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fabs of -0 returns -0 on Cortex-M4 #24465

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llvmbot opened this issue Jul 11, 2015 · 6 comments
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fabs of -0 returns -0 on Cortex-M4 #24465

llvmbot opened this issue Jul 11, 2015 · 6 comments
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@MatzeB
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bugzilla Issues migrated from bugzilla clang Clang issues not falling into any other category

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@llvmbot
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llvmbot commented Jul 11, 2015

Bugzilla Link 24091
Resolution FIXED
Resolved on Nov 11, 2015 19:05
Version unspecified
OS Linux
Attachments test case, function that calls fabs, assembly output with clang, assembly output with gcc
Reporter LLVM Bugzilla Contributor
CC @MatzeB

Extended Description

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
@llvmbot
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llvmbot commented Jul 11, 2015

assigned to @MatzeB

@llvmbot
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llvmbot commented Jul 11, 2015

Also note, trying to workaround this with "-fno-builtin-fabs" does not work. Even for the simple case when no C headers are included and instead a declaration of fabs is given with "extern double fabs(double);" (so the headers could not be replacing it with __builtin_fabs). On the other hand, "-fno-builtin" does work (but is not really a workaround considering it affects other builtins).

@MatzeB
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MatzeB commented Jul 13, 2015

The legalization in LegalizeDAG.cpp looks wrong to me, I'll take a stab at fixing it.

@MatzeB
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MatzeB commented Jul 14, 2015

I proposed a patch here: http://reviews.llvm.org/D11172

Could you test if it works for you?

@llvmbot
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llvmbot commented Jul 16, 2015

I have tested this patch (on top of a8eaf29, doesn't apply to latest llvm anymore). It appears to work. Thank you!

@MatzeB
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MatzeB commented Nov 12, 2015

Finally got back to this. The proposed fix landed in 252839.

@llvmbot llvmbot transferred this issue from llvm/llvm-bugzilla-archive Dec 10, 2021
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