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<div class="moz-cite-prefix">On 12/06/2017 01:32 PM, Martin J.
O'Riordan wrote:<br>
</div>
<blockquote cite="mid:004401d36ec8$eded1ad0$c9c75070$@theheart.ie"
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<div class="WordSection1">
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">I
think that the key concept goes all the way back to the
original C++ Standard (C++98), where section 5.2.2 “Function
Call” states:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"
style="margin-left:36.0pt;text-autospace:none"><span
style="font-size:10.0pt;color:windowtext">The first
expression in the postfix expression is then called the <i>object
expression</i>, and the call<o:p></o:p></span></p>
<p class="MsoNormal"
style="margin-left:36.0pt;text-autospace:none"><span
style="font-size:10.0pt;color:windowtext">is as a member of
the object pointed to or referred to. In the case of an
implicit class member access, the<o:p></o:p></span></p>
<p class="MsoNormal"
style="margin-left:36.0pt;text-autospace:none"><span
style="font-size:10.0pt;color:windowtext">implied object is
the one pointed to by </span><span
style="font-size:10.0pt;font-family:Courier;color:windowtext">this</span><span
style="font-size:10.0pt;color:windowtext">. [<i>Note: </i>a
member function call of the form </span><span
style="font-size:10.0pt;font-family:Courier;color:windowtext">f()
</span><span style="font-size:10.0pt;color:windowtext">is
interpreted<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-size:10.0pt;color:windowtext">as </span><span
style="font-size:10.0pt;font-family:Courier;color:windowtext">(*this).f()
</span><span style="font-size:10.0pt;color:windowtext">(see
9.3.1). ]</span><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">A
NULL pointer does not point to an object, so two things make
a NULL ‘</span><span style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">this</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
invalid - first the </span><i><span
style="font-family:"Book
Antiqua",serif;color:black;mso-fareast-language:EN-US">object
expression</span></i><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">
does not refer to an object and is thus undefined; and
second, the note which clarifies that it is equivalent to ‘</span><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">(*this).</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
means that if ‘</span><span style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">this</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
is NULL, then it is a NULL pointer dereference which is
already undefined behaviour elsewhere in the Standard. I
can’t remember if “notes” are normative.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">In
“very old C++”, i.e. prior to the introduction of static
member functions (circa 1987), the following idiom was not
unusual to get the semantic intent of a static member
provide that ‘</span><span style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">this</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
was neither implicitly nor explicitly used:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">class T {<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">public:<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US"> void
wishIwasStatic();<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">};<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">...<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">((T*)0)->wishIwasStatic();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">Such
functions had access to all object of type ‘</span><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">T</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’,
but without the need for ‘</span><span
style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">friend</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
declarations. But this was only a stop-gap until the
introduction of static member functions was devised.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">I
very much doubt that pre-C++98 code such as this is still
part of any production application, and if it is, it really
ought to be rewritten.</span></p>
</div>
</blockquote>
<br>
Regardless, this is not legal C++ code, and we don't need to support
it.<br>
<br>
<blockquote cite="mid:004401d36ec8$eded1ad0$c9c75070$@theheart.ie"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">Making
‘</span><span style="font-family:"Courier
New";color:black;mso-fareast-language:EN-US">this</span><span
style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">’
not de-referenceable seems to me to be a really good idea
semantically, and if it yields performance advantages too,
then this is a really good thing. Perhaps it might be a
good idea to ping either Bjarne Stroustrup or the C++
Standards committee to be sure - though I expect many of the
committee’s members are also participants in this forum.</span></p>
</div>
</blockquote>
<br>
Yes, many of us are here :-)<br>
<br>
-Hal<br>
<br>
<blockquote cite="mid:004401d36ec8$eded1ad0$c9c75070$@theheart.ie"
type="cite">
<div class="WordSection1">
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US">
MartinO<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Book
Antiqua",serif;color:#943634;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<div>
<div style="border:none;border-top:solid #E1E1E1
1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US">From:</span></b><span
style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:windowtext"
lang="EN-US"> cfe-dev
[<a class="moz-txt-link-freetext" href="mailto:cfe-dev-bounces@lists.llvm.org">mailto:cfe-dev-bounces@lists.llvm.org</a>] <b>On Behalf Of
</b>Hal Finkel via cfe-dev<br>
<b>Sent:</b> 06 December 2017 13:07<br>
<b>To:</b> Lei Huang <a class="moz-txt-link-rfc2396E" href="mailto:lei@ca.ibm.com"><lei@ca.ibm.com></a>;
<a class="moz-txt-link-abbreviated" href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a><br>
<b>Cc:</b> LLVM on Power <a class="moz-txt-link-rfc2396E" href="mailto:powerllvm@ca.ibm.com"><powerllvm@ca.ibm.com></a><br>
<b>Subject:</b> Re: [cfe-dev] [RFC] Setting
dereferenceable flag on the implicit this parameter for
non-static member functions<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p><o:p> </o:p></p>
<div>
<p class="MsoNormal">On 12/05/2017 01:47 PM, Lei Huang via
cfe-dev wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"> <o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif">Hello,</span><span
style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif"> </span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif">In
the discussion on bugzilla 30729, it is mentioned
that the 'this' pointer needs to be valid upon entry
to a non-static method. Does the standard
guarantee </span><span style="font-family:Courier">this</span><span
style="font-family:"Arial",sans-serif"> is
non-null on entry?</span><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"><o:p></o:p></span></p>
</div>
</div>
</div>
</blockquote>
<p class="MsoNormal"><br>
Yes. You have to call a non-static member function on a valid
object.<br>
<br>
<br>
<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif"> If
so, is there a reason we can't use that fact to mark
'</span><span style="font-family:"Courier
New"">this'</span><span
style="font-family:"Arial",sans-serif"> as
'</span><span style="font-family:"Courier
New"">dereferenceable(sizeof(*this))'</span><span
style="font-family:"Arial",sans-serif">?</span><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"><o:p></o:p></span></p>
</div>
</div>
</div>
</blockquote>
<p class="MsoNormal"><br>
Yes, this seems like a good idea.<br>
<br>
-Hal<br>
<br>
<br>
<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<div>
<div>
<p class="MsoNormal"><span
style="font-family:"Arial",sans-serif"> </span><span
style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif">There
are LICM optimizations we can do based on the
knowledge that '</span><span
style="font-family:"Courier New"">this' </span><span
style="font-family:"Arial",sans-serif">is
non-null on entry to a non-static member function. </span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif"> </span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif">eg.
