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<div class="moz-cite-prefix">On 12/05/2017 01:47 PM, Lei Huang via
cfe-dev wrote:<br>
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cite="mid:OFB022F28E.4057B6DB-ON002581ED.006C5D8F-002581ED.006CC1F5@notes.na.collabserv.com"
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<div dir="ltr"><font style="outline: none;" size="3"
face="Arial">Hello,</font><br style="outline: none;
font-family: "Helvetica Neue", Helvetica, Arial,
"Lucida Grande", sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial"> </font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial">In the
discussion on bugzilla 30729, it is mentioned that the <span
style="outline: none; font-family: "Courier
New", Courier, monospace;">'</span></font><span
style="outline: none; font-family: "Courier
New", Courier, monospace;"><font style="outline:
none;" size="3">this'</font></span><font style="outline:
none;" size="3" face="Arial"> pointer needs to be valid
upon entry to a non-static method. Does the standard
guarantee </font><font style="outline: none;" size="3"
face="Courier">this</font><font style="outline: none;"
size="3" face="Arial"> is non-null on entry?</font></div>
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Yes. You have to call a non-static member function on a valid
object.<br>
<br>
<blockquote
cite="mid:OFB022F28E.4057B6DB-ON002581ED.006C5D8F-002581ED.006CC1F5@notes.na.collabserv.com"
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<div dir="ltr"><font style="outline: none;" size="3"
face="Arial"> If so, is there a reason we can't use that
fact to mark '</font><font style="outline: none;" size="3"
face="Courier New">this'</font><font style="outline:
none;" size="3" face="Arial"> as '</font><font
style="outline: none;" size="3" face="Courier New">dereferenceable(sizeof(*this))'</font><font
style="outline: none;" size="3" face="Arial">?</font><br
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Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
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Yes, this seems like a good idea.<br>
<br>
-Hal<br>
<br>
<blockquote
cite="mid:OFB022F28E.4057B6DB-ON002581ED.006C5D8F-002581ED.006CC1F5@notes.na.collabserv.com"
type="cite">
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style="font-family:Arial, Helvetica,
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style="font-family:Arial, Helvetica,
sans-serif;font-size:10.5pt">
<div dir="ltr"><font style="outline: none;" size="3"
face="Arial"> </font><br style="outline: none;
font-family: "Helvetica Neue", Helvetica, Arial,
"Lucida Grande", sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial">There are
LICM optimizations we can do based on the knowledge that '</font><font
style="outline: none;" size="3" face="Courier New">this' </font><font
style="outline: none;" size="3" face="Arial">is non-null
on entry to a non-static member function. </font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial"> </font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial">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 '</font><font
style="outline: none;" size="3" face="Courier New">this</font><font
style="outline: none;" size="3" face="Arial">' 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.</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial"> </font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial">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 '</font><font style="outline: none;" size="3"
face="Courier New">dereferenceable(N)'</font><font
style="outline: none;" size="3" face="Arial">).</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial"> </font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<b style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;"><font style="outline: none;"
size="3" face="Courier New">$ cat a.ll</font></b><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">target
datalayout = "e-m:e-i64:64-n32:64"<br style="outline:
none;">
target triple = "powerpc64le-unknown-linux-gnu"</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">%struct.S
= type { <4 x i32>, <4 x i32> }</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">;
Function Attrs: norecurse nounwind readonly<br
style="outline: none;">
define <4 x i32>
@_ZNK1S20constShouldBeHoistedEmDv4_i(%struct.S* nocapture
readonly %this, i64 %n, <4 x<br style="outline: none;">
i32> %x) align 2 {<br style="outline: none;">
entry:<br style="outline: none;">
%tobool9 = icmp eq i64 %n, 0<br style="outline: none;">
br i1 %tobool9, label %for.end, label %for.body.lr.ph</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">for.body.lr.ph:
; preds = %entry<br
style="outline: none;">
%k1 = getelementptr inbounds %struct.S, %struct.S* %this,
i64 0, i32 0<br style="outline: none;">
%k2 = getelementptr inbounds %struct.S, %struct.S* %this,
i64 0, i32 1<br style="outline: none;">
br label %for.body</font><br style="outline: none;
font-family: "Helvetica Neue", Helvetica, Arial,
"Lucida Grande", sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">for.body:
; preds =
%for.body.lr.ph, %if.end<br style="outline: none;">
%n.addr.011 = phi i64 [ %n, %for.body.lr.ph ], [ %div,
%if.end ]<br style="outline: none;">
%x.addr.010 = phi <4 x i32> [ %x, %for.body.lr.ph
], [ %x.addr.1, %if.end ]<br style="outline: none;">
%rem = and i64 %n.addr.011, 15<br style="outline: none;">
%cmp = icmp eq i64 %rem, 0<br style="outline: none;">
br i1 %cmp, label %if.end, label %if.then</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">if.then:
; preds =
%for.body<br style="outline: none;">
</font><b style="outline: none; font-family:
"Helvetica Neue", Helvetica, Arial, "Lucida
Grande", sans-serif; font-size: 12px;"><font
style="outline: none;" size="3" face="Courier New">%0 =
load <4 x i32>, <4 x i32>* %k1, align 16</font></b><br
style="outline: none;">
<font style="outline: none;" size="3" face="Courier New"> %add
= add <4 x i32> %0, %x.addr.010<br style="outline:
none;">
</font><b style="outline: none; font-family:
"Helvetica Neue", Helvetica, Arial, "Lucida
Grande", sans-serif; font-size: 12px;"><font
style="outline: none;" size="3" face="Courier New">%1 =
load <4 x i32>, <4 x i32>* %k2, align 16</font></b><br
style="outline: none;">
<font style="outline: none;" size="3" face="Courier New"> %xor
= xor <4 x i32> %add, %1<br style="outline: none;">
br label %if.end</font><br style="outline: none;
font-family: "Helvetica Neue", Helvetica, Arial,
"Lucida Grande", sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">if.end:
; preds =
%for.body, %if.then<br style="outline: none;">
%x.addr.1 = phi <4 x i32> [ %xor, %if.then ], [
%x.addr.010, %for.body ]<br style="outline: none;">
%div = lshr i64 %n.addr.011, 4<br style="outline: none;">
%tobool = icmp eq i64 %div, 0<br style="outline: none;">
br i1 %tobool, label %for.end, label %for.body</font><br
style="outline: none; font-family: "Helvetica
Neue", Helvetica, Arial, "Lucida Grande",
sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Courier New">for.end:
; preds =
%if.end, %entry<br style="outline: none;">
%x.addr.0.lcssa = phi <4 x i32> [ %x, %entry ], [
%x.addr.1, %if.end ]<br style="outline: none;">
ret <4 x i32> %x.addr.0.lcssa<br style="outline:
none;">
}</font><br style="outline: none; font-family:
"Helvetica Neue", Helvetica, Arial, "Lucida
Grande", sans-serif; font-size: 12px;">
<font style="outline: none;" size="3" face="Arial"> </font></div>
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<div dir="ltr"><br>
Regards,<br>
Lei Huang</div>
<div dir="ltr"> </div>
<div dir="ltr"> </div>
<div dir="ltr">LLVM Development on POWER</div>
<div dir="ltr">Internal mail: C2/YGK/8200/MKM<br>
Phone: (905) 413-4419<br>
TieLine: 969-4419<br>
E-mail: <a class="moz-txt-link-abbreviated" href="mailto:lei@ca.ibm.com">lei@ca.ibm.com</a></div>
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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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