[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.

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
 
   Kind = getWasmKindForNamedSection(Name, Kind);
 
   StringRef Group = "";
   if (const Comdat *C = getWasmComdat(GO)) {
     Group = C->getName();
   }
 
+  if (isa<Function>(GO)) {
+    return SelectSectionForGlobal(GO, Kind, TM);
+  }
+
   return getContext().getWasmSection(Name, Kind, Group,
                                      MCContext::GenericSectionID);
 }
 
 static MCSectionWasm *selectWasmSectionForGlobal(
     MCContext &Ctx, const GlobalObject *GO, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM, bool EmitUniqueSection, unsigned *NextUniqueID) {
   StringRef Group = "";

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
+