[Bug target/85328] New: accessing ymm16 with non-avx512 instruction form

[zsojka at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85328

            Bug ID: 85328
           Summary: accessing ymm16 with non-avx512 instruction form
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zsojka at seznam dot cz
  Target Milestone: ---

Created attachment 43902
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43902&action=edit
reduced testcase

Compiler output:
$ x86_64-pc-linux-gnu-gcc -O3 -fno-caller-saves -mavx512f testcase.c
/tmp/ccNXhuu3.s: Assembler messages:
/tmp/ccNXhuu3.s:418: Error: unsupported instruction `vpand'

The failing instruction is:
...
        vpand   %ymm16, %ymm1, %ymm1
...

I tried both most recent GNU as and NASM (both from most recent GIT), but
neither accept this form. Accessing ymm >= 16 is allowed only with the
EVEX-prefixed VPANDD or VPANDQ.

I am really not an expert in AVX512 instruction encoding, but gcc seems to be
wrong here according to the assemblers I tested and Intel's SDM vol.2:
only vpandd/vpandq are using the EVEX prefix, and you can use ymm >= 16 only
with the EVEX prefix.

(this can be generalized to other instructions, such as vpor, and maybe to
other registers, such as xmm/zmm)

$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-259207-checking-yes-rtl-df-extra-nobootstrap-pr85177-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/8.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++
--enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra
--disable-bootstrap --with-cloog --with-ppl --with-isl
--build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld
--with-as=/usr/bin/x86_64-pc-linux-gnu-as --disable-libstdcxx-pch
--prefix=/repo/gcc-trunk//binary-trunk-259207-checking-yes-rtl-df-extra-nobootstrap-pr85177-amd64
Thread model: posix
gcc version 8.0.1 20180407 (experimental) (GCC)