit9696]

Got it. To be honest, I cannot imagine a situation where one would prefer to keep separate behaviour, and with GCC it would be exactly just one. In my view, if we introduce -fprofile-prefix-map, it should rather mirror the entire -fcoverage-prefix-map behaviour, not some parts of it.

clang/lib/CodeGen/CodeGenModule.cpp

if (getCodeGenOpts().CoverageDataFile.empty() &&
     return;
 
   llvm::NamedMDNode *CUNode = TheModule.getNamedMetadata("llvm.dbg.cu");
   if (!CUNode)
     return;
 
   llvm::NamedMDNode *GCov = TheModule.getOrInsertNamedMetadata("llvm.gcov");
   llvm::LLVMContext &Ctx = TheModule.getContext();
-  auto *CoverageDataFile =
+  llvm::MDString *CoverageDataFile = nullptr;
+  auto &PrefixMap = getCodeGenOpts().CoveragePrefixMap;
+  llvm::SmallString<256> File;
+  if (!PrefixMap.empty()) {
+    File = getCodeGenOpts().CoverageDataFile;
+    llvm::sys::path::remove_dots(File, /*remove_dot_dot=*/true);
+    for (const auto &Entry : PrefixMap) {
+      if (llvm::sys::path::replace_path_prefix(File, Entry.first, Entry.second))
+        break;
+    }
+    CoverageDataFile = llvm::MDString::get(Ctx, File.c_str());
+  } else {
+    CoverageDataFile =
       llvm::MDString::get(Ctx, getCodeGenOpts().CoverageDataFile);
+  }
   auto *CoverageNotesFile =
       llvm::MDString::get(Ctx, getCodeGenOpts().CoverageNotesFile);
   for (int i = 0, e = CUNode->getNumOperands(); i != e; ++i) {
     llvm::MDNode *CU = CUNode->getOperand(i);
     llvm::Metadata *Elts[] = {CoverageNotesFile, CoverageDataFile, CU};
     GCov->addOperand(llvm::MDNode::get(Ctx, Elts));
   }
 }