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<div class="moz-cite-prefix">On 06/29/2017 12:43 PM, Peter Lawrence
wrote:<br>
</div>
<blockquote
cite="mid:D41961C0-953F-403B-8C09-A9192DD4DABB@sbcglobal.net"
type="cite">
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<div class="">On Jun 29, 2017, at 9:32 AM, Hal Finkel <<a
moz-do-not-send="true" href="mailto:hfinkel@anl.gov"
class="">hfinkel@anl.gov</a>> wrote:</div>
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On 06/29/2017 10:41 AM, Peter Lawrence wrote:<br class="">
</div>
<blockquote
cite="mid:8D1216F2-8E1C-4895-8DA2-19932A92BD1B@sbcglobal.net"
type="cite" style="font-family: Helvetica; font-size:
12px; font-style: normal; font-variant-caps: normal;
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255, 255);" class=""><br class="">
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<blockquote type="cite" class="">
<div class="">On Jun 29, 2017, at 4:39 AM, Hal Finkel
<<a moz-do-not-send="true"
href="mailto:hfinkel@anl.gov" class="">hfinkel@anl.gov</a>>
wrote:</div>
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<div class="">
<div bgcolor="#FFFFFF" text="#000000" class="">
<div class="moz-cite-prefix">On 06/28/2017 05:33
PM, Peter Lawrence wrote:<br class="">
</div>
<blockquote
cite="mid:37027DE0-B3CC-47C2-907C-C5A7A5A97BD1@sbcglobal.net"
type="cite" class="">Chandler,
<div class=""> where we disagree
is in whether the current project is moving
the issue</div>
<div class="">forward. It is not. It is making
the compiler more complex for no additional
value.</div>
<div class=""><br class="">
</div>
<div class="">The current project is not based
in evidence, I have asked for any SPEC
benchmark</div>
<div class="">that shows performance gain by the
compiler taking advantage of “undefined
behavior”</div>
<div class="">and no one can show that.</div>
</blockquote>
<br class="">
I can't comment on SPEC, but this does remind me
of code I was working on recently. To abstract the
relevant parts, it looked something like this:<br
class="">
<br class="">
template <typename T><br class="">
int do_something(T mask, bool cond) {<br class="">
<span class="Apple-converted-space"> </span>if
(mask & 2)<br class="">
<span class="Apple-converted-space"> </span>return
1;<br class="">
<br class="">
<span class="Apple-converted-space"> </span>if
(cond) {<br class="">
<span class="Apple-converted-space"> </span>T
high_mask = mask >> 48;<br class="">
<span class="Apple-converted-space"> </span>if
(high_mask > 5)<br class="">
<span class="Apple-converted-space"> </span>do_something_1(high_mask);<br
class="">
<span class="Apple-converted-space"> </span>else
if (high_mask > 3)<br class="">
<span class="Apple-converted-space"> </span>do_something_2();<br
class="">
<span class="Apple-converted-space"> </span>}<br
class="">
<br class="">
<span class="Apple-converted-space"> </span>return
0;<br class="">
}<br class="">
<br class="">
This function ended up being instantiated on
different types T (e.g. unsigned char, unsigned
int, unsigned long, etc.) and, dynamically, cond
was always false when T was char. The question is:
Can the compiler eliminate all of the code
predicated on cond for the smaller types? In this
case, this code was hot, and moreover, performance
depended on the fact that, for T = unsigned char,
the function was inlined and the branch on cond
was eliminated. In the relevant translation unit,
however, the compiler would never see how cond was
set.<br class="">
<br class="">
Luckily, we do the right thing here currently. In
the case where T = unsigned char, we end up
folding both of the high_mask tests as though they
were false. That entire part of the code is
eliminated, the function is inlined, and everyone
is happy.<br class="">
<br class="">
Why was I looking at this? As it turns out, if the
'else if' in this example is just 'else', we don't
actually eliminate both sides of the branch. The
same is true for many other variants of the
conditionals (i.e. we don't recognize all of the
code as dead).</div>
</div>
</blockquote>
<div class="">
<div class=""><br class="">
</div>
<div class=""><br class="">
</div>
<div class="">I apologize in advance if I have missed
something here and am misreading your example...</div>
<div class=""><br class="">
</div>
</div>
<div class="">This doesn’t make sense to me, a shift
amount of 48 is “undefined” for unsigned char,</div>
<div class="">How do we know this isn’t a source code
bug,</div>
<div class="">What makes us think the the user intended
the result to be “0”.</div>
</div>
</blockquote>
<br style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant-caps: normal;
font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px; text-transform:
none; white-space: normal; widows: auto; word-spacing:
0px; -webkit-text-stroke-width: 0px; background-color:
rgb(255, 255, 255);" class="">
<span style="font-family: Helvetica; font-size: 12px;
font-style: normal; font-variant-caps: normal;
font-weight: normal; letter-spacing: normal; orphans:
auto; text-align: start; text-indent: 0px; text-transform:
none; white-space: normal; widows: auto; word-spacing:
0px; -webkit-text-stroke-width: 0px; background-color:
rgb(255, 255, 255); float: none; display: inline
!important;" class="">As I said, this is representation of
what the real code did, and looked like, after other
inlining had taken place, etc. In the original form, the
user's intent was clear. That code is never executed when
T is a small integer type.</span><br style="font-family:
Helvetica; font-size: 12px; font-style: normal;
font-variant-caps: normal; font-weight: normal;
letter-spacing: normal; orphans: auto; text-align: start;
text-indent: 0px; text-transform: none; white-space:
normal; widows: auto; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255,
255, 255);" class="">
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</blockquote>
<br class="">
</div>
<div><br class="">
</div>
<div>I will still have a hard time believing this until I see a
real example, can you fill in the details ?</div>
</blockquote>
<br>
As I recall, it was roughly like this: There is some collection of
objects that have some flags (stored in a bit mask). Some of these
objects optionally supported tracing, and for such objects, the
tracing flags are stored in the high bits of the bit mask. Some of
the low bits had universal meanings. For objects that didn't support
tracing at all, they used a smaller bit mask (to save on space,
etc.). External logic guaranteed that the tracing bits were only
checked for objects that supported tracing (in addition to other
conditions being true). There was a template class that handled the
bit masks and interpreted some of the tracing bits (on supported
objects, but only if tracing was enabled). This function was a
member of that template class. The 48 was not directly specified in
the function like this, but was a parameter, and so only appeared
like this after inlining.<br>
<br>
-Hal<br>
<br>
<blockquote
cite="mid:D41961C0-953F-403B-8C09-A9192DD4DABB@sbcglobal.net"
type="cite">
<div><br class="">
</div>
<div><br class="">
</div>
<div>Peter Lawrence.</div>
<div><br class="">
</div>
<div><br class="">
</div>
<div><br class="">
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</blockquote>
<br>
<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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