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<p class="MsoNormal" style="margin-left:.5in">So I wouldn't personally worry too much about performance degredation when built with MSVC - if, when building a stage 2 on Windows (building Clang with MSVC build Clang) you do end up with a compiler with the desired
performance characteristics - then that's probably sufficient.<span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><a name="_MailEndCompose"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></a></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hold on there—we deliver an MSVC-built Clang to our licensees, and I would really rather not pessimize it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">--paulr<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>David Blaikie via llvm-dev<br>
<b>Sent:</b> Friday, November 25, 2016 8:52 AM<br>
<b>To:</b> Mueller-Roemer, Johannes Sebastian; Malcolm Parsons; Hal Finkel; llvm-dev@lists.llvm.org<br>
<b>Subject:</b> Re: [llvm-dev] RFC: Constructing StringRefs at compile time<o:p></o:p></span></p>
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<p class="MsoNormal">On Fri, Nov 25, 2016 at 6:10 AM Mueller-Roemer, Johannes Sebastian via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">What about going for<br>
<br>
template<unsigned N><br>
constexpr StringRef(const char (&Str)[N])<br>
<br>
and avoiding strlen entirely for string literals?<o:p></o:p></p>
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<p class="MsoNormal">You'd at least want an assert in there (that N - 1 == strlen(Str)) in case a StringRef is ever constructed from a non-const char buffer that's only partially filled.<br>
<br>
But if we can write this in such a way that it performs well on good implementations - that seems sufficient. If getting good performance out of the compiler means bootstrapping - that's pretty much the status quo already, as I understand it.<br>
<br>
So I wouldn't personally worry too much about performance degredation when built with MSVC - if, when building a stage 2 on Windows (building Clang with MSVC build Clang) you do end up with a compiler with the desired performance characteristics - then that's
probably sufficient.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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-----Original Message-----<br>
From: llvm-dev [mailto:<a href="mailto:llvm-dev-bounces@lists.llvm.org" target="_blank">llvm-dev-bounces@lists.llvm.org</a>] On Behalf Of Malcolm Parsons via llvm-dev<br>
Sent: Friday, November 25, 2016 13:34<br>
To: Hal Finkel <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>><br>
Cc: <a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a><br>
Subject: Re: [llvm-dev] RFC: Constructing StringRefs at compile time<br>
<br>
On 24 November 2016 at 15:04, Hal Finkel <<a href="mailto:hfinkel@anl.gov" target="_blank">hfinkel@anl.gov</a>> wrote:<br>
>> Creating constexpr StringRefs isn't trivial as strlen isn't portably<br>
>> constexpr and std::char_traits<char>::length is only constexpr in<br>
>> C++17.<br>
><br>
> Why don't we just create our own traits class that has a constexpr length, and then we can switch over to the standard one when we switch to C++17?<br>
<br>
GCC and Clang treat __builtin_strlen as constexpr.<br>
MSVC 2015 doesn't support C++14 extended constexpr. I don't know how well it optimises a recursive strlen.<br>
<br>
This works as an optimisation for GCC and Clang, and doesn't make things worse for MSVC:<br>
<br>
/// Construct a string ref from a cstring.<br>
LLVM_ATTRIBUTE_ALWAYS_INLINE<br>
+#if __has_builtin(__builtin_strlen)<br>
+ /*implicit*/ constexpr StringRef(const char *Str)<br>
+ : Data(Str), Length(Str ? __builtin_strlen(Str) : 0) {} #else<br>
/*implicit*/ StringRef(const char *Str)<br>
: Data(Str), Length(Str ? ::strlen(Str) : 0) {}<br>
+#endif<br>
<br>
--<br>
Malcolm Parsons<br>
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