<div dir="ltr"><br><br><div class="gmail_quote"><div dir="ltr">On Fri, Nov 30, 2018 at 12:00 PM Gábor Horváth <<a href="mailto:xazax.hun@gmail.com">xazax.hun@gmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div dir="ltr"><div><div><div><div>Unfortunately, we do not have real world experience on a large codebase for the following reasons:<br></div>* The implementation we have does not support annotations yet, and large projects are likely to contain exceptions to the default rules (even if only a few)<br></div>* The analysis has an expected coding style, e.g. it does not reason about arithmetic on raw pointers<br><br></div>We did run it on some projects like LLVM and there were a lot of noise mainly due to the unsupported language features like pointer arithmetic.<br></div><div>The rest of the false positives were mostly due to the basic assumption of the analysis that every path is feasible.<br></div><div>We did find some true positives with a similar analysis looking for use after moves in LLVM. Those true positives were redundant std::forward calls that are not going to cause any runtime error but still removing them would make the code cleaner and less likely to misbehave in the future. See: <a href="https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Support/Error.h#L894" target="_blank">https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Support/Error.h#L894</a> (The same arguments forwarded multiple times in a loop. Since the callee never actually moves the arguments we will not end up using a moved from object. But if we never move the argument, why would we use std::forward in the first place?)<br><br></div><div>But even if the cfg-based lifetime analysis end up being a coding style specific check, the rest of the (cfg-less) clang warnings would become much more powerful after type categories are upstreamed. They could catch a lot of errors without having any false positives by default. Thus, I do see the value going forward, even if we do not have much real world experience yet. <br></div></div></div></blockquote><div><br></div><div>Could you run the restricted checks on llvm?</div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div dir="ltr"><div></div><div><br></div><div>Regards,<br></div><div>Gabor<br></div><div><div><div><div><div><div><br><div class="gmail_quote"><div dir="ltr">On Thu, 29 Nov 2018 at 20:58, George Karpenkov <<a href="mailto:ekarpenkov@apple.com" target="_blank">ekarpenkov@apple.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div>Thanks, the idea looks great!<div><br></div><div>The heuristic described in a talk on matching returned and accepted references looked fragile to me initially,</div><div>but after a bit of time I could not find obvious and common counterexamples which would confuse it.</div><div><br></div><div>Has the evaluation been done on any large project? E.g. LLVM itself, Chrome, Webkit, or any other large C++ codebase?</div><div>What is the false positive rate? What is the annotation burden to suppress those?</div><div>How many useful bugs were found?</div><div><br></div><div>I’m also very happy that more dataflow analyses are being implemented in Clang,</div><div>and I could help with reviews (though obviously I’m not the code owner for most of the components).</div><div><div><br><blockquote type="cite"><div>On Nov 29, 2018, at 8:02 AM, Gábor Horváth via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>> wrote:</div><br class="m_-1186342185363738695gmail-m_-7100975426903406487Apple-interchange-newline"><div><div dir="ltr"><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Hi!</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">This is a proposal to implement Lifetime Analysis [1] defined by Herb Sutter in Clang.</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Summary from the paper:</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">“This analysis shows how to efficiently diagnose many common cases of dangling (use-after-free) in C++ code, using only local analysis to report them as deterministic readable errors at compile time. The approach is to identify variables that are of generalized “Owner” types (e.g., smart pointers, containers, string) and “Pointer” types (e.g., int*, string_view, span, iterators, and ranges), and then use a local simple acyclic control flow graph (ACFG) analysis to track what each Pointer points to and identify when modifying an Owner invalidates a Pointer. The analysis leverages C++’s existing strong notions of scopes, object lifetimes, and const that carry rich information already available in reasonably modern C++ source code. Interestingly, it appears that with minor extension this analysis can also detect uses of local moved-from variables (use-after-move), which are a form of dangling.”</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">More details can be found in the paper [1] or in the CppCon keynote [3].</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Matthias Gehre and myself had been working on a prototype in Clang [2]. The changes are rather large, so we are planning to take an incremental approach to upstreaming the features should the community want to see this upstream.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><b><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Plans for upstreaming</span></b></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">1. Upstream Type Categorization</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Clang already performs statement-local lifetime analyses that would benefit from type categorization even before adding any other analysis.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">This includes annotating types as Owners and Pointers, and automatically inferring Owner or Point without annotation to minimize annotation burden.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Consider the following code example:</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">std::reference_wrapper<</span><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">const</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> </span><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">int</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">> get_data() {</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">    const</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> </span><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">int</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> i = </span><span style="font-size:11pt;color:rgb(9,136,90);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">3</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">;</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">    return</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> {i};</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">}</span></span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Unfortunately, today compilers do not warn on this case of returning a dangling reference. They do warn if we return a raw pointer or reference, but the compiler does not know that std::reference_wrapper also is a non-owning indirection. In the Lifetime analysis, this is diagnosed because std::reference_wrapper is recognized as a Pointer type.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">As a first step we would upstream the type categorization part of the analysis and make some clang warnings optionally use it. We would also upstream a set of annotations to give the users a way to fix potential false positives due to miscategorization. (This should be very rare according to our experience so far). By default, we could constrain the categorization for std types, whose semantics are known.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">2. Extensions of existing CFG-less analyses</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">2a. Initialization from temporaries</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">The goal is to detect Pointers that dangle on initialization, such as</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">std::string_view sv = “test”s;</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">By restricting the analysis to single statements, it has a low false-positive rate and can be done without building a CFG (i.e. faster).