<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 Apr 17, 2019, at 10:42 AM, Amara Emerson <<a href="mailto:aemerson@apple.com" class="">aemerson@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><br class="Apple-interchange-newline"><br class="" 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;"><blockquote type="cite" class="" 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-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;"><div class="">On Apr 17, 2019, at 9:35 AM, JF Bastien via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="" 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;"><blockquote type="cite" class=""><div class=""><br class="Apple-interchange-newline">On Apr 17, 2019, at 5:02 AM, Arsenault, Matthew via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-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 class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;">This won’t happen with volatile load/store</div></div></div></blockquote><div class=""><br class=""></div><div class="">This is mostly true today, but AFAICT the LLVM memory model doesn’t actually offer this guarantee. It merely says that LLVM treats volatile like C / C++ treats volatile… which isn’t much of a guarantee because C / C++ volatile doesn’t normatively mean anything. Specifically, we cannot really honor this when volatile bitfields are used for which memory operations don’t exist:</div><div class=""><br class=""></div></div><blockquote class="" 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; margin: 0px 0px 0px 40px; border: none; padding: 0px;"><div class=""><div class=""><font face="Courier" class="">struct {</font></div></div><div class=""><div class=""><font face="Courier" class=""> volatile int a : 12;</font></div></div><div class=""><div class=""><font face="Courier" class=""> volatile int b : 4;</font></div></div><div class=""><div class=""><font face="Courier" class="">} s;</font></div></div></blockquote><div class="" 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;"><div class=""><br class=""></div><div class="">As things stand, we haven’t promised that we won’t combine adjacent volatile stores, and C / C++ certainly allow us to do so. I don’t think it would be a good idea to do so, but we certainly could.</div></div></div></blockquote><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="">Is this really true? The writes to two volatile variables are sequenced and can’t be re-ordered since they can have side effects, and so merging adjacent volatile stores would break this.</span></div></blockquote><div><br class=""></div><div>There’s further discussion of what we promise downthread. It’s definitely true of C / C++, a handwavy promise in LLVM LangRef, and the LLVM memory model just says “same as C / C++".</div><div><br class=""></div><br class=""><blockquote type="cite" class=""><div class=""><blockquote type="cite" class="" 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-size-adjust: auto; -webkit-text-stroke-width: 0px; text-decoration: none;"><div class=""><div class="" 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;"><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="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="font-size: 11pt;"> </span><span class="" style="font-size: 11pt;">-Matt</span> </div></div></div></blockquote><blockquote type="cite" 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="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p class=""></o:p></div><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p class=""> </o:p></div><div class="" style="border-style: solid none none; border-top-width: 1pt; border-top-color: rgb(181, 196, 223); padding: 3pt 0in 0in;"><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><b class=""><span class="" style="font-size: 12pt;">From:<span class="Apple-converted-space"> </span></span></b><span class="" style="font-size: 12pt;">llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org" class="">llvm-dev-bounces@lists.llvm.org</a>> on behalf of llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>><br class=""><b class="">Reply-To:<span class="Apple-converted-space"> </span></b>Neil Ryan <<a href="mailto:neilryan@cs.washington.edu" class="">neilryan@cs.washington.edu</a>><br class=""><b class="">Date:<span class="Apple-converted-space"> </span></b>Tuesday, April 16, 2019 at 9:01 PM<br class=""><b class="">To:<span class="Apple-converted-space"> </span></b>llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="">llvm-dev@lists.llvm.org</a>><br class=""><b class="">Subject:<span class="Apple-converted-space"> </span></b>[llvm-dev] Disable combining of loads and stores in instcombine<o:p class=""></o:p></span></div></div><div class=""><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><o:p class=""> </o:p></div></div><div name="messageBodySection" class=""><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="font-size: 10.5pt; font-family: Arial, sans-serif;"> LLVM's optimizer combines stores to consecutive characters into a write of a single word. For instance, if I have char A[4] and I write some static value to each element, these writes would be combined into a single 32-bit word write. I found<span class="Apple-converted-space"> </span><a href="http://llvm.1065342.n5.nabble.com/disabling-combining-load-stores-in-optimizer-td37560.html" target="_top" class="" style="color: purple; text-decoration: underline;">this thread</a> from 2009 -- it seems like it wasn't possible then. Has anything changed since? <o:p class=""></o:p></span></div></div><div name="messageSignatureSection" class=""><div class="" style="margin: 0in 0in 0.0001pt; font-size: 11pt; font-family: Calibri, sans-serif;"><span class="" style="font-size: 10.5pt; font-family: Arial, sans-serif;"><br class="">Neil<o:p class=""></o:p></span></div></div></div><span class="" 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;">_______________________________________________</span><br class="" 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; 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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;"><span class="" 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;">_______________________________________________</span><br class="" 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; 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