For the following IR, the two highlighted loads
are not being hoisted out of the for loop because we
are not able to guarantee that the pointer is
non-null. If the '</span><span
style="font-family:"Courier New"">this</span><span
style="font-family:"Arial",sans-serif">' pointer
is guaranteed to be non-null on entry and we mark it
thus, then the 2 loads within the for-loop body can
then be hoisted out into the loop preheader.</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif"> </span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif">This
is of course just one example of an optimization we
could perform based on this knowledge, but there are
probably a number of others (i.e. anything that
relies on a pointer being '</span><span
style="font-family:"Courier New"">dereferenceable(N)'</span><span
style="font-family:"Arial",sans-serif">).</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif"> </span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><b><span style="font-family:"Courier
New"">$ cat a.ll</span></b><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New"">target datalayout = "e-m:e-i64:64-n32:64"<br>
target triple = "powerpc64le-unknown-linux-gnu"</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New"">%struct.S = type { <4 x i32>, <4
x i32> }</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New"">; Function Attrs: norecurse nounwind
readonly<br>
define <4 x i32>
@_ZNK1S20constShouldBeHoistedEmDv4_i(%struct.S*
nocapture readonly %this, i64 %n, <4 x<br>
i32> %x) align 2 {<br>
entry:<br>
%tobool9 = icmp eq i64 %n, 0<br>
br i1 %tobool9, label %for.end, label
%for.body.lr.ph</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New"">for.body.lr.ph:
; preds = %entry<br>
%k1 = getelementptr inbounds %struct.S, %struct.S*
%this, i64 0, i32 0<br>
%k2 = getelementptr inbounds %struct.S, %struct.S*
%this, i64 0, i32 1<br>
br label %for.body</span><span
style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
</span><span style="font-family:"Courier
New"">for.body:
; preds = %for.body.lr.ph, %if.end<br>
%n.addr.011 = phi i64 [ %n, %for.body.lr.ph ], [
%div, %if.end ]<br>
%x.addr.010 = phi <4 x i32> [ %x,
%for.body.lr.ph ], [ %x.addr.1, %if.end ]<br>
%rem = and i64 %n.addr.011, 15<br>
%cmp = icmp eq i64 %rem, 0<br>
br i1 %cmp, label %if.end, label %if.then</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New"">if.then:
; preds = %for.body<br>
<b>%0 = load <4 x i32>, <4 x i32>* %k1,
align 16</b></span><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New""> %add = add <4 x i32> %0,
%x.addr.010<br>
<b>%1 = load <4 x i32>, <4 x i32>* %k2,
align 16</b></span><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New""> %xor = xor <4 x i32> %add, %1<br>
br label %if.end</span><span
style="font-size:9.0pt;font-family:"Helvetica",sans-serif"><br>
</span><span style="font-family:"Courier
New"">if.end:
; preds = %for.body, %if.then<br>
%x.addr.1 = phi <4 x i32> [ %xor, %if.then ],
[ %x.addr.010, %for.body ]<br>
%div = lshr i64 %n.addr.011, 4<br>
%tobool = icmp eq i64 %div, 0<br>
br i1 %tobool, label %for.end, label %for.body</span><span
style="font-size:9.0pt;font-family:"Arial",sans-serif"><br>
</span><span style="font-family:"Courier
New"">for.end:
; preds = %if.end, %entry<br>
%x.addr.0.lcssa = phi <4 x i32> [ %x, %entry
], [ %x.addr.1, %if.end ]<br>
ret <4 x i32> %x.addr.0.lcssa<br>
}</span><span
style="font-size:10.5pt;font-family:"Helvetica",sans-serif"><br>
</span><span
style="font-family:"Arial",sans-serif"> </span><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"><o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"> <o:p></o:p></span></p>
</div>
<div>
<div>
<div>
<div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"><br>
Regards,<br>
Lei Huang<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"> <o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif"> <o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif">LLVM
Development on POWER<o:p></o:p></span></p>
</div>
<div>
<p class="MsoNormal"><span
style="font-size:10.5pt;font-family:"Arial",sans-serif">Internal
mail: C2/YGK/8200/MKM<br>
Phone: (905) 413-4419<br>
TieLine: 969-4419<br>
E-mail: <a moz-do-not-send="true"
href="mailto:lei@ca.ibm.com">lei@ca.ibm.com</a><o:p></o:p></span></p>
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<pre>_______________________________________________<o:p></o:p></pre>
<pre>cfe-dev mailing list<o:p></o:p></pre>
<pre><a moz-do-not-send="true" href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a><o:p></o:p></pre>
<pre><a moz-do-not-send="true" href="http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev">http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><o:p></o:p></pre>
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<pre>-- <o:p></o:p></pre>
<pre>Hal Finkel<o:p></o:p></pre>
<pre>Lead, Compiler Technology and Programming Languages<o:p></o:p></pre>
<pre>Leadership Computing Facility<o:p></o:p></pre>
<pre>Argonne National Laboratory<o:p></o:p></pre>
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<pre class="moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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