<div dir="ltr"><div class="gmail_extra"><div class="gmail_quote">On 25 May 2018 at 12:15, Hal Finkel via cfe-dev <span dir="ltr"><<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<div class="gmail-m_-7615144704090997817moz-cite-prefix">On 05/25/2018 02:01 PM, Friedman, Eli
via cfe-dev wrote:<br>
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<div class="gmail-m_-7615144704090997817moz-cite-prefix">On 5/25/2018 11:46 AM, JF Bastien
wrote:<br>
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<div>On May 25, 2018, at 11:38 AM, Friedman, Eli
<<a href="mailto:efriedma@codeaurora.org" target="_blank">efriedma@codeaurora.org</a>>
wrote:</div>
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<div class="gmail-m_-7615144704090997817moz-cite-prefix">On 5/25/2018 11:29 AM, JF
Bastien via cfe-dev wrote:<br>
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<li>Teach the target infrastructure
that hardware interference size is something
they can specify (in tablegen files
somewhere).</li>
<li>Allow overriding the value in
sub-targets using -march or -mcpu (the
sub-target defines the numeric value, and the
user gets the overriden one by using -march or
-mcpu).</li>
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We can't change the value based on -mcpu. We generally
allow mixing code built with different values of -mcpu.
And any code which is linked together must use the same
value for hardware_destructive_<wbr>interference_size, or
else we violate ODR.<br>
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<div>Interesting point. The case I’d like to cover is one
where the developer wants to get the exact right value for
their particular CPU, instead of a conservative answer with
extra padding. How do you think we should meet this use
case?</div>
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Go back to the standards committee and ask for a function that
isn't constexpr? I can't think of any other reasonable solution.<br>
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Unfortunately, to define structure layouts they need to be constant.<br>
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The best solution I've thought of is to extend the abi_tag support
to force the mangling of interfaces depending on values of these
constructs to be different.<br></div></blockquote><div><br></div><div>abi_tag is not an effective way of maintaining ABI, because it needs to be "viral" / transitive, and can't be (at least, not without huge developer effort).</div><div><br></div><div>Perhaps we could add an attribute to hardware_{con,de}structive_interference_size that produces a warning if they are used outside the main source file? We'd also need to make them non-inline, which is an observable conformance break, but seems unlikely to be important compared to the other issues.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div bgcolor="#FFFFFF">
-Hal <br>
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-Eli<br>
<pre class="gmail-m_-7615144704090997817moz-signature" cols="72">--
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project</pre>
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<pre class="gmail-m_-7615144704090997817moz-signature" cols="72">--
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory</pre>
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