[PATCH] D17392: Embed bitcode in object file (clang cc1 part)

[Steven Wu via cfe-commits]

steven_wu added a comment.

Attach a new patch with the comments


================
Comment at: lib/CodeGen/BackendUtil.cpp:799-831
@@ +798,35 @@
+  // Embed the bitcode for the llvm module.
+  std::string Data;
+  ArrayRef<uint8_t> ModuleData;
+  Triple T(M->getTargetTriple());
+  if (CGOpts.getEmbedBitcode() != CodeGenOptions::Embed_Marker) {
+    if (!isBitcode((const unsigned char *)Buf.getBufferStart(),
+                   (const unsigned char *)Buf.getBufferEnd())) {
+      // If the input is LLVM Assembly, bitcode is produced by serializing
+      // the module. Use-lists order need to be perserved in this case.
+      llvm::raw_string_ostream OS(Data);
+      llvm::WriteBitcodeToFile(M, OS, /* ShouldPreserveUseListOrder */ true);
+      ModuleData =
+          ArrayRef<uint8_t>((const uint8_t *)OS.str().data(), OS.str().size());
+    } else
+      // If the input is LLVM bitcode, write the input byte stream directly.
+      ModuleData = ArrayRef<uint8_t>((const uint8_t *)Buf.getBufferStart(),
+                                     Buf.getBufferSize());
+  }
+  llvm::Constant *ModuleConstant =
+      llvm::ConstantDataArray::get(M->getContext(), ModuleData);
+  // Use Appending linkage so it doesn't get optimized out.
+  llvm::GlobalVariable *GV = new llvm::GlobalVariable(
+      *M, ModuleConstant->getType(), true, llvm::GlobalValue::AppendingLinkage,
+      ModuleConstant);
+  GV->setSection(getSectionNameForBitcode(T));
+  if (llvm::GlobalVariable *Old =
+          M->getGlobalVariable("llvm.embedded.module")) {
+    assert(Old->use_empty() && "llvm.embedded.module must have no uses");
+    GV->takeName(Old);
+    Old->eraseFromParent();
+  } else {
+    GV->setName("llvm.embedded.module");
+  }
+
+  // Return if only bitcode needs to be embedded.
----------------
rsmith wrote:
> I see, the "marker" is an empty `llvm.embedded.module` constant. Please add a comment to that effect; that's not obvious.
Sure. 

================
Comment at: lib/Driver/Tools.cpp:5710-5714
@@ -5709,4 +5709,7 @@
   // by the frontend.
-  if (C.getDriver().isSaveTempsEnabled() && isa<CompileJobAction>(JA))
+  // When -fembed-bitcode is enabled, optimized bitcode is emitted because it
+  // has slightly different breakdown between stages.
+  if (C.getDriver().isSaveTempsEnabled() &&
+      !C.getDriver().embedBitcodeEnabled() && isa<CompileJobAction>(JA))
     CmdArgs.push_back("-disable-llvm-passes");
 
----------------
rsmith wrote:
> This means that `-fembed-bitcode -save-temps` will save different intermediate IR than `-save-temps` alone; that seems wrong. I would expect that process to save the unoptimized IR but embed the optimized IR. Getting that right will probably require adding another action to the pipeline for that combination of flags.
That is exactly the case. I will add a FIXME here.


http://reviews.llvm.org/D17392