These relocations represent offsets from the __tls_base symbol.
Previously we were just using normal MEMORY_ADDR relocations and relying
on the linker to select a segment-offset rather and absolute value in
Symbol::getVirtualAddress(). Using an explicit relocation type allows
allow us to clearly distinguish absolute from relative relocations based
on the relocation information alone.
One place this is useful is being able to reject absolute relocation in
the PIC case, but still accept TLS relocations.
Differential Revision: https://reviews.llvm.org/D91276
Summary:
Currently, on Emscripten, dynamic linking is not supported with threads.
This means that if thread-local storage is used, it must be used in a
statically-linked executable. Hence, local-exec is the only possible model.
This diff compiles all TLS variables to use local-exec on Emscripten as a
temporary measure until dynamic linking is supported with threads.
The goal for this is to allow C++ types with constructors to be thread-local.
Currently, when `clang` compiles a `thread_local` variable with a constructor,
it generates `__tls_guard` variable:
@__tls_guard = internal thread_local global i8 0, align 1
As no TLS model is specified, this is treated as general-dynamic, which we do
not support (and cannot support without implementing dynamic linking support
with threads in Emscripten). As a result, any C++ constructor in `thread_local`
variables would not compile.
By compiling all `thread_local` as local-exec, `__tls_guard` will compile and
we can support C++ constructors with TLS without implementing dynamic linking
with threads.
Depends on D64537
Reviewers: tlively, aheejin, sbc100
Reviewed By: aheejin
Subscribers: dschuff, jgravelle-google, hiraditya, sunfish, llvm-commits
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D64776
llvm-svn: 366275
Sam Clegg