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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW " title="NEW --- - fabs of -0 returns -0 on Cortex-M4" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D24091&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=CYd-YnDIhPiVZuBQQ1_DIH4fyCXR1HSWGVlBGcfj_Zw&s=T4v-2EEAsDimg6iP-42UguKy4lTqNsExPfNhrmPukTU&e=">24091</a>
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<th>Summary</th>
<td>fabs of -0 returns -0 on Cortex-M4
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>-New Bugs
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>ambrop7@gmail.com
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=14574" name="attach_14574" title="test case, function that calls fabs">attachment 14574</a> <a href="attachment.cgi?id=14574&action=edit" title="test case, function that calls fabs">[details]</a></span>
test case, function that calls fabs
While trying to use Clang for a microcontroller project (Cortex-M4 with FPU), I
am seeing fabs(-0) returning -0. But the correct result is 0. Note, this is for
calculation with doubles not floats.
Attached is a simple test case that can be compiled to assembly, and the
resulting assembly, produced with the command below.
I cannot read ARM assembly well but I suspect the inlined implementation of
fabs really is incorrect with respect to -0. I have also attached the result of
compiling with GCC (same command line), which seems to be a much more optimized
implementation.
arm-none-eabi-clang -mcpu=cortex-m4 -mthumb -O2 -fno-math-errno
-fno-trapping-math -fshort-enums -mfpu=fpv4-sp-d16 -mfloat-abi=hard -fno-rtti
-fno-exceptions abs.c -S -o abs.S
Clang was configured as follows:
../configure --enable-targets=arm --target=arm-none-eabi --enable-cxx11
--disable-docs \
--with-gcc-toolchain=${gcc-arm-embedded}/lib/gcc/arm-none-eabi/$gccVersion \
--with-c-include-dirs=${gcc-arm-embedded}/arm-none-eabi/include</pre>
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