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auto merge of #11711 : alexcrichton/rust/issue-11683, r=brson

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There's lots of fun rationale in the comments of the diff.

Closes #11683
This commit is contained in:
bors 2014-01-22 00:51:20 -08:00
commit 750d48b0ad
1 changed files with 58 additions and 4 deletions
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62
src/libstd/rt/local_ptr.rs
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@ -94,12 +94,63 @@ pub mod compiled {
pub unsafe fn cleanup() {}
// Rationale for all of these functions being inline(never)
//
// The #[thread_local] annotation gets propagated all the way through to
// LLVM, meaning the global is specially treated by LLVM to lower it to an
// efficient sequence of instructions. This also involves dealing with fun
// stuff in object files and whatnot. Regardless, it turns out this causes
// trouble with green threads and lots of optimizations turned on. The
// following case study was done on linux x86_64, but I would imagine that
// other platforms are similar.
//
// On linux, the instruction sequence for loading the tls pointer global
// looks like:
//
// mov %fs:0x0, %rax
// mov -0x8(%rax), %rbx
//
// This code leads me to believe that (%fs:0x0) is a table, and then the
// table contains the TLS values for the process. Hence, the slot at offset
// -0x8 is the task TLS pointer. This leads us to the conclusion that this
// table is the actual thread local part of each thread. The kernel sets up
// the fs segment selector to point at the right region of memory for each
// thread.
//
// Optimizations lead me to believe that this code is lowered to these
// instructions in the LLVM codegen passes, because you'll see code like
// this when everything is optimized:
//
// mov %fs:0x0, %r14
// mov -0x8(%r14), %rbx
// // do something with %rbx, the rust Task pointer
//
// ... // <- do more things
//
// mov -0x8(%r14), %rbx
// // do something else with %rbx
//
// Note that the optimization done here is that the first load is not
// duplicated during the lower instructions. This means that the %fs:0x0
// memory location is only dereferenced once.
//
// Normally, this is actually a good thing! With green threads, however,
// it's very possible for the code labeled "do more things" to context
// switch to another thread. If this happens, then we *must* re-load %fs:0x0
// because it's changed (we're on a different thread). If we don't re-load
// the table location, then we'll be reading the original thread's TLS
// values, not our thread's TLS values.
//
// Hence, we never inline these functions. By never inlining, we're
// guaranteed that loading the table is a local decision which is forced to
// *always* happen.
/// Give a pointer to thread-local storage.
///
/// # Safety note
///
/// Does not validate the pointer type.
#[inline]
#[inline(never)] // see comments above
pub unsafe fn put<T>(sched: ~T) {
RT_TLS_PTR = cast::transmute(sched)
}
@ -109,7 +160,7 @@ pub mod compiled {
/// # Safety note
///
/// Does not validate the pointer type.
#[inline]
#[inline(never)] // see comments above
pub unsafe fn take<T>() -> ~T {
let ptr = RT_TLS_PTR;
rtassert!(!ptr.is_null());
@ -124,7 +175,7 @@ pub mod compiled {
/// # Safety note
///
/// Does not validate the pointer type.
#[inline]
#[inline(never)] // see comments above
pub unsafe fn try_take<T>() -> Option<~T> {
let ptr = RT_TLS_PTR;
if ptr.is_null() {
@ -143,18 +194,20 @@ pub mod compiled {
///
/// Does not validate the pointer type.
/// Leaves the old pointer in TLS for speed.
#[inline]
#[inline(never)] // see comments above
pub unsafe fn unsafe_take<T>() -> ~T {
cast::transmute(RT_TLS_PTR)
}
/// Check whether there is a thread-local pointer installed.
#[inline(never)] // see comments above
pub fn exists() -> bool {
unsafe {
RT_TLS_PTR.is_not_null()
}
}
#[inline(never)] // see comments above
pub unsafe fn unsafe_borrow<T>() -> *mut T {
if RT_TLS_PTR.is_null() {
rtabort!("thread-local pointer is null. bogus!");
@ -162,6 +215,7 @@ pub mod compiled {
RT_TLS_PTR as *mut T
}
#[inline(never)] // see comments above
pub unsafe fn try_unsafe_borrow<T>() -> Option<*mut T> {
if RT_TLS_PTR.is_null() {
None