[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, 4 insertions, 0 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp
index 312b203..2552e91 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyFastISel.cpp
@@ -233,6 +233,8 @@ bool WebAssemblyFastISel::computeAddress(const Value *Obj, Address &Addr) {
       return false;
     if (Addr.getGlobalValue())
       return false;
+    if (GV->isThreadLocal())
+      return false;
     Addr.setGlobalValue(GV);
     return true;
   }
@@ -614,6 +616,8 @@ unsigned WebAssemblyFastISel::fastMaterializeConstant(const Constant *C) {
   if (const GlobalValue *GV = dyn_cast<GlobalValue>(C)) {
     if (TLI.isPositionIndependent())
       return 0;
+    if (GV->isThreadLocal())
+      return 0;
     unsigned ResultReg =
         createResultReg(Subtarget->hasAddr64() ? &WebAssembly::I64RegClass
                                                : &WebAssembly::I32RegClass);