<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="">Chandler,<div class=""> Please give some citations, I’ve search the llvm-dev archives and didn't find any.</div><div class=""><br class=""></div><div class="">Peter Lawrence.</div><div class=""><br class=""></div><div class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jun 28, 2017, at 12:01 PM, Chandler Carruth <<a href="mailto:chandlerc@gmail.com" class="">chandlerc@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class="gmail_quote"><div dir="ltr" class="">On Wed, Jun 28, 2017 at 9:39 AM Peter Lawrence <<a href="mailto:peterl95124@sbcglobal.net" class="">peterl95124@sbcglobal.net</a>> wrote:<br class=""></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div dir="auto" style="word-wrap:break-word" class=""><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><div style="margin:0px;line-height:normal;min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Preface: This paper shows that "poison" was never actually necessary</span></div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">in the first place. “Poison"</span>s existence is based on incorrect assumptions </div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">that are being explored for the first time.</span></div></div></div></div></blockquote><div class=""><br class=""></div><div class="">Just so you are aware, there have been numerous people in the community who have been exploring the issues you bring up for many years. So I don't think it is correct to say that they are being explored for the first time.</div><div class=""> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div style="word-wrap:break-word" class=""><div dir="auto" style="word-wrap:break-word" class=""><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><div style="margin:0px;line-height:normal;min-height:16px" class=""><br class=""></div><div style="margin:0px;line-height:normal;min-height:16px" class=""><br class=""></div><div style="margin:0px;line-height:normal;min-height:16px" class=""><br class=""></div></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">I have been re-reading Dan Gohman's original post "the nsw story" [1]</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">and have come to the conclusion that Dan got it wrong in some respects.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">He came up with "no signed wrap" ("nsw") which in his words means</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">"The purpose of the nsw flag is to indicate instructions which are</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">known to have no overflow". The key operative word here is "known".</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">This means literally an operation with an additional "llvm.assume".</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">For example (a +nsw b) when a and b are i32 is really</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> ((llvm.assume((INT_MIN <= ((i64)a+(i64)b) <= INT_MAX) , (a + b))</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">It is this "assume" that justifies the transformation that Dan does</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">to optimize sign-extension of i32 induction variables out of loops</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">on LP64 targets. So far so good.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Note that there is no "undef" in the IR either before or after the</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">transform, this doesn't just fall out because of a clever definition</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">of IR "undef".</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Note that even the concept of "undefined" never enters into the </span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">justification, only that "nsw" ==> "assume" ==> the loop iteration</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">bounds don't wrap ==> i64 arithmetic will generate the exact same</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">iterations as i32 arithmetic ==> the induction variable can be</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">promoted ==> there is no longer any sign-extend inside the loop.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Note that clang can generate "+nsw" for signed “+" regardless</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">of whether the precise C standard wording is "undefined behavior"</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">or more simply "unspecified value"</span>.</div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Where Dan goes wrong is in thinking that "+nsw" is an operation</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">rather than an operation with an assume, and therefore he feels</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">the need to define what the result of this operation is when it</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">overflows, which then seems to require a new "poison" instruction</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">because "undef" isn't good enough.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">But there is no need to ask what the result of overflow is</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">because "+nsw" is like a "+" inside of an if-statement whose</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">condition precludes overflow, and if it can't overflow then</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">asking about it is a non sequitor.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">And speculatively hoisting the "+nsw" doesn't cause problems</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">because hoisting a "+nsw" is like taking a "+" outside of the</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">if-statement that guarantees no overflow, it is then simply</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">a plain old un-attributed "+" operation which has no undefined</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">behavior.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Dan's follow on email "nsw is still inconsistent" [2] shows by</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">example why it is illegal to hoist the "nsw" attribute along </span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">with the "+" operation.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">It therefore makes no sense to discuss the result of "+nsw" as</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">ever being either "undef" or "poison", and so the need for "poison"</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">is gone.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Here's what Dan thought at the time about this "poison" creation</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><div style="margin:0px;line-height:normal;min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> "I wrote up a description of this concept, and it's been in</span></div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> LangRef ever since. It sticks out though, because it got pretty</span></div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> big and complex, especially in view of its relative obscurity.</span></div><div style="margin:0px;line-height:normal" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""> Realistically speaking, it's probably not fully watertight yet."</span></div></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">I agree with Dan here "it's probably not fully watertight yet", and</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">apparently other folks agree because yet another instruction,</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">"freeze", is being proposed to fix "poison"s problems. My guess</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">is that "freeze is probably not fully watertight yet" either, but</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">since "poison" isn't needed it is time to delete it from the LangRef,</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">and we can therefore stop considering "freeze".</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">Peter Lawrence.</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">References</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">[1. llvm-dev, Dan Gohman, Tue Nov 29 15:21:58 PST 2011 ]</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255)" class=""><span style="font-variant-ligatures:no-common-ligatures" class="">[2. llvm-dev, Dan Gohman, Mon Dec 12 12:58:31 PST 2011 ]</span></div><div style="margin:0px;font-size:14px;line-height:normal;font-family:Menlo;background-color:rgb(255,255,255);min-height:16px" class=""><span style="font-variant-ligatures:no-common-ligatures" class=""></span><br class=""></div></div></div></blockquote></div></div>
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