<html><head><meta http-equiv="Content-Type" content="text/html charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Apr 7, 2016, at 1:21 PM, Peter Collingbourne <<a href="mailto:peter@pcc.me.uk" class="">peter@pcc.me.uk</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="Apple-interchange-newline"><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: 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;" class=""><div class="gmail_quote" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: 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 Thu, Apr 7, 2016 at 12:52 PM, Mehdi Amini<span class="Apple-converted-space"> </span><span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span><span class="Apple-converted-space"> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><br class=""><div class=""><div class=""><div class="h5"><blockquote type="cite" class=""><div class="">On Apr 7, 2016, at 12:39 PM, Peter Collingbourne <<a href="mailto:peter@pcc.me.uk" target="_blank" class="">peter@pcc.me.uk</a>> wrote:</div><br class=""><div class=""><br class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><div class="gmail_quote" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;">On Thu, Apr 7, 2016 at 12:29 PM, Mehdi Amini<span class=""> </span><span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><br class=""><div class=""><div class=""><div class=""><blockquote type="cite" class=""><div class="">On Apr 7, 2016, at 11:59 AM, Xinliang David Li <<a href="mailto:davidxl@google.com" target="_blank" class="">davidxl@google.com</a>> wrote:</div><br class=""><div class=""><br class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><div class="gmail_quote" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;">On Thu, Apr 7, 2016 at 11:26 AM, Mehdi Amini<span class=""> </span><span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><br class=""><div class=""><span class=""><blockquote type="cite" class=""><div class="">On Apr 7, 2016, at 10:58 AM, Xinliang David Li <<a href="mailto:davidxl@google.com" target="_blank" class="">davidxl@google.com</a>> wrote:</div><br class=""><div class=""><br class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><div class="gmail_quote" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;">On Wed, Apr 6, 2016 at 9:53 PM, Mehdi Amini<span class=""> </span><span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><br class=""><div class=""><span class=""><blockquote type="cite" class=""><div class="">On Apr 6, 2016, at 9:40 PM, Teresa Johnson <<a href="mailto:tejohnson@google.com" target="_blank" class="">tejohnson@google.com</a>> wrote:</div><br class=""><div class=""><br class=""><br style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;" class=""><div class="gmail_quote" style="font-family: Helvetica; font-size: 12px; font-style: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px;">On Wed, Apr 6, 2016 at 5:13 PM, Peter Collingbourne<span class=""> </span><span dir="ltr" class=""><<a href="mailto:peter@pcc.me.uk" target="_blank" class="">peter@pcc.me.uk</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div dir="ltr" class=""><br class=""><div class="gmail_extra"><br class=""><div class="gmail_quote"><span class="">On Wed, Apr 6, 2016 at 4:53 PM, Mehdi Amini<span class=""> </span><span dir="ltr" class=""><<a href="mailto:mehdi.amini@apple.com" target="_blank" class="">mehdi.amini@apple.com</a>></span><span class=""> </span>wrote:<br class=""><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><br class=""><div class=""><span class=""><blockquote type="cite" class=""><div class="">On Apr 6, 2016, at 4:41 PM, Peter Collingbourne <<a href="mailto:peter@pcc.me.uk" target="_blank" class="">peter@pcc.me.uk</a>> wrote:</div><br class=""><div class=""><div dir="ltr" class="">Hi all,<div class=""><br class=""></div><div class="">I'd like to propose changes to how we do promotion of global values in ThinLTO. The goal here is to make it possible to pre-compile parts of the translation unit to native code at compile time. For example, if we know that:</div><div class=""><br class=""></div><div class="">1) A function is a leaf function, so it will never import any other functions, and</div></div></div></blockquote><div class=""><br class=""></div></span><div class="">It still may be imported somewhere else right?</div><span class=""><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="">2) The function's instruction count falls above a threshold specified at compile time, so it will never be imported.</div></div></div></blockquote><div class=""><br class=""></div></span><div class="">It won’t be imported, but unless it is a “leaf” it may import and inline itself.</div></div></div></blockquote><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class=""><span class=""><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="">or</div><div class="">3) The compile-time threshold is zero, so there is no possibility of functions being imported (What's the utility of this? Consider a program transformation that requires whole-program information, such as CFI. During development, the import threshold may be set to zero in order to minimize the incremental link time while still providing the same CFI enforcement that would be used in production builds of the application.)</div><div class=""><br class=""></div><div class="">then the function's body will not be affected by link-time decisions, and we might as well produce its object code at compile time.<span class=""> </span></div></div></div></blockquote><div class=""><br class=""></div></span><div class="">Reading this last sentence, it seems exactly the “non-LTO” case?