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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US">Hi David and Julian,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US">I just checked it with GCC (<a href="https://godbolt.org/z/qcYjn6dn9">https://godbolt.org/z/qcYjn6dn9</a>) and Clang (<a href="https://godbolt.org/z/zvfnba5Tf">https://godbolt.org/z/zvfnba5Tf</a>)
in Godbolt: GCC without any optimizations discards the loop, Clang replaces it with a single jump, MSVC (<a href="https://godbolt.org/z/saj1sreen">https://godbolt.org/z/saj1sreen</a>) and ICC (<a href="https://godbolt.org/z/saj1sreenproduce">https://godbolt.org/z/saj1sreenproduce</a>)
a full loop.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US">The change is therefore only a minor optimization on MSVC and ICC.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US">Regards<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;mso-fareast-language:EN-US">Johannes<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="font-size:12.0pt;font-family:"Aptos",sans-serif;color:black">From:
</span></b><span style="font-size:12.0pt;font-family:"Aptos",sans-serif;color:black">hotspot-dev <hotspot-dev-retn@openjdk.org> on behalf of David Holmes <david.holmes@oracle.com><br>
<b>Date: </b>Monday, 15. January 2024 at 03:49<br>
<b>To: </b>hotspot-dev@openjdk.org <hotspot-dev@openjdk.org><br>
<b>Subject: </b>Re: static_cast<void>(0) vs do while<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt">[You don't often get email from david.holmes@oracle.com. Learn why this is important at
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<br>
Hi Julian,<br>
<br>
On 12/01/2024 3:24 pm, Julian Waters wrote:<br>
> Hi all,<br>
><br>
> In my personal fork of HotSpot I have the following commit<br>
> <a href="https://github.com/TheShermanTanker/jdk/commit/54131b70d40a88ab4176d23821f4c32044c0043d">
https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2FTheShermanTanker%2Fjdk%2Fcommit%2F54131b70d40a88ab4176d23821f4c32044c0043d&data=05%7C02%7Cjohannes.bechberger%40sap.com%7C87b0fb3aa6d44bb13f8208dc15749167%7C42f7676cf455423c82f6dc2d99791af7%7C0%7C0%7C638408837602750422%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=dTWgX8aIQsZuS33M%2F047H4uu3L4ZRzQGIiwh%2FfKqdhc%3D&reserved=0</a><br>
> which replaces some occurrences of do while with a discarding<br>
> static_cast (static_cast<void>(0) is a nop), to avoid inefficiencies<br>
> in the compiled code for debug mode, when optimizations are turned off<br>
> (Namely a compare and jump back to the start of the loop on a<br>
> condition that is always false). Is this minor optimization worth<br>
> committing upstream to HotSpot? It should also mean that the compiled<br>
> code is clearer when the methods containing asserts are disassembled.<br>
<br>
Sorry that just looks weird to me - I'm thinking "what on earth is a<br>
cast doing here?". `do { ... } while (0)` is a long standing idiomatic<br>
way to write a multi-line macro so that it can be used as a statement. I<br>
would think any compiler worth its salt would see the loop never repeats<br>
and just discard it.<br>
<br>
Cheers,<br>
David<br>
<br>
> best regards,<br>
> Julian<o:p></o:p></span></p>
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