<div dir="ltr">OK - good to know. (not sure we're talking about pessimizing it - just not adding a new/possible optimization, to be clear)<br><br>Just out of curiosity - are there particular reasons you prefer or need to ship an MSVC built version, rather than a bootstrapped Clang?</div><br><div class="gmail_quote"><div dir="ltr">On Mon, Nov 28, 2016 at 9:24 AM Robinson, Paul <<a href="mailto:paul.robinson@sony.com">paul.robinson@sony.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div lang="EN-US" link="blue" vlink="purple" class="gmail_msg"><div class="m_5859138549290728135WordSection1 gmail_msg">
<p class="MsoNormal gmail_msg" 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" class="gmail_msg"><u class="gmail_msg"></u><u class="gmail_msg"></u></span></p>
<p class="MsoNormal gmail_msg"><a name="m_5859138549290728135__MailEndCompose" class="gmail_msg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class="gmail_msg"><u class="gmail_msg"></u> <u class="gmail_msg"></u></span></a></p>
</div></div><div lang="EN-US" link="blue" vlink="purple" class="gmail_msg"><div class="m_5859138549290728135WordSection1 gmail_msg"><p class="MsoNormal gmail_msg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class="gmail_msg">Hold on there—we deliver an MSVC-built Clang to our licensees, and I would really rather not pessimize it.<u class="gmail_msg"></u><u class="gmail_msg"></u></span></p>
<p class="MsoNormal gmail_msg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class="gmail_msg">--paulr<u class="gmail_msg"></u><u class="gmail_msg"></u></span></p>
<p class="MsoNormal gmail_msg"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d" class="gmail_msg"><u class="gmail_msg"></u> <u class="gmail_msg"></u></span></p>
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<p class="MsoNormal gmail_msg"><b class="gmail_msg"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" class="gmail_msg">From:</span></b><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" class="gmail_msg"> llvm-dev [mailto:<a href="mailto:llvm-dev-bounces@lists.llvm.org" class="gmail_msg" target="_blank">llvm-dev-bounces@lists.llvm.org</a>]
<b class="gmail_msg">On Behalf Of </b>David Blaikie via llvm-dev<br class="gmail_msg">
<b class="gmail_msg">Sent:</b> Friday, November 25, 2016 8:52 AM<br class="gmail_msg">
<b class="gmail_msg">To:</b> Mueller-Roemer, Johannes Sebastian; Malcolm Parsons; Hal Finkel; <a href="mailto:llvm-dev@lists.llvm.org" class="gmail_msg" target="_blank">llvm-dev@lists.llvm.org</a></span></p></div></div></div></div></div><div lang="EN-US" link="blue" vlink="purple" class="gmail_msg"><div class="m_5859138549290728135WordSection1 gmail_msg"><div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt" class="gmail_msg"><div class="gmail_msg"><div style="border:none;border-top:solid #b5c4df 1.0pt;padding:3.0pt 0in 0in 0in" class="gmail_msg"><p class="MsoNormal gmail_msg"><span style="font-size:10.0pt;font-family:"Tahoma","sans-serif"" class="gmail_msg"><br class="gmail_msg">
<b class="gmail_msg">Subject:</b> Re: [llvm-dev] RFC: Constructing StringRefs at compile time<u class="gmail_msg"></u><u class="gmail_msg"></u></span></p></div></div></div></div></div><div lang="EN-US" link="blue" vlink="purple" class="gmail_msg"><div class="m_5859138549290728135WordSection1 gmail_msg"><div style="border:none;border-left:solid blue 1.5pt;padding:0in 0in 0in 4.0pt" class="gmail_msg">
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<p class="MsoNormal gmail_msg">On Fri, Nov 25, 2016 at 6:10 AM Mueller-Roemer, Johannes Sebastian via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" class="gmail_msg" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<u class="gmail_msg"></u><u class="gmail_msg"></u></p>
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<p class="MsoNormal gmail_msg">What about going for<br class="gmail_msg">
<br class="gmail_msg">
template<unsigned N><br class="gmail_msg">
constexpr StringRef(const char (&Str)[N])<br class="gmail_msg">
<br class="gmail_msg">
and avoiding strlen entirely for string literals?<u class="gmail_msg"></u><u class="gmail_msg"></u></p>
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<p class="MsoNormal gmail_msg">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 class="gmail_msg">
<br class="gmail_msg">
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 class="gmail_msg">
<br class="gmail_msg">
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.<u class="gmail_msg"></u><u class="gmail_msg"></u></p>
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<p class="MsoNormal gmail_msg"><br class="gmail_msg">
-----Original Message-----<br class="gmail_msg">
From: llvm-dev [mailto:<a href="mailto:llvm-dev-bounces@lists.llvm.org" class="gmail_msg" target="_blank">llvm-dev-bounces@lists.llvm.org</a>] On Behalf Of Malcolm Parsons via llvm-dev<br class="gmail_msg">
Sent: Friday, November 25, 2016 13:34<br class="gmail_msg">
To: Hal Finkel <<a href="mailto:hfinkel@anl.gov" class="gmail_msg" target="_blank">hfinkel@anl.gov</a>><br class="gmail_msg">
Cc: <a href="mailto:llvm-dev@lists.llvm.org" class="gmail_msg" target="_blank">llvm-dev@lists.llvm.org</a><br class="gmail_msg">
Subject: Re: [llvm-dev] RFC: Constructing StringRefs at compile time<br class="gmail_msg">
<br class="gmail_msg">
On 24 November 2016 at 15:04, Hal Finkel <<a href="mailto:hfinkel@anl.gov" class="gmail_msg" target="_blank">hfinkel@anl.gov</a>> wrote:<br class="gmail_msg">
>> Creating constexpr StringRefs isn't trivial as strlen isn't portably<br class="gmail_msg">
>> constexpr and std::char_traits<char>::length is only constexpr in<br class="gmail_msg">
>> C++17.<br class="gmail_msg">
><br class="gmail_msg">
> 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 class="gmail_msg">
<br class="gmail_msg">
GCC and Clang treat __builtin_strlen as constexpr.<br class="gmail_msg">
MSVC 2015 doesn't support C++14 extended constexpr. I don't know how well it optimises a recursive strlen.<br class="gmail_msg">
<br class="gmail_msg">
This works as an optimisation for GCC and Clang, and doesn't make things worse for MSVC:<br class="gmail_msg">
<br class="gmail_msg">
/// Construct a string ref from a cstring.<br class="gmail_msg">
LLVM_ATTRIBUTE_ALWAYS_INLINE<br class="gmail_msg">
+#if __has_builtin(__builtin_strlen)<br class="gmail_msg">
+ /*implicit*/ constexpr StringRef(const char *Str)<br class="gmail_msg">
+ : Data(Str), Length(Str ? __builtin_strlen(Str) : 0) {} #else<br class="gmail_msg">
/*implicit*/ StringRef(const char *Str)<br class="gmail_msg">
: Data(Str), Length(Str ? ::strlen(Str) : 0) {}<br class="gmail_msg">
+#endif<br class="gmail_msg">
<br class="gmail_msg">
--<br class="gmail_msg">
Malcolm Parsons<br class="gmail_msg">
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