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<p class="MsoNormal"><span style="color:#1F497D">This definitely looks like a bug to me – C++11 section 7.1.2 paragraph 4 clearly states<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">“A string literal in the body of an extern inline function is the same object in different translation units.”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Data for extern inline functions should be placed in a comdat group, and it looks like we get this<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">correct if an actual variable is used, e.g.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> extern inline const char *fn()<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> static const char str[] = "example";<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> return str;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> }</span><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">generates<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> $_Z2fnv = comdat any<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> $_ZZ2fnvE3str = comdat any<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> @_ZZ2fnvE3str = linkonce_odr constant [8 x i8] c"example\00", comdat, align 1</span><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> define linkonce_odr arm_aapcscc i8* @_Z2fnv() #1 comdat {<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> entry:<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> ret i8* getelementptr inbounds ([8 x i8], [8 x i8]* @_ZZ2fnvE3str, i32 0, i32 0)<o:p></o:p></span></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New""> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">so I think string literals should be handled similarly.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">John<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> llvm-dev [mailto:llvm-dev-bounces@lists.llvm.org]
<b>On Behalf Of </b>Sameer AbuAsal via llvm-dev<br>
<b>Sent:</b> 22 March 2017 01:02<br>
<b>To:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> [llvm-dev] [ARM] [C++ standard] Correct linkage type for string literals in extern inline functions<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">Hi, <o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">I came across this behavior irregularity in LLVM for ARM backend (-target arm-linux-gnueabi) with the constant promotion optimization in arm (-arm-promote-constant=true).
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">For the attached source files compiling with the following:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">clang++ -target arm-linux-gnueabi A.cpp B.cpp –o a.out<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The addresses returned from bar() and foo() are not the same (string literals live in different memory locations) however, when we turn off the constant pool optimization<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">clang-arm-x++ -Ofast -mllvm -arm-promote-constant=false A.cpp B.cpp -o test_case.exe –o a.out
<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">we are getting the same addresses for string literals.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Looking into the ll files , the strings are created as “private unnamed_addr constant” so the constant pool optimization pass is promoting them to constant pools and causing them to have different addresses, which seems
fine.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Is this behavior in line with the C++ standard for strings in extern inline functions? If not, what should be the correct linkage type emitted for this constant? Is this a potential clang bug?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thank you,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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