</div></div></div></blockquote><div class=""><br class=""></div></span><div class="">Yes, basically the point of this proposal is to be able to split the linkage unit into LTO and non-LTO parts.</div><span class=""><div class=""><br class=""></div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class=""><span class=""><br class=""><blockquote type="cite" class=""><div class=""><div dir="ltr" class=""><div class="">This will also allow the object code to be shared between linkage units (this should hopefully help solve a major scalability problem for Chromium, as that project contains a large number of test binaries based on common libraries).</div><div class=""><br class=""></div><div class="">This can be done with a change to the intermediate object file format. We can represent object files as native code containing statically compiled functions and global data in the .text,. data, .rodata (etc.) sections, with an .llvmbc section (or, I suppose, "__LLVM, __bitcode" when targeting Mach-O) containing bitcode for functions to be compiled at link time.</div><div class=""><br class=""></div><div class="">In order to make this work, we need to make sure that references from link-time compiled functions to statically compiled functions work correctly in the case where the statically compiled function has internal linkage. We can do this by promoting every global value with internal linkage, using a hash of the external names (as I mentioned in [1]).</div></div></div></blockquote></span></div></div></blockquote></span></div></div></div></blockquote><div class=""><br class=""></div><div class="">Mehdi - I know you were keen to reduce the amount of promotion. Is that still an issue for you assuming linker GC (dead stripping)?</div></div></div></blockquote><div class=""><br class=""></div></span><div class="">Yes: we do better optimization on internal function in general.<span class=""> </span></div></div></div></blockquote><div class=""><br class=""></div><div class="">Inliner is one of the affected optimization -- however this sounds like a matter of tuning to teach inliner about promoted static functions.<span class=""> </span></div></div></div></blockquote><div class=""><br class=""></div></span><div class="">The inliner compute a tradeoff between pseudo runtime cost and binary size, the existing bonus for static functions is when there is a single call site because it makes the binary increase inexistant (dropping the static after inline). We promote function because we think we are likely to introduce a reference to it somewhere else, so “lying” to the inliner is not necessarily a good idea.</div></div></div></blockquote><div class=""><br class=""></div><div class="">It is not lying to the inliner. If a static (before promotion) function is a candidate to be inlined in the original defining module, it is probably more likely to inlined in other importing modules where more context is available. In other words, the inliner can apply the same bonus to 'promoted' static functions as if references in other modules will also disappear. Of course, we can not assume it has single callsite.</div><div class=""><br class=""></div><div class="">Comdat functions can be handled similarly. </div><div class=""><br class=""></div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class=""><div class=""><div class="">That said we (actually Bruno did) prototyped it already with somehow good results :)</div><div class="">I’m not convinced yet that it should be independent of promoted or not promoted though.</div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Generally true (see the comdat case).</div><div class=""> </div><blockquote class="gmail_quote" style="margin: 0px 0px 0px 0.8ex; border-left-width: 1px; border-left-color: rgb(204, 204, 204); border-left-style: solid; padding-left: 1ex;"><div style="word-wrap: break-word;" class=""><div class=""><div class=""><div class=""><br class=""></div></div><div class="">Assuming we solve the inliner issue, then remain the “optimizations other than inliner”. We can probably solve most but I suspect it won’t be “trivial” either.</div></div></div></blockquote><div class=""><br class=""></div><div class=""><br class=""></div><div class="">Any such optimizations in mind?</div></div></div></blockquote><div class=""><br class=""></div></div></div><div class="">I don’t have the details, but in short:</div><div class=""><br class=""></div><div class="">For promoted functions: IPSCCP, dead arg elimination</div><div class="">For promoted global variables: anything that is impacted somehow by aliasing</div></div></div></blockquote><div class=""><br class=""></div><div class="">When are you imagining that promotion would happen? If it happens just before codegen (or bitcode emission), it wouldn't inhibit these optimizations, right?</div></div></div></blockquote><div class=""><br class=""></div></div></div><div class="">For ThinLTO it has to happen before the link-time optimizations, because of cross-module importing.</div></div></div></blockquote><div class=""> </div><div class="">Are you referring to the fact that these optimizations would be inhibited versus regular LTO, since we cannot internalize? Yes, that does seem like an issue.</div></div></div></blockquote><div><br class=""></div></div>Yes, this is an issue I'm fighting with currently with ThinLTO.<div class="">(And I haven't reach the tuning stage yet because I can't nail the infrastructure these days...)</div><div class=""><br class=""></div><div class="">-- </div><div class="">Mehdi</div><div class=""><br class=""></div></body></html>