[WebAssembly] Emit __llvm_covmap and __llvm_covfun as custom sections

This patch makes `__llvm_covmap` and `__llvm_covfun` as custom sections
in the wasm object file because they will not be referenced at runtime
but just used by `llvm-cov` post-processing tools. The same approach is
used in the ELF object file to emit them without `SHF_ALLOC` flag.

Those sections have their associated comdat group symbols, which are
section symbols (not data symbols) in the wasm object file. This patch
also sets the symbol type explicitly not to hit an assertion in the
`WasmObjectWriter::writeOneObject` ("data symbols must live in a data
section").

Author: kateinoigakukun

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -2171,7 +2171,11 @@ MCSection *TargetLoweringObjectFileWasm::getExplicitSectionGlobal(
   // This could be avoided if all data segements (the wasm sense) were
   // represented as their own sections (in the llvm sense).
   // TODO(sbc): https://github.com/WebAssembly/tool-conventions/issues/138
-  if (Name == ".llvmcmd" || Name == ".llvmbc")
+  if (Name == getInstrProfSectionName(IPSK_covmap, Triple::Wasm,
+                                      /*AddSegmentInfo=*/false) ||
+      Name == getInstrProfSectionName(IPSK_covfun, Triple::Wasm,
+                                      /*AddSegmentInfo=*/false) ||
+      Name == ".llvmbc" || Name == ".llvmcmd")
     Kind = SectionKind::getMetadata();
 
   StringRef Group = "";

llvm/lib/MC/MCContext.cpp

@@ -757,6 +757,11 @@ MCSectionWasm *MCContext::getWasmSection(const Twine &Section, SectionKind K,
   if (!Group.isTriviallyEmpty() && !Group.str().empty()) {
     GroupSym = cast<MCSymbolWasm>(getOrCreateSymbol(Group));
     GroupSym->setComdat(true);
+    if (K.isMetadata() && !GroupSym->getType().has_value()) {
+      // Comdat group symbol associated with a custom section is a section
+      // symbol (not a data symbol).
+      GroupSym->setType(wasm::WASM_SYMBOL_TYPE_SECTION);
+    }
   }
 
   return getWasmSection(Section, K, Flags, GroupSym, UniqueID);

llvm/test/CodeGen/WebAssembly/profile.ll

@@ -0,0 +1,47 @@
+; RUN: llc < %s --filetype=obj | obj2yaml | FileCheck %s
+
+target triple = "wasm32-unknown-unknown"
+
+$__covrec_A = comdat any
+$__covrec_B = comdat any
+
+@__covrec_A = linkonce_odr hidden constant <{ i64, i32, i64, i64, [4 x i8] }> <{
+    i64 -1978722966671112904,
+    i32 4,
+    i64 0,
+    i64 -8102528905418564625,
+    [4 x i8] c"\01\01\04\11"
+}>, section "__llvm_covfun", comdat, align 8
+@__covrec_B = linkonce_odr hidden constant <{ i64, i32, i64, i64, [4 x i8] }> <{
+    i64 8006510647218728891,
+    i32 9,
+    i64 0,
+    i64 -8102528905418564625,
+    [4 x i8] c"\01\01\00\01"
+}>, section "__llvm_covfun", comdat, align 8
+@__llvm_coverage_mapping = private constant { { i32, i32, i32, i32 }, [4 x i8] } {
+    { i32, i32, i32, i32 } { i32 0, i32 87, i32 0, i32 5 },
+    [4 x i8] c"\01\01\00\02"
+}, section "__llvm_covmap", align 8
+
+; CHECK:      - Type:            CUSTOM
+; CHECK-NEXT:   Name:            __llvm_covfun
+; CHECK-NEXT:   Payload:         3845A90EF2298AE4040000000000000000000000EF1B31BAE3088E8F01010411
+; CHECK-NEXT: - Type:            CUSTOM
+; CHECK-NEXT:   Name:            __llvm_covfun
+; CHECK-NEXT:   Payload:         BBEFDA6903D71C6F090000000000000000000000EF1B31BAE3088E8F01010001
+; CHECK-NEXT: - Type:            CUSTOM
+; CHECK-NEXT:   Name:            __llvm_covmap
+; CHECK-NEXT:   Payload:         '0000000057000000000000000500000001010002'
+; CHECK-NEXT: - Type:            CUSTOM
+; CHECK-NEXT:   Name:            linking
+; CHECK-NEXT:   Version:         2
+; CHECK-NEXT:   Comdats:
+; CHECK-NEXT:     - Name:            __covrec_A
+; CHECK-NEXT:       Entries:
+; CHECK-NEXT:         - Kind:            SECTION
+; CHECK-NEXT:           Index:           1
+; CHECK-NEXT:     - Name:            __covrec_B
+; CHECK-NEXT:       Entries:
+; CHECK-NEXT:         - Kind:            SECTION
+; CHECK-NEXT:           Index:           2