<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Jul 30, 2019, at 5:21 PM, Eli Friedman via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" class="">cfe-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><b class="">From:</b><span class="Apple-converted-space"> </span>cfe-dev <<a href="mailto:cfe-dev-bounces@lists.llvm.org" style="color: purple; text-decoration: underline;" class="">cfe-dev-bounces@lists.llvm.org</a>><span class="Apple-converted-space"> </span><b class="">On Behalf Of<span class="Apple-converted-space"> </span></b>Reid Kleckner via cfe-dev<br class=""><b class="">Sent:</b><span class="Apple-converted-space"> </span>Tuesday, July 30, 2019 4:54 PM<br class=""><b class="">To:</b><span class="Apple-converted-space"> </span>Billy O'Neal (VC LIBS) <<a href="mailto:bion@microsoft.com" style="color: purple; text-decoration: underline;" class="">bion@microsoft.com</a>><br class=""><b class="">Cc:</b><span class="Apple-converted-space"> </span>cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" style="color: purple; text-decoration: underline;" class="">cfe-dev@lists.llvm.org</a>><br class=""><b class="">Subject:</b><span class="Apple-converted-space"> </span>[EXT] Re: [cfe-dev] FYI, Intel folks might be looking to add the __iso_volatile_Xxx family for MSVC STL <atomic> soon<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div class=""><div class=""><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class="">On Tue, Jul 30, 2019 at 4:15 PM Billy O'Neal (VC LIBS) <<a href="mailto:bion@microsoft.com" style="color: purple; text-decoration: underline;" class="">bion@microsoft.com</a>> wrote:<o:p class=""></o:p></div></div><div class=""><blockquote style="border-style: none none none solid; border-left-width: 1pt; border-left-color: rgb(204, 204, 204); padding: 0in 0in 0in 6pt; margin-left: 4.8pt; margin-right: 0in;" class=""><div class=""><div class=""><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class="">> we can worry about optimization, perhaps using _Atomic, later<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class=""> <o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class="">We can’t use _Atomic because it would not be ABI compatible with our std::atomic; in particular, because we put the spinlock for non-lock-free atomics inside the atomic, for instance. And because that isn’t a thing for some of our supported frontends.<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class=""> <o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class="">It is possible that there are different intrinsics we could call for Clang that would make you folks happier, but we don’t know what those are or even if they exist at present.<o:p class=""></o:p></div></div></div></blockquote><div class=""><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div></div><div class=""><div style="margin: 0in 0in 0.0001pt 0.5in; font-size: 11pt; font-family: Calibri, sans-serif;" class="">I was thinking that perhaps the _Atomic_address_as template would do the necessary casts to use it when necessary without changing the storage type inside the std::atomic object.<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">There are actually multiple APIs to access C++11 atomics in clang, since we invented APIs in parallel with gcc. The __atomic_* builtins (<a href="https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html" style="color: purple; text-decoration: underline;" class="">https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html</a>) don’t require the use of _Atomic types.<o:p class=""></o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class=""><o:p class=""> </o:p></div><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">Using volatile loads and stores with barriers probably prevents the compiler from performing any breaking optimizations, assuming the RMW atomics are protected appropriately, and the user doesn’t call any of the clang atomic builtins directly. But it’s not a good idea; it won’t optimize well. For example, on AArch64, it will prevent the compiler from using the dedicated load-acquire and store-release instructions.</div></div></div></div></div></div></blockquote><div><br class=""></div><div>TL;DR: what are these _iso_ things? I read the bug report, I don’t see the details I’d expect.</div><div>I think the __atomic_* builtins should work as Eli suggests.</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><div class="WordSection1" style="page: WordSection1; caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;"><div class=""><div class=""><div class=""><div style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;" class="">-Eli<o:p class=""></o:p></div></div></div></div></div><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">_______________________________________________</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><span style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none; float: none; display: inline !important;" class="">cfe-dev mailing list</span><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><a href="mailto:cfe-dev@lists.llvm.org" style="color: purple; text-decoration: underline; 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">cfe-dev@lists.llvm.org</a><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""><a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" style="color: purple; text-decoration: underline; 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-size-adjust: auto; -webkit-text-stroke-width: 0px;" class="">https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><br style="caret-color: rgb(0, 0, 0); font-family: Helvetica; font-size: 12px; font-style: normal; font-variant-caps: normal; font-weight: normal; letter-spacing: normal; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration: none;" class=""></div></blockquote></div><br class=""></body></html>