[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, 12 insertions, 0 deletions

diff --git a/llvm/include/llvm/BinaryFormat/Wasm.h b/llvm/include/llvm/BinaryFormat/Wasm.h
index 7848be1cbfa..443c2a5d2c6 100644
--- a/llvm/include/llvm/BinaryFormat/Wasm.h
+++ b/llvm/include/llvm/BinaryFormat/Wasm.h
@@ -48,6 +48,11 @@ struct WasmProducerInfo {
   std::vector<std::pair<std::string, std::string>> SDKs;
 };
 
+struct WasmFeatureEntry {
+  uint8_t Prefix;
+  std::string Name;
+};
+
 struct WasmExport {
   StringRef Name;
   uint8_t Kind;
@@ -253,6 +258,13 @@ enum : unsigned {
   WASM_SEGMENT_HAS_MEMINDEX = 0x02,
 };
 
+// Feature policy prefixes used in the custom "target_features" section
+enum : uint8_t {
+  WASM_FEATURE_PREFIX_USED = '+',
+  WASM_FEATURE_PREFIX_REQUIRED = '=',
+  WASM_FEATURE_PREFIX_DISALLOWED = '-',
+};
+
 // Kind codes used in the custom "name" section
 enum : unsigned {
   WASM_NAMES_FUNCTION = 0x1,