</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">2b. Return of locals</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">The goal is to detect returning Pointers to local variables, e.g.</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">std::reference_wrapper<</span><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">const</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> </span><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">int</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">> get_data() {</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">    const</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> </span><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">int</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> i = </span><span style="font-size:11pt;color:rgb(9,136,90);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">3</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">;</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;color:rgb(0,0,255);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">    return</span><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap"> {i};</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">}</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Similar to 2a also restricted to single statement.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">2c. Member pointer that dangles once construction is complete</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">struct X {</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">    std::string_view sv;</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">    X() : sv("test"s) {} // warning: string_view member bound to string temporary whose lifetime ends within the constructor</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">};</span></span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">2d. New of a Pointer that dangles after the end of the full-expression</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">new string_view("test"s) // warning: dynamically-allocated string_view refers to string whose lifetime ends at the end of the full-expression</span></span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">3. Intra-function analysis across basic blocks, excluding function call expressions</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Propagate point-to sets of Pointers across branches/loops intra-function, e.g. analysing</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">int* p = &i;</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">if(condition)</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">  p = nullptr;</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">*p; // ERROR: p is possibly null</span></span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">We have some CFG patches and some code traversing the CFG and propagating the analysis state. With the type categories already in place, this patch should be smaller. We could split these patches further by implementing null tracking in a separate patch.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">4. Function calls</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">auto find(const string& needle, const string& haystack) -> string_view [[gsl::lifetime(haystack)]];</span></span></div><span style="font-family:monospace,monospace"><br></span><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">string_view sv = find(“needle”, haystack);   </span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">sv[0]; // OK</span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">string_view sv = find(needle, “temporaryhaystack”);   </span></span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-family:monospace,monospace"><span style="font-size:11pt;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">sv[0]; // ERROR: sv is dangling</span></span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">This includes the following subparts.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">4a. Precondition checks</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Check that the psets of the arguments are valid at call site according to the lifetime annotations of the callee.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">4b. Postcondition checks</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Check that the psets returned from a function adhere to its advertised return/output psets.</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Rigorous checking of not just the function arguments but also the returned values is crucial part of the analysis.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">4c. Lifetimes annotations</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">The analysis gets pretty usable at this point. Most of the time the user does not need any annotations, but it is crucial to have them before a project can adapt it. For example, the user will occasionally want to explicitly state that a member function is “const as far as Lifetime is concerned” even though the function itself is not actually declared const (e.g., vector::operator[] does not invalidate any Pointers, such as iterators or raw pointers).</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">5. Implementing use after move analysis and exception support</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">These parts are not implemented yet in our prototype, but they will be useful additions for the analysis.</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><b><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Questions</span></b></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Does that make sense? What is the criteria for this work to be upstreamed? Who is willing to participate in reviewing the patches?</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Thanks in advance,</span></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">Gabor, Matthias, and Herb</span></div><br><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">[1] </span><a href="https://github.com/isocpp/CppCoreGuidelines/blob/master/docs/Lifetime.pdf" style="text-decoration:none" target="_blank"><span style="font-size:11pt;font-family:Arial;color:rgb(17,85,204);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:underline;vertical-align:baseline;white-space:pre-wrap">https://github.com/isocpp/CppCoreGuidelines/blob/master/docs/Lifetime.pdf</span></a></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">[2] </span><a href="https://github.com/mgehre/clang" style="text-decoration:none" target="_blank"><span style="font-size:11pt;font-family:Arial;color:rgb(17,85,204);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:underline;vertical-align:baseline;white-space:pre-wrap">https://github.com/mgehre/clang</span></a></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">[3] </span><a href="https://www.youtube.com/watch?v=80BZxujhY38" style="text-decoration:none" target="_blank"><span style="font-size:11pt;font-family:Arial;color:rgb(17,85,204);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:underline;vertical-align:baseline;white-space:pre-wrap">https://www.youtube.com/watch?v=80BZxujhY38</span></a></div><div style="line-height:1.38;margin-top:0pt;margin-bottom:0pt"><span style="font-size:11pt;font-family:Arial;background-color:transparent;font-weight:400;font-style:normal;font-variant-ligatures:normal;font-variant-caps:normal;font-variant-east-asian:normal;text-decoration:none;vertical-align:baseline;white-space:pre-wrap">[4] </span><a href="https://godbolt.org/z/90puuu" style="text-decoration:none" target="_blank"><span style="font-size:11pt;font-family:Arial;color:rgb(17,85,204);background-color:transparent;font-weight:400;font-style:normal;font-variant:normal;text-decoration:underline;vertical-align:baseline;white-space:pre-wrap">https://godbolt.org/z/90puuu</span></a></div><br>

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