[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
diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
index b0a80b0..c8a53eb 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyAsmPrinter.cpp
@@ -21,8 +21,10 @@
#include "WebAssemblyMCInstLower.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblyRegisterInfo.h"
+#include "WebAssemblyTargetMachine.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/StringExtras.h"
+#include "llvm/BinaryFormat/Wasm.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/CodeGen/MachineConstantPool.h"
@@ -159,6 +161,7 @@
}
EmitProducerInfo(M);
+ EmitTargetFeatures();
}
void WebAssemblyAsmPrinter::EmitProducerInfo(Module &M) {
@@ -212,6 +215,61 @@
}
}
+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"},
+ };
+
+ struct FeatureEntry {
+ uint8_t Prefix;
+ StringRef Name;
+ };
+
+ FeatureBitset UsedFeatures =
+ static_cast<WebAssemblyTargetMachine &>(TM).getUsedFeatures();
+
+ // Calculate the features and linkage policies to emit
+ SmallVector<FeatureEntry, 4> EmittedFeatures;
+ for (auto &F : FeaturePairs) {
+ 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);
+ }
+ }
+ }
+
+ // 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);
+
+ OutStreamer->EmitULEB128IntValue(EmittedFeatures.size());
+ for (auto &F : EmittedFeatures) {
+ OutStreamer->EmitIntValue(F.Prefix, 1);
+ OutStreamer->EmitULEB128IntValue(F.Name.size());
+ OutStreamer->EmitBytes(F.Name);
+ }
+
+ OutStreamer->PopSection();
+}
+
void WebAssemblyAsmPrinter::EmitConstantPool() {
assert(MF->getConstantPool()->getConstants().empty() &&
"WebAssembly disables constant pools");