[WebAssembly] Merge used feature sets, update atomics linkage policy

Summary:
It does not currently make sense to use WebAssembly features in some functions
but not others, so this CL adds an IR pass that takes the union of all used
feature sets and applies it to each function in the module. This allows us to
prevent atomics from being lowered away if some function has opted in to using
them. When atomics is not enabled anywhere, we detect whether there exists any
atomic operations or thread local storage that would be stripped and disallow
linking with objects that contain atomics if and only if atomics or tls are
stripped. When atomics is enabled, mark it as used but do not require it of
other objects in the link. These changes allow libraries that do not use atomics
to be built once and linked into both single-threaded and multithreaded
binaries.

Reviewers: aheejin, sbc100, dschuff

Subscribers: jgravelle-google, hiraditya, sunfish, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59625

llvm-svn: 357226

1 files changed, 27 insertions, 32 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
index e4129276b84..2e6de3cd9f2 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -33,6 +33,7 @@
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/DebugInfoMetadata.h"
 #include "llvm/IR/GlobalVariable.h"
+#include "llvm/IR/Metadata.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCSectionWasm.h"
 #include "llvm/MC/MCStreamer.h"
@@ -161,7 +162,7 @@ void WebAssemblyAsmPrinter::EmitEndOfAsmFile(Module &M) {
   }
 
   EmitProducerInfo(M);
-  EmitTargetFeatures();
+  EmitTargetFeatures(M);
 }
 
 void WebAssemblyAsmPrinter::EmitProducerInfo(Module &M) {
@@ -215,46 +216,40 @@ void WebAssemblyAsmPrinter::EmitProducerInfo(Module &M) {
   }
 }
 
-void WebAssemblyAsmPrinter::EmitTargetFeatures() {
-  static const std::pair<unsigned, const char *> FeaturePairs[] = {
-      {WebAssembly::FeatureAtomics, "atomics"},
-      {WebAssembly::FeatureBulkMemory, "bulk-memory"},
-      {WebAssembly::FeatureExceptionHandling, "exception-handling"},
-      {WebAssembly::FeatureNontrappingFPToInt, "nontrapping-fptoint"},
-      {WebAssembly::FeatureSignExt, "sign-ext"},
-      {WebAssembly::FeatureSIMD128, "simd128"},
-  };
-
+void WebAssemblyAsmPrinter::EmitTargetFeatures(Module &M) {
   struct FeatureEntry {
     uint8_t Prefix;
     StringRef Name;
   };
 
-  FeatureBitset UsedFeatures =
-      static_cast<WebAssemblyTargetMachine &>(TM).getUsedFeatures();
-
-  // Calculate the features and linkage policies to emit
+  // Read target features and linkage policies from module metadata
   SmallVector<FeatureEntry, 4> EmittedFeatures;
-  for (auto &F : FeaturePairs) {
+  for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
+    std::string MDKey = (StringRef("wasm-feature-") + KV.Key).str();
+    Metadata *Policy = M.getModuleFlag(MDKey);
+    if (Policy == nullptr)
+      continue;
+
     FeatureEntry Entry;
-    Entry.Name = F.second;
-    if (F.first == WebAssembly::FeatureAtomics) {
-      // "atomics" is special: code compiled without atomics may have had its
-      // atomics lowered to nonatomic operations. Such code would be dangerous
-      // to mix with proper atomics, so it is always Required or Disallowed.
-      Entry.Prefix = UsedFeatures[F.first]
-                         ? wasm::WASM_FEATURE_PREFIX_REQUIRED
-                         : wasm::WASM_FEATURE_PREFIX_DISALLOWED;
-      EmittedFeatures.push_back(Entry);
-    } else {
-      // Other features are marked Used or not mentioned
-      if (UsedFeatures[F.first]) {
-        Entry.Prefix = wasm::WASM_FEATURE_PREFIX_USED;
-        EmittedFeatures.push_back(Entry);
-      }
-    }
+    Entry.Prefix = 0;
+    Entry.Name = KV.Key;
+
+    if (auto *MD = cast<ConstantAsMetadata>(Policy))
+      if (auto *I = cast<ConstantInt>(MD->getValue()))
+        Entry.Prefix = I->getZExtValue();
+
+    // Silently ignore invalid metadata
+    if (Entry.Prefix != wasm::WASM_FEATURE_PREFIX_USED &&
+        Entry.Prefix != wasm::WASM_FEATURE_PREFIX_REQUIRED &&
+        Entry.Prefix != wasm::WASM_FEATURE_PREFIX_DISALLOWED)
+      continue;
+
+    EmittedFeatures.push_back(Entry);
   }
 
+  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());