[WebAssembly] support Memory64 in target_features section

[aardappel]

Summary

This largely to let Binaryen consistently detect features.

[tlively]

Thanks!

llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp

void WebAssemblyAsmPrinter::EmitTargetFeatures(Module &M) {
   };
 
   for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
     EmitFeature(KV.Key);
   }
   // This pseudo-feature tells the linker whether shared memory would be safe
   EmitFeature("shared-mem");
 
+  // This is an "architecture", not a "feature", but we emit it as such for
+  // the benefit of tools like Binaryen and consistency with other producers.
+  // FIXME: Subtarget is null here, so can't Subtarget->hasAddr64() ?
+  if (M.getDataLayout().getMaxPointerSize() == 8) {
+    // Can't use EmitFeature since "wasm-feature-memory64" is not a module
+    // flag.
+    EmittedFeatures.push_back({wasm::WASM_FEATURE_PREFIX_USED, "memory64"});
+  }
+
   if (EmittedFeatures.size() == 0)
     return;
 
   // Emit features and linkage policies into the "target_features" section
   MCSectionWasm *FeaturesSection = OutContext.getWasmSection(
       ".custom_section.target_features", SectionKind::getMetadata());
   OutStreamer->PushSection();
   OutStreamer->SwitchSection(FeaturesSection);

llvm/test/CodeGen/WebAssembly/memory64-feature.ll

+; RUN: llc < %s | FileCheck %s
+
+; Test that wasm64 is properly emitted into the target features section
+
+target triple = "wasm64-unknown-unknown"
+
+define void @foo() {
+  ret void
+}
+
+; CHECK-LABEL: .custom_section.target_features
+; CHECK-NEXT: .int8 1
+; CHECK-NEXT: .int8 43
+; CHECK-NEXT: .int8 8
+; CHECK-NEXT: .ascii "memory64"

llvm/test/MC/WebAssembly/debug-info64.ll

 ; CHECK-NEXT:     Name: reloc..debug_line
 ; CHECK-NEXT:   }
 ; CHECK-NEXT:   Section {
 ; CHECK-NEXT:     Type: CUSTOM (0x0)
 ; CHECK-NEXT:     Size: 77
 ; CHECK-NEXT:     Offset: 1073
 ; CHECK-NEXT:     Name: producers
 ; CHECK-NEXT:   }
+; CHECK-NEXT:   Section {
+; CHECK-NEXT:     Type: CUSTOM (0x0)
+; CHECK-NEXT:     Size: 11
+; CHECK-NEXT:     Offset: 1166
+; CHECK-NEXT:     Name: target_features
+; CHECK-NEXT:   }
 ; CHECK-NEXT: ]
 ; CHECK-NEXT: Relocations [
 ; CHECK-NEXT:   Section (7) DATA {
 ; CHECK-NEXT:     0x6 R_WASM_MEMORY_ADDR_I64 myextern 0
 ; CHECK-NEXT:     0x13 R_WASM_TABLE_INDEX_I64 f2
 ; CHECK-NEXT:   }
 ; CHECK-NEXT:   Section (9) .debug_info {
 ; CHECK-NEXT:     0x6 R_WASM_SECTION_OFFSET_I32 .debug_abbrev 0