[WebAssembly] Allow for the creation of user-defined custom sections

This patch adds a way for users to create their own custom sections to
be added to wasm files. At the LLVM IR layer, they are defined through
the "wasm.custom_sections" named metadata. The expected use case for
this is bindings generators such as wasm-bindgen.

Patch by Dan Gohman

Differential Revision: https://reviews.llvm.org/D45297

llvm-svn: 329315

1 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
index 4f7c68d4882..e8144add3f7 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -31,10 +31,11 @@
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/MC/MCContext.h"
+#include "llvm/MC/MCSectionWasm.h"
 #include "llvm/MC/MCStreamer.h"
 #include "llvm/MC/MCSymbol.h"
-#include "llvm/MC/MCSymbolWasm.h"
 #include "llvm/MC/MCSymbolELF.h"
+#include "llvm/MC/MCSymbolWasm.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Support/raw_ostream.h"
@@ -107,6 +108,26 @@ void WebAssemblyAsmPrinter::EmitEndOfAsmFile(Module &M) {
       }
     }
   }
+
+  if (const NamedMDNode *Named = M.getNamedMetadata("wasm.custom_sections")) {
+    for (const Metadata *MD : Named->operands()) {
+      const MDTuple *Tuple = dyn_cast<MDTuple>(MD);
+      if (!Tuple || Tuple->getNumOperands() != 2)
+        continue;
+      const MDString *Name = dyn_cast<MDString>(Tuple->getOperand(0));
+      const MDString *Contents = dyn_cast<MDString>(Tuple->getOperand(1));
+      if (!Name || !Contents)
+        continue;
+
+      OutStreamer->PushSection();
+      std::string SectionName = (".custom_section." + Name->getString()).str();
+      MCSectionWasm *mySection =
+          OutContext.getWasmSection(SectionName, SectionKind::getMetadata());
+      OutStreamer->SwitchSection(mySection);
+      OutStreamer->EmitBytes(Contents->getString());
+      OutStreamer->PopSection();
+    }
+  }
 }
 
 void WebAssemblyAsmPrinter::EmitConstantPool() {