<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;"><div>The addition about the undefined state of the current context is a very good point. Although I wouldn’t like to shut the door completely and allow for future extensions to relax this constraint for APIs that explicitly state that they are safe to use during a yield callback.</div><div><br></div><div>I agree with Andy that we shouldn’t mention multi-threaded context at all in the API docs, because we are not even supporting it right now. I think this feature should be opt-in anyways, so extending the API documentation once we support it should be sufficient.</div><div><br></div><div>What I would like to see of course is that we would still be able to support this callback for a multi-threaded context/pass manager design in such a way, that it is possible to suspend any thread at a suspension point and it won’t block any other thread even in the same context. But I don’t want to write this in stone now. This gives us all the flexibility we need for developing a good concurrent design.</div><div><br></div><div>Having the ability to dynamically add and remove threads from an active compilation would be a nice feature that we should try to incorporate when possible and that would work nicely with the yield callback.</div><div><br></div><div>Anyways, I am digressing to much. As I said before we should only specify the multi-threaded context constrains in the API doc once they are actually available. For now this is simply undefined behavior and not even supported.</div><div><br></div><div>I extended the API with Philip’s suggestions and updated the doc. Please ignore proper 80col formatting for now ;-)</div><div><br></div><div>Does this look good to everyone?</div><div><br></div><div>-Juergen</div><div><br></div><div><div><font face="Menlo">Core.h:</font></div><font face="Menlo">typedef void (*LLVMYieldCallback)(LLVMContextRef, void *);</font><div><font face="Menlo"><br></font></div><div><font face="Menlo">/**</font></div><div><font face="Menlo"> * Set the yield callback function for this context.</font></div><div><font face="Menlo"> *<br> * @see LLVMContext::setYieldCallback()<br> */<br>void LLVMContextSetYieldCallback(LLVMContextRef C,<br> LLVMYieldCallback 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 yield callback with the given context.<br>///</font></div><div><font face="Menlo">/// The yield callback function may be called by LLVM to transfer</font></div><div><font face="Menlo">/// control back to the client that invoked the LLVM compilation. This can</font></div><div><font face="Menlo">/// be used to yield control of the thread, or perform periodic work needed</font></div><div><font face="Menlo">/// by the client. There is no guaranteed frequency at which callbacks must</font></div><div><font face="Menlo">/// occur; in fact, the client is not guaranteed to ever receive this callback.</font></div><div><font face="Menlo">/// It is at the sole discretion of LLVM to do so and only if it can guarantee</font></div><div><font face="Menlo">/// that suspending the thread won't block any forward progress in other LLVM</font></div><div><font face="Menlo">/// contexts in the same process.</font></div><div><font face="Menlo">///</font></div><div><font face="Menlo">/// At a suspend point, the state of the current LLVM context is intentionally</font></div><div><font face="Menlo">/// undefined. No assumptions about it can or should be made. Only LLVM context</font></div><div><font face="Menlo">/// API calls </font><span style="font-family: Menlo;">that explicitly state that they can be used during a yield callback</span></div><div><span style="font-family: Menlo;">/// are </span><span style="font-family: Menlo;">allowed to be used. Any other API calls into the context are not supported</span></div><div><font face="Menlo">/// until the yield callback function returns control to LLVM. Other LLVM contexts</font></div><div><font face="Menlo">/// are unaffected by this restriction.</font></div><div><font face="Menlo">void setYieldCallback(YieldCallbackTy Callback, void *OpaqueHandle);<br><br>/// \brief Calls the yield 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 yieldToClient();</font></div></div><div><br></div><br><div><div>On May 15, 2014, at 11:32 AM, Andrew Trick <<a href="mailto:atrick@apple.com">atrick@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><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;"><br class="Apple-interchange-newline">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" 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;"><blockquote type="cite" 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;"><div text="#000000" bgcolor="#FFFFFF"><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? <span class="Apple-converted-space"> </span><br>- You hint at this already, but clarifying the state of the current context at a suspend point would be helpful. <span class="Apple-converted-space"> </span><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. <span class="Apple-converted-space"> </span></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 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;"><br></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;">Great.</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;"><br></div><blockquote type="cite" 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;"><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. <span class="Apple-converted-space"> </span><br></div></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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br></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;">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 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;"><blockquote type="cite" 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;"><div text="#000000" bgcolor="#FFFFFF"><div class="moz-cite-prefix">p.s. Bikeshed wise, might "yield" be a better term than "suspend" here? <span class="Apple-converted-space"> </span><br></div></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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br></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 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 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;"><br></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;">Chandler likes “yield” too, so lets go with that unless anyone else wants to weigh in.</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;"><br></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;">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 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;"><br></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;">-Andy</div><br 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;"><blockquote type="cite" 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;"><div text="#000000" bgcolor="#FFFFFF"><div class="moz-cite-prefix">Philip<br><br>On 05/13/2014 11:49 AM, Juergen Ributzka wrote:<br></div><blockquote cite="mid:0A13BA19-4D1C-40D5-A31E-99C1909B82B0@apple.com" type="cite"><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></div><div>Cheers,</div><div>Juergen </div><div><br></div><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></div><div><br><div><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><br class="Apple-interchange-newline"><blockquote type="cite"><blockquote class="gmail_quote" 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; margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex; position: 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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br></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; 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;"><br></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; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;"><br></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></blockquote></div><br></div><br><fieldset class="mimeAttachmentHeader"></fieldset><br><pre wrap="">_______________________________________________
LLVM Developers mailing list
<a class="moz-txt-link-abbreviated" href="mailto:LLVMdev@cs.uiuc.edu">LLVMdev@cs.uiuc.edu</a> <a class="moz-txt-link-freetext" href="http://llvm.cs.uiuc.edu/">http://llvm.cs.uiuc.edu</a>
<a class="moz-txt-link-freetext" href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a>
</pre></blockquote><br></div>_______________________________________________<br>LLVM Developers mailing list<br><a href="mailto:LLVMdev@cs.uiuc.edu">LLVMdev@cs.uiuc.edu</a><span class="Apple-converted-space"> </span> <a href="http://llvm.cs.uiuc.edu/">http://llvm.cs.uiuc.edu</a><br><a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a></blockquote></blockquote></div><br></body></html>