[Bug c/86393] New: GCC-8 appears to not detect AVX512 on iMac Pro 2018

["mrslevinsky at hotmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86393

            Bug ID: 86393
           Summary: GCC-8 appears to not detect AVX512 on iMac Pro 2018
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mrslevinsky at hotmail dot com
  Target Milestone: ---

On GCC-8, build details:

dhcp-10-45-0-78:~ mikael$ gcc-8 -v
Using built-in specs.
COLLECT_GCC=gcc-8
COLLECT_LTO_WRAPPER=/usr/local/Cellar/gcc/8.1.0/libexec/gcc/x86_64-apple-darwin17.5.0/8.1.0/lto-wrapper
Target: x86_64-apple-darwin17.5.0
Configured with: ../configure --build=x86_64-apple-darwin17.5.0
--prefix=/usr/local/Cellar/gcc/8.1.0
--libdir=/usr/local/Cellar/gcc/8.1.0/lib/gcc/8
--enable-languages=c,c++,objc,obj-c++,fortran --program-suffix=-8
--with-gmp=/usr/local/opt/gmp --with-mpfr=/usr/local/opt/mpfr
--with-mpc=/usr/local/opt/libmpc --with-isl=/usr/local/opt/isl
--with-system-zlib --enable-checking=release --with-pkgversion='Homebrew GCC
8.1.0' --with-bugurl=https://github.com/Homebrew/homebrew-core/issues
--disable-nls
Thread model: posix
gcc version 8.1.0 (Homebrew GCC 8.1.0)

the compiler appears to fail to detect that the iMac Pro 2018 with a Skylake W
generation Intel Xeon W processor has AVX512 vectorization capabilities,
turning many compiler optimizations off:

dhcp-10-45-0-78:~ mikael$ gcc-8 -march=native -E -v - </dev/null 2>&1 | grep
cc1
 /usr/local/Cellar/gcc/8.1.0/libexec/gcc/x86_64-apple-darwin17.5.0/8.1.0/cc1 -E
-quiet -v -D__DYNAMIC__ - -march=skylake-avx512 -mmmx -mno-3dnow -msse -msse2
-msse3 -mssse3 -mno-sse4a -mcx16 -msahf -mmovbe -maes -mno-sha -mpclmul
-mpopcnt -mabm -mno-lwp -mfma -mno-fma4 -mno-xop -mbmi -mno-sgx -mbmi2
-mno-pconfig -mno-wbnoinvd -mno-tbm -mavx -mavx2 -msse4.2 -msse4.1 -mlzcnt
-mrtm -mhle -mrdrnd -mf16c -mfsgsbase -mrdseed -mprfchw -madx -mfxsr -mxsave
-mxsaveopt -mno-avx512f -mno-avx512er -mno-avx512cd -mno-avx512pf
-mno-prefetchwt1 -mclflushopt -mxsavec -mxsaves -mno-avx512dq -mno-avx512bw
-mno-avx512vl -mno-avx512ifma -mno-avx512vbmi -mno-avx5124fmaps
-mno-avx5124vnniw -mclwb -mno-mwaitx -mno-clzero -mno-pku -mno-rdpid -mno-gfni
-mno-shstk -mno-avx512vbmi2 -mno-avx512vnni -mno-vaes -mno-vpclmulqdq
-mno-avx512bitalg -mno-movdiri -mno-movdir64b --param l1-cache-size=32 --param
l1-cache-line-size=64 --param l2-cache-size=25344 -mtune=skylake-avx512 -fPIC
-mmacosx-version-min=10.13.0

On the contrary, clang-6 appears to enable AVX512:

dhcp-10-45-0-78:~ mikael$ /usr/local/opt/llvm/bin/clang -march=native -E -v -
</dev/null 2>&1 | grep cc1
 "/usr/local/Cellar/llvm/6.0.0/bin/clang-6.0" -cc1 -triple
x86_64-apple-macosx10.13.0 -Wdeprecated-objc-isa-usage
-Werror=deprecated-objc-isa-usage -E -disable-free -disable-llvm-verifier
-discard-value-names -main-file-name - -mrelocation-model pic -pic-level 2
-mthread-model posix -mdisable-fp-elim -masm-verbose -munwind-tables
-target-cpu skylake-avx512 -target-feature +sse2 -target-feature +cx16
-target-feature -tbm -target-feature -avx512ifma -target-feature -gfni
-target-feature -sha -target-feature -fma4 -target-feature -vpclmulqdq
-target-feature +prfchw -target-feature +bmi2 -target-feature +xsavec
-target-feature +fsgsbase -target-feature +popcnt -target-feature +aes
-target-feature -avx512bitalg -target-feature +xsaves -target-feature -avx512er
-target-feature -avx512vnni -target-feature -avx512vpopcntdq -target-feature
+clwb -target-feature -avx512f -target-feature -clzero -target-feature -pku
-target-feature +mmx -target-feature -lwp -target-feature -xop -target-feature
+rdseed -target-feature -ibt -target-feature -sse4a -target-feature -avx512bw
-target-feature +clflushopt -target-feature +xsave -target-feature -avx512vbmi2
-target-feature -avx512vl -target-feature -avx512cd -target-feature +avx
-target-feature -vaes -target-feature +rtm -target-feature +fma -target-feature
+bmi -target-feature +rdrnd -target-feature -mwaitx -target-feature +sse4.1
-target-feature +sse4.2 -target-feature +avx2 -target-feature +sse
-target-feature +lzcnt -target-feature +pclmul -target-feature -prefetchwt1
-target-feature +f16c -target-feature +ssse3 -target-feature -sgx
-target-feature -shstk -target-feature +cmov -target-feature -avx512vbmi
-target-feature +movbe -target-feature +xsaveopt -target-feature -avx512dq
-target-feature +adx -target-feature -avx512pf -target-feature +sse3
-dwarf-column-info -debugger-tuning=lldb -target-linker-version 305 -v
-resource-dir /usr/local/Cellar/llvm/6.0.0/lib/clang/6.0.0
-fdebug-compilation-dir /Users/mikael -ferror-limit 19 -fmessage-length 0
-stack-protector 1 -fblocks -fencode-extended-block-signature
-fobjc-runtime=macosx-10.13.0 -fmax-type-align=16 -fdiagnostics-show-option -o
- -x c -
clang -cc1 version 6.0.0 based upon LLVM 6.0.0 default target
x86_64-apple-darwin17.4.0