<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=""><div dir="auto" style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" 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><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=""><br class=""></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="">Part I.</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="">The original LangRef appeared to be “nice and pretty”</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 originally ‘undef’ did not seem to stick out.</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="">Then evidence came to light in the form of bug reports, 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="">in the form of Dan Gohman’s email “the nsw story”, that all</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="">was not good for ‘undef’ [1,2].</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="">A proposal was made based on that evidence.</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 presentation was made at an llvm gathering.</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 paper was written. The proposal has even been</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="">implemented and tested.</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="">The IR might not be so “nice and pretty” anymore,</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="">but at least all the known bugs could be closed.</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="">I think everyone agreed that the case could be closed</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="">based on the evidence at hand.</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="">However new evidence has come to light,</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 function-inlining example for one,</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="">which the proposal does not address.</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 the case must be re-opened.</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="">Now some folks seem upset over the fact that the new</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="">evidence implies that the original bugs had not been</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="">fully root-caused and that a simpler solution exists.</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 this is not a reason to be upset, rather it is a reason to</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="">be pleased that the IR can be restored to its original “nice</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 pretty” status,</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 I think Dan Gohman would be pleased.</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="">Part II.</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="">That the current LangRef IR definition of "undef" is incorrect</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 not immediately obvious, it has gone unnoticed for a long</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="">time, and has become lodged in peoples minds as unquestionable.</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="">Instead folks have viewed it as insufficient, and piled on</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="">fixes in the form of "poison" and "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);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">Here are examples of why the current definition of "undef",</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="">which explicitly assumes that it is correct to copy-propagate</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="">"x = undef" everywhere "x" occurs, is incorrect:</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="">From 3.2 in "Taming Undefined Behavior in LLVM" [3]</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 shows that the current definition blocks a</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="">highly desirable and otherwise legal optimization. ***</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="">if (k != 0) {</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=""> while (cond) {</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=""> t = 1/k;</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=""> use(t);</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=""> }</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="">}</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="">-----> hoist loop invariant -----></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 (k != 0) {</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=""> t = 1/k;</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=""> while (cond) {</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=""> use(t);</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=""> }</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="">}</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 becomes illegal when later "k" is replaced</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 "undef" because then "(undef != 0)" 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="">"t = 1/undef" need not be consistent,and 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="">program can incorrectly divide-by-zero. So this</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 an example of why copy-propagation of "undef"</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 undesirable.</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="">From "Function Inlining and undef / poison question" [4]</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 shows that the current definition creates illegal</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="">translations. ***</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 function always executes statement S</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="">foo(a)</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="">{</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 (a == a)</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=""> S;</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="">}</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="">but in llvm when foo is inlined and "a" is replaced</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 "undef" then nothing can be said about whether</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="">statement S is executed because "(undef == undef)"</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="">can go either way with the current definition. So this</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 an example of why copy-propagation of "undef" 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="">illegal.</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="">These problems cannot be fixed by adding "poison" or "freeze",</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="">it is "undef" itself that is inherently flawed.</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="">Part III.</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="">Here is the offending paragraph in the LangRef, from "Undefined Values" :</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>This example points out that two ‘undef‘ operands are not necessarily</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>the same. This can be surprising to people (and also matches C</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>semantics) where they assume that “X^X” is always zero, even if X 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=""><span class="Apple-tab-span" style="white-space:pre"> </span>undefined. This isn’t true for a number of reasons, but the short</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>answer is that an ‘undef‘ “variable” can arbitrarily change its value</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>over its “live range”. This is true because the variable doesn’t</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>actually have a live range. Instead, the value is logically read from</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>arbitrary registers that happen to be around when needed, so the value</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=""><span class="Apple-tab-span" style="white-space:pre"> </span>is not necessarily consistent over time.</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="">The purported benefit of this definition comes down to a register allocation</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="">argument in that you don't need a register for an uninitialized variable. </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="">But this benefit has never been shown, and if it ever does become important 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="">it should be handled in the register allocator, not in the IR.</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="">While the evidence of the harm of this definition is overwhelming. It leads to</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="">translations like “x ^ x” not necessarily being 0 that are non-sensical (no</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="">matter what you think the C standard says), the inability to hoist a loop</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="">invariant divide out of a loop, and the function-inlining example which 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="">patently illegal.</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="">The LangRef's definition of “undef” needs to be rewritten so that “x = undef”</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="">simply means that this is where the live range of x starts, and that it 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="">uninitialized. Another way to think of it is as an incoming argument register,</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="">which similarly has an unknown (ie "undef") but stable bit pattern.</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="">(in fact you'll know the compiler correctly implements "undef" when you</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=""> can insert "x = undef" into a function's entry block for each incomming</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=""> argument, and (without any inlining or other IPA of course) it still compiles</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=""> AOK at the IR level (you'd have to strip them back out before codegen))</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="">The LangRef's mathematical examples of "undef" all need to be deleted because</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="">while technically correct thay all presume copy-propagation, but you cannot </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="">copy-propagate the “undef” symbol itself because doing so leads to absurdities</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 mis-compiles.</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="">Once the definition of "undef" is changed then there are no longer any</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="">examples of optimizatitons that need "poison" instead of "undef", so</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="">"poison" needs to be deleted, and "freeze" no longer needs to be considered.</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); 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);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[3. <a href="http://www.cs.utah.edu/~regehr/papers/undef-pldi17.pdf" class="">http://www.cs.utah.edu/~regehr/papers/undef-pldi17.pdf</a> ]</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="">[4. llvm-dev, Peter Lawrence, Thu Jun 15 10:27:40 PDT 2017 ]</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></body></html>