[llvm-dev] Restrict global constructors to base ISA (original) (raw)

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 3 11:30:57 PST 2018

On 12/1/2018 2:17 AM, Jeffrey Walton via llvm-dev wrote:

I'm testing on older OS X 10.8 with older SSE4 hardware from about 2010. I've got updated gear from MacPorts and it includes GCC and Clang. GCC is the compiler, and Clang is the assembler.

We perform a compile/link on a test file to ensure an ISA is supported by the toolchain. If an ISA is available then we compile a source file to the ISA as needed. Then, we guard the higher ISAs at runtime to avoid SIGILLs. It worked well until we added AVX2. For AVX2 we see this as expected: $ CXX=/opt/local/bin/clang++-mp-5.0 make /opt/local/bin/clang++-mp-5.0 ... -c chacha.cpp /opt/local/bin/clang++-mp-5.0 ... -mavx2 -c chachaavx.cpp /opt/local/bin/clang++-mp-5.0 ... -msse2 -c chachasimd.cpp ... At runtime we catch a SIGILL due to chachaavx.cpp as shown below. It looks like global constructors are using instructions from AVX (vxorps), which is beyond what the machine supports. How do we tell Clang to use the base ISA for global constructors?

There isn't any way to specifically restrict the ISA for global constructors/inline functions/etc.  The inverse works, though: you can specify the base ISA for the whole file, then mark specific functions using attribute((target("avx2"))).

-Eli

-- Employee of Qualcomm Innovation Center, Inc. Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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