[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, 116 insertions, 23 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
index 6d9fd00..8a768db 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
@@ -26,6 +26,7 @@
 #include "llvm/Support/TargetRegistry.h"
 #include "llvm/Target/TargetOptions.h"
 #include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Scalar/LowerAtomic.h"
 #include "llvm/Transforms/Utils.h"
 using namespace llvm;
 
@@ -117,10 +118,6 @@ WebAssemblyTargetMachine::WebAssemblyTargetMachine(
 
   initAsmInfo();
 
-  // Create a subtarget using the unmodified target machine features to
-  // initialize the used feature set with explicitly enabled features.
-  getSubtargetImpl(getTargetCPU(), getTargetFeatureString());
-
   // Note that we don't use setRequiresStructuredCFG(true). It disables
   // optimizations than we're ok with, and want, such as critical edge
   // splitting and tail merging.
@@ -134,7 +131,6 @@ WebAssemblyTargetMachine::getSubtargetImpl(std::string CPU,
   auto &I = SubtargetMap[CPU + FS];
   if (!I) {
     I = llvm::make_unique<WebAssemblySubtarget>(TargetTriple, CPU, FS, *this);
-    UsedFeatures |= I->getFeatureBits();
   }
   return I.get();
 }
@@ -160,21 +156,123 @@ WebAssemblyTargetMachine::getSubtargetImpl(const Function &F) const {
 }
 
 namespace {
-class StripThreadLocal final : public ModulePass {
-  // The default thread model for wasm is single, where thread-local variables
-  // are identical to regular globals and should be treated the same. So this
-  // pass just converts all GlobalVariables to NotThreadLocal
+
+class CoalesceFeaturesAndStripAtomics final : public ModulePass {
+  // Take the union of all features used in the module and use it for each
+  // function individually, since having multiple feature sets in one module
+  // currently does not make sense for WebAssembly. If atomics are not enabled,
+  // also strip atomic operations and thread local storage.
   static char ID;
+  WebAssemblyTargetMachine *WasmTM;
 
 public:
-  StripThreadLocal() : ModulePass(ID) {}
+  CoalesceFeaturesAndStripAtomics(WebAssemblyTargetMachine *WasmTM)
+      : ModulePass(ID), WasmTM(WasmTM) {}
+
   bool runOnModule(Module &M) override {
-    for (auto &GV : M.globals())
-      GV.setThreadLocalMode(GlobalValue::ThreadLocalMode::NotThreadLocal);
+    FeatureBitset Features = coalesceFeatures(M);
+
+    std::string FeatureStr = getFeatureString(Features);
+    for (auto &F : M)
+      replaceFeatures(F, FeatureStr);
+
+    bool Stripped = false;
+    if (!Features[WebAssembly::FeatureAtomics]) {
+      Stripped |= stripAtomics(M);
+      Stripped |= stripThreadLocals(M);
+    }
+
+    recordFeatures(M, Features, Stripped);
+
+    // Conservatively assume we have made some change
     return true;
   }
+
+private:
+  FeatureBitset coalesceFeatures(const Module &M) {
+    FeatureBitset Features =
+        WasmTM
+            ->getSubtargetImpl(WasmTM->getTargetCPU(),
+                               WasmTM->getTargetFeatureString())
+            ->getFeatureBits();
+    for (auto &F : M)
+      Features |= WasmTM->getSubtargetImpl(F)->getFeatureBits();
+    return Features;
+  }
+
+  std::string getFeatureString(const FeatureBitset &Features) {
+    std::string Ret;
+    for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
+      if (Features[KV.Value])
+        Ret += (StringRef("+") + KV.Key + ",").str();
+    }
+    return Ret;
+  }
+
+  void replaceFeatures(Function &F, const std::string &Features) {
+    F.removeFnAttr("target-features");
+    F.removeFnAttr("target-cpu");
+    F.addFnAttr("target-features", Features);
+  }
+
+  bool stripAtomics(Module &M) {
+    // Detect whether any atomics will be lowered, since there is no way to tell
+    // whether the LowerAtomic pass lowers e.g. stores.
+    bool Stripped = false;
+    for (auto &F : M) {
+      for (auto &B : F) {
+        for (auto &I : B) {
+          if (I.isAtomic()) {
+            Stripped = true;
+            goto done;
+          }
+        }
+      }
+    }
+
+  done:
+    if (!Stripped)
+      return false;
+
+    LowerAtomicPass Lowerer;
+    FunctionAnalysisManager FAM;
+    for (auto &F : M)
+      Lowerer.run(F, FAM);
+
+    return true;
+  }
+
+  bool stripThreadLocals(Module &M) {
+    bool Stripped = false;
+    for (auto &GV : M.globals()) {
+      if (GV.getThreadLocalMode() !=
+          GlobalValue::ThreadLocalMode::NotThreadLocal) {
+        Stripped = true;
+        GV.setThreadLocalMode(GlobalValue::ThreadLocalMode::NotThreadLocal);
+      }
+    }
+    return Stripped;
+  }
+
+  void recordFeatures(Module &M, const FeatureBitset &Features, bool Stripped) {
+    for (const SubtargetFeatureKV &KV : WebAssemblyFeatureKV) {
+      std::string MDKey = (StringRef("wasm-feature-") + KV.Key).str();
+      if (KV.Value == WebAssembly::FeatureAtomics && Stripped) {
+        // "atomics" is special: code compiled without atomics may have had its
+        // atomics lowered to nonatomic operations. In that case, atomics is
+        // disallowed to prevent unsafe linking with atomics-enabled objects.
+        assert(!Features[WebAssembly::FeatureAtomics]);
+        M.addModuleFlag(Module::ModFlagBehavior::Error, MDKey,
+                        wasm::WASM_FEATURE_PREFIX_DISALLOWED);
+      } else if (Features[KV.Value]) {
+        // Otherwise features are marked Used or not mentioned
+        M.addModuleFlag(Module::ModFlagBehavior::Error, MDKey,
+                        wasm::WASM_FEATURE_PREFIX_USED);
+      }
+    }
+  }
 };
-char StripThreadLocal::ID = 0;
+char CoalesceFeaturesAndStripAtomics::ID = 0;
 
 /// WebAssembly Code Generator Pass Configuration Options.
 class WebAssemblyPassConfig final : public TargetPassConfig {
@@ -222,16 +320,11 @@ FunctionPass *WebAssemblyPassConfig::createTargetRegisterAllocator(bool) {
 //===----------------------------------------------------------------------===//
 
 void WebAssemblyPassConfig::addIRPasses() {
-  if (static_cast<WebAssemblyTargetMachine *>(TM)
-          ->getUsedFeatures()[WebAssembly::FeatureAtomics]) {
-    // Expand some atomic operations. WebAssemblyTargetLowering has hooks which
-    // control specifically what gets lowered.
-    addPass(createAtomicExpandPass());
-  } else {
-    // If atomics are not enabled, they get lowered to non-atomics.
-    addPass(createLowerAtomicPass());
-    addPass(new StripThreadLocal());
-  }
+  // Runs LowerAtomicPass if necessary
+  addPass(new CoalesceFeaturesAndStripAtomics(&getWebAssemblyTargetMachine()));
+
+  // This is a no-op if atomics are not used in the module
+  addPass(createAtomicExpandPass());
 
   // Add signatures to prototype-less function declarations
   addPass(createWebAssemblyAddMissingPrototypes());