[WebAssembly] Ignore explict section names for functions

[sbc100]

Summary

WebAssembly doesn't support more than one function per section
and we rely on function sections being unique. This change ignores
the section provided by the function to avoid two functions being
in the same section.

Without this change the object writer produces the following
error for this test:
LLVM ERROR: section already has a defining function: baz

[sbc100]

add comment

[dschuff]

This does seem a little odd, but I think it makes sense given the special nature of functions in the wasm object format.

[sbc100]

I agree... if you can think of a more sensible solution please let me know.

llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

static SectionKind getWasmKindForNamedSection(StringRef Name, SectionKind K) {
 
   // Otherwise, ignore whatever section type the generic impl detected and use
   // a plain data section.
   return SectionKind::getData();
 }
 
 MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
+  // We don't support explict section names for functions in the wasm object
+  // format.  Each function has to be in its own unique section.
+  if (isa<Function>(GO)) {
+    return SelectSectionForGlobal(GO, Kind, TM);
+  }
+
   StringRef Name = GO->getSection();
 
   Kind = getWasmKindForNamedSection(Name, Kind);
 
   StringRef Group = "";
   if (const Comdat *C = getWasmComdat(GO)) {
     Group = C->getName();
   }

llvm/trunk/test/MC/WebAssembly/function-sections.ll

+; RUN: llc -filetype=obj %s -o - | obj2yaml | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+define hidden i32 @foo() section "baz" {
+entry:
+  ret i32 2
+}
+
+define hidden i32 @bar() section "baz" {
+entry:
+  ret i32 1
+}
+
+; CHECK:        - Type:            CUSTOM
+; CHECK-NEXT:     Name:            linking
+; CHECK-NEXT:     Version:         1
+; CHECK-NEXT:     SymbolTable:     
+; CHECK-NEXT:       - Index:           0
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Name:            foo
+; CHECK-NEXT:         Flags:           [ VISIBILITY_HIDDEN ]
+; CHECK-NEXT:         Function:        0
+; CHECK-NEXT:       - Index:           1
+; CHECK-NEXT:         Kind:            FUNCTION
+; CHECK-NEXT:         Name:            bar
+; CHECK-NEXT:         Flags:           [ VISIBILITY_HIDDEN ]
+; CHECK-NEXT:         Function:        1