[WebAssembly] Target features section

Summary:
Implements a new target features section in assembly and object files
that records what features are used, required, and disallowed in
WebAssembly objects. The linker uses this information to ensure that
all objects participating in a link are feature-compatible and records
the set of used features in the output binary for use by optimizers
and other tools later in the toolchain.

The "atomics" feature is always required or disallowed to prevent
linking code with stripped atomics into multithreaded binaries. Other
features are marked used if they are enabled globally or on any
function in a module.

Future CLs will add linker flags for ignoring feature compatibility
checks and for specifying the set of allowed features, implement using
the presence of the "atomics" feature to control the type of memory
and segments in the linked binary, and add front-end flags for
relaxing the linkage policy for atomics.

Reviewers: aheejin, sbc100, dschuff

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59173

llvm-svn: 356610

1 files changed, 36 insertions, 1 deletions

diff --git a/llvm/lib/Object/WasmObjectFile.cpp b/llvm/lib/Object/WasmObjectFile.cpp
index 2ff2c555d5e..14b17bcfa5e 100644
--- a/llvm/lib/Object/WasmObjectFile.cpp
+++ b/llvm/lib/Object/WasmObjectFile.cpp
@@ -715,6 +715,36 @@ Error WasmObjectFile::parseProducersSection(ReadContext &Ctx) {
   return Error::success();
 }
 
+Error WasmObjectFile::parseTargetFeaturesSection(ReadContext &Ctx) {
+  llvm::SmallSet<std::string, 8> FeaturesSeen;
+  uint32_t FeatureCount = readVaruint32(Ctx);
+  for (size_t I = 0; I < FeatureCount; ++I) {
+    wasm::WasmFeatureEntry Feature;
+    Feature.Prefix = readUint8(Ctx);
+    switch (Feature.Prefix) {
+    case wasm::WASM_FEATURE_PREFIX_USED:
+    case wasm::WASM_FEATURE_PREFIX_REQUIRED:
+    case wasm::WASM_FEATURE_PREFIX_DISALLOWED:
+      break;
+    default:
+      return make_error<GenericBinaryError>("Unknown feature policy prefix",
+                                            object_error::parse_failed);
+    }
+    Feature.Name = readString(Ctx);
+    if (!FeaturesSeen.insert(Feature.Name).second)
+      return make_error<GenericBinaryError>(
+          "Target features section contains repeated feature \"" +
+              Feature.Name + "\"",
+          object_error::parse_failed);
+    TargetFeatures.push_back(Feature);
+  }
+  if (Ctx.Ptr != Ctx.End)
+    return make_error<GenericBinaryError>(
+        "Target features section ended prematurely",
+        object_error::parse_failed);
+  return Error::success();
+}
+
 Error WasmObjectFile::parseRelocSection(StringRef Name, ReadContext &Ctx) {
   uint32_t SectionIndex = readVaruint32(Ctx);
   if (SectionIndex >= Sections.size())
@@ -816,6 +846,9 @@ Error WasmObjectFile::parseCustomSection(WasmSection &Sec, ReadContext &Ctx) {
   } else if (Sec.Name == "producers") {
     if (Error Err = parseProducersSection(Ctx))
       return Err;
+  } else if (Sec.Name == "target_features") {
+    if (Error Err = parseTargetFeaturesSection(Ctx))
+      return Err;
   } else if (Sec.Name.startswith("reloc.")) {
     if (Error Err = parseRelocSection(Sec.Name, Ctx))
       return Err;
@@ -1528,6 +1561,7 @@ int WasmSectionOrderChecker::getSectionOrder(unsigned ID,
         .StartsWith("reloc.", WASM_SEC_ORDER_RELOC)
         .Case("name", WASM_SEC_ORDER_NAME)
         .Case("producers", WASM_SEC_ORDER_PRODUCERS)
+        .Case("target_features", WASM_SEC_ORDER_TARGET_FEATURES)
         .Default(WASM_SEC_ORDER_NONE);
   case wasm::WASM_SEC_TYPE:
     return WASM_SEC_ORDER_TYPE;
@@ -1584,7 +1618,8 @@ int WasmSectionOrderChecker::DisallowedPredecessors[WASM_NUM_SEC_ORDERS][WASM_NU
   {WASM_SEC_ORDER_LINKING, WASM_SEC_ORDER_RELOC, WASM_SEC_ORDER_NAME}, // WASM_SEC_ORDER_LINKING,
   {}, // WASM_SEC_ORDER_RELOC (can be repeated),
   {WASM_SEC_ORDER_NAME, WASM_SEC_ORDER_PRODUCERS}, // WASM_SEC_ORDER_NAME,
-  {WASM_SEC_ORDER_PRODUCERS}, // WASM_SEC_ORDER_PRODUCERS,
+  {WASM_SEC_ORDER_PRODUCERS, WASM_SEC_ORDER_TARGET_FEATURES}, // WASM_SEC_ORDER_PRODUCERS,
+  {WASM_SEC_ORDER_TARGET_FEATURES}  // WASM_SEC_ORDER_TARGET_FEATURES
 };
 
 bool WasmSectionOrderChecker::isValidSectionOrder(unsigned ID,