[WebAssembly] Implement thread-local storage (local-exec model)

Summary:
Thread local variables are placed inside a `.tdata` segment. Their symbols are
offsets from the start of the segment. The address of a thread local variable
is computed as `__tls_base` + the offset from the start of the segment.

`.tdata` segment is a passive segment and `memory.init` is used once per thread
to initialize the thread local storage.

`__tls_base` is a wasm global. Since each thread has its own wasm instance,
it is effectively thread local. Currently, `__tls_base` must be initialized
at thread startup, and so cannot be used with dynamic libraries.

`__tls_base` is to be initialized with a new linker-synthesized function,
`__wasm_init_tls`, which takes as an argument a block of memory to use as the
storage for thread locals. It then initializes the block of memory and sets
`__tls_base`. As `__wasm_init_tls` will handle the memory initialization,
the memory does not have to be zeroed.

To help allocating memory for thread-local storage, a new compiler intrinsic
is introduced: `__builtin_wasm_tls_size()`. This instrinsic function returns
the size of the thread-local storage for the current function.

The expected usage is to run something like the following upon thread startup:

    __wasm_init_tls(malloc(__builtin_wasm_tls_size()));

Reviewers: tlively, aheejin, kripken, sbc100

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

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D64537

llvm-svn: 366272

1 files changed, 16 insertions, 7 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
index a75df34..7e65368 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyTargetMachine.cpp
@@ -186,13 +186,21 @@ public:
     for (auto &F : M)
       replaceFeatures(F, FeatureStr);
 
-    bool Stripped = false;
-    if (!Features[WebAssembly::FeatureAtomics]) {
-      Stripped |= stripAtomics(M);
-      Stripped |= stripThreadLocals(M);
-    }
+    bool StrippedAtomics = false;
+    bool StrippedTLS = false;
+
+    if (!Features[WebAssembly::FeatureAtomics])
+      StrippedAtomics = stripAtomics(M);
+
+    if (!Features[WebAssembly::FeatureBulkMemory])
+      StrippedTLS = stripThreadLocals(M);
+
+    if (StrippedAtomics && !StrippedTLS)
+      stripThreadLocals(M);
+    else if (StrippedTLS && !StrippedAtomics)
+      stripAtomics(M);
 
-    recordFeatures(M, Features, Stripped);
+    recordFeatures(M, Features, StrippedAtomics || StrippedTLS);
 
     // Conservatively assume we have made some change
     return true;
@@ -271,7 +279,8 @@ private:
         // "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]);
+        assert(!Features[WebAssembly::FeatureAtomics] ||
+               !Features[WebAssembly::FeatureBulkMemory]);
         M.addModuleFlag(Module::ModFlagBehavior::Error, MDKey,
                         wasm::WASM_FEATURE_PREFIX_DISALLOWED);
       } else if (Features[KV.Value]) {