[Bug c++/89929] New: __attribute__((target("avx512bw"))) doesn't work on non avx512bw systems

[nheart at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89929

            Bug ID: 89929
           Summary: __attribute__((target("avx512bw"))) doesn't work on
                    non avx512bw systems
           Product: gcc
           Version: 8.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nheart at gmail dot com
  Target Milestone: ---

Created attachment 46075
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=46075&action=edit
testcase for attribute avx512bw

Hey,

I was trying to use function multi-versioning and it turns out that if I
specify the attribute to __attribute__((target("avx512bw"))), I get a
compilation error on non-avx512bw systems that reads:

g++ test.cpp                                :(
test.cpp: In function ‘_Z3fooi.resolver’:
test.cpp:1:41: error: No dispatcher found for avx512bw
 __attribute__((target("avx512bw"))) int foo(int i) {

This works fine if I specify -mavx512f in a sense that it compiles, but
dispatches to the wrong function at runtime (as it probably ignores the avx2
target in the compilation)

I change avx512bw to avx512f the program compiles correctly and at runtime
dispatches to the correct function version for my processor (avx2). The problem
doesn't occur in clang v8, not that it is relevant.

In addition, it seems that gcc recognizes this as valid syntax:

__attribute__((target("avx512bw", "avx512f")))

But actually ignores everything after the comma in target's arguments. Not sure
if I should open another bug for that. Please find a small testcase attached.