<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><br><div><div>On May 15, 2014, at 9:50 AM, Philip Reames <<a href="mailto:listmail@philipreames.com">listmail@philipreames.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite">
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<div class="moz-cite-prefix">Given the use case (user mode
scheduling), I'm not going to oppose this proposal. I would like
to see a couple of things clarified documentation wise:<br>
- When is this interface valid? (i.e. the single thread case)<br>
- If a context does have multiple threads, is this called once per
thread? Or once per thread group after internal coordination?
(you can write this out of scope if desired)<br>
- If we later introduce multiple threads, and this mechanism
doesn't support it, what will happen? Will the function just not
be called? <br>
- You hint at this already, but clarifying the state of the
current context at a suspend point would be helpful. <br>
<br>
Here's a possible draft that includes the above:<br>
<font face="Menlo">The may-suspend callback function may be called
by LLVM to transfer control back to the client that invoked the
LLVM compilation. This can be used to yield control of the
thread, or perform periodic work needed by the client. There is
no guaranteed frequency at which callbacks must occur; in fact,
the client is not guaranteed to ever receive this callback. It
is at the sole discretion of LLVM to do so and only if it
can guarantee that suspending the thread won't block any forward
progress in other LLVM contexts. </font><br>
<br>
<font face="Menlo">At a suspend point, the state of the current
LLVM context is intentionally undefined. No assumptions about
it can or should be made. In particular, call backs into the
context are not supported until the suspend function returns
control to LLVM. Other LLVM contexts are unaffected.</font></div></div></blockquote><div><br></div><div>Great.</div><div><br></div><blockquote type="cite"><div text="#000000" bgcolor="#FFFFFF"><div class="moz-cite-prefix">Currently, LLVM assumes one thread per LLVM context. If,
or when, we introduce multiple threads, this interface will not be
available for contexts which opt-in to the thread pool model. We
may extend this interface at a later time to support thread pools,
but for the moment, that use case is explicitly unsupported. <br></div></div></blockquote><div><br></div><div>Correct. We should avoid mentioning multi-thread contexts in the API docs. It is very misleading to describe a feature that LLVM does not support. We can instead add a statement to the docs explaining that the callback is only synchronous with respect to the calling thread and places no guarantee on the state of other threads, regardless of their context.</div><br><blockquote type="cite"><div text="#000000" bgcolor="#FFFFFF"><div class="moz-cite-prefix">
p.s. Bikeshed wise, might "yield" be a better term than "suspend"
here? <br></div></div></blockquote><div><br></div><div>I personally like calling it “yield” because it is more intuitive and describes the use case. I proposed maySuspend because I wanted to be accurate. It is really the client deciding what to do with the callback. LLVM should make no assumption that it’s actually yielding.</div><div><br></div><div>Chandler likes “yield” too, so lets go with that unless anyone else wants to weigh in.</div><div><br></div><div>On the commits list, Juergen introduced our current use case, along with a couple other future use cases for this API. I’m sorry I neglected to clearly reiterate our usage, but when it comes to documenting the C API I intentionally try not to limit its potential to a narrow use case.</div><div><br></div><div>-Andy</div><br><blockquote type="cite"><div text="#000000" bgcolor="#FFFFFF"><div class="moz-cite-prefix">
Philip<br>
<br>
On 05/13/2014 11:49 AM, Juergen Ributzka wrote:<br>
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<div>Sounds good. Lets get started by nailing down the C API and
semantics for this first.</div>
<div><br>
</div>
<div>I mirrored the C API for the LLVM context diagnostic handler
and used Andy’s suggested name for the callback.</div>
<div>The opaque handle was suggested by Duncan and can provide
optional user specified information that is</div>
<div>provided back during the callback (i.e. barrier, etc).</div>
<div><br>
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<div>Cheers,</div>
<div>Juergen </div>
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<div>
<div><font face="Menlo">Core.h:</font></div>
<font face="Menlo">typedef void
(*LLVMMaySuspendCallback)(LLVMContextRef, void *);</font>
<div><font face="Menlo"><br>
</font></div>
<div><font face="Menlo">/**</font></div>
<div><font face="Menlo"> * Set the may-suspend callback function
for this context.<br>
*<br>
* @see LLVMContext::setMaySuspendCallback()<br>
*/<br>
void LLVMContextSetMaySuspendCallback(LLVMContextRef C,<br>
LLVMMaySuspendCallback
Callback,<br>
void *OpaqueHandle);</font></div>
<div><font face="Menlo"><br>
</font></div>
<div><font face="Menlo">LLVMContext.h:</font></div>
<div><font face="Menlo">/// \brief Registers a may-suspend
callback with the context.<br>
///<br>
/// The may-suspend callback function may be called by LLVM
to transfer<br>
/// control back to the client that invoked the LLVM
compilation. The client<br>
/// is not garantueed to ever receive this callback. It is
at the sole<br>
/// discretion of LLVM to do so and only if it
can guarantee that suspending<br>
/// the thread won't block any forward progress in other
LLVM contexts.<br>
void setMaySuspendCallback(MaySuspendCallbackTy Callback,
void *OpaqueHandle);<br>
<br>
/// \brief Calls the may-suspend callback (if applicable).<br>
///<br>
/// This transfers control back to the client, which may
suspend the current<br>
/// thread. Only call this method when LLVM doesn't hold any
global mutex or<br>
/// cannot block the execution in another LLVM context.<br>
void callMaySuspendCallback();</font></div>
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<div>On May 12, 2014, at 5:26 PM, Nick Lewycky <<a moz-do-not-send="true" href="mailto:nlewycky@google.com">nlewycky@google.com</a>>
wrote:</div>
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<blockquote type="cite">
<blockquote class="gmail_quote" style="font-family:
Helvetica; font-size: 12px; font-style: normal;
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static; z-index: auto;">
<div style="word-wrap: break-word;">Would you (or
anyone) oppose a simple maySuspendContext() callback
API? It would mean nothing more than the thread(s) for
a given LLVM context can be suspended independent from
other contexts.</div>
</blockquote>
<div style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant: normal; font-weight:
normal; letter-spacing: normal; line-height: normal;
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</div>
<div style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant: normal; font-weight:
normal; letter-spacing: normal; line-height: normal;
orphans: auto; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: auto;
word-spacing: 0px; -webkit-text-stroke-width: 0px;">I
think this is the right approach. So a given thread hits
a safe point, it optionally calls a "suspend check" or
"i an safe to suspend right now" callback if set. It
doesn't stop other threads, it doesn't continue until
the function returns.</div>
<div style="font-family: Helvetica; font-size: 12px;
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normal; letter-spacing: normal; line-height: normal;
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</div>
<div style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant: normal; font-weight:
normal; letter-spacing: normal; line-height: normal;
orphans: auto; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: auto;
word-spacing: 0px; -webkit-text-stroke-width: 0px;">If
you want to stop all threads then the user callback may
contain a barrier and count down how many threads have
stopped until it sees all of them.</div>
<div style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant: normal; font-weight:
normal; letter-spacing: normal; line-height: normal;
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</div>
<div style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant: normal; font-weight:
normal; letter-spacing: normal; line-height: normal;
orphans: auto; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: auto;
word-spacing: 0px; -webkit-text-stroke-width: 0px;">Nick</div>
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