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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - C++ math functions and errno dependencies not captured correctly in LLVM builtins"
href="http://llvm.org/bugs/show_bug.cgi?id=21635">21635</a>
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<th>Summary</th>
<td>C++ math functions and errno dependencies not captured correctly in LLVM builtins
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>lawrence@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13383" name="attach_13383" title="IR Dump for the testcase">attachment 13383</a> <a href="attachment.cgi?id=13383&action=edit" title="IR Dump for the testcase">[details]</a></span>
IR Dump for the testcase
The problem is exposed by a very small testcase which do a log(-4) then check
errno, the program doesn’t run correctly if I compile it with llvm for arm 32
bit at O1 and above, the reason is the call to the log function is moved down
after errno checking; If I replace std::log with log, program can run
correctly, see my command line and testcase below and IR dump in attached t.ll:
clang -mcpu=cortex-a9 -O1 -static -Wno-return-type -fmath-errno -static
-trigraphs -fexceptions t.C -o t
./t
//t.C
#ifdef NO_CPP_C_HEADERS
#include <math.h>
#else
#include <cmath>
#endif
#include <errno.h>
#include <stdio.h>
float x3 = -4.0f;
int locflg=0;
int
main( void )
{
float val;
val = std::log(x3); // clang generated a call to _ZSt3logf()，which
contains a call to logf(), because std::log is just a wrapper of
__builtin_logf()
//val = log(x3); // clang generated a call to log() directly
if (errno != EDOM) {
locflg = 1;
printf( "std::log: GOT %f, EXPECTED EDOM\n", val);
}
return( locflg );
}
After some debugging, I think the root cause is because of the attribute “c” of
“Fnc” in the following line of tools/clang/include/clang/Basic/Builtins.def:
BUILTIN(__builtin_logf , "dd" , "Fnc")
“c” means const, then later clang will set the function attribute to readnone
based on that (CodeGenModule::ConstructAttributeList of
tools/clang/lib/CodeGen/CGCall.cpp), that will enable optimizer to do a lot of
unsafe optimization such as move logf pass errno checking, that should not
happen since I have –fmath-errno option.
For C version log function, the attribute is “Fne” according to the following
line in the same Builtins.def
LIBBUILTIN(log, "dd", "fne", "math.h", ALL_LANGUAGES)
Note that “e” mean “const, but only when -fmath-errno=0”, that’s the correct
attribute, and that’s why the optimizer doesn’t do the unsafe optimization.
During filing this bug, I noticed there were related <a class="bz_bug_link
bz_status_RESOLVED bz_closed"
title="RESOLVED INVALID - clang should not default to -fno-math-errno"
href="show_bug.cgi?id=11858">bug #11858</a> and #5971,
however I think mine is a little bit different since I have -fmath-errno:
so my questions are:
1. why does __builtin_logf and its brothers has different attributes than log
and its brothers?
2. If __builtin_logfs and logs should all have "Fnc", then there should be a
later mechanism in clang to remove readnone if -fmath-errno is present, but I
found none.</pre>
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