[Bug rtl-optimization/105175] New: [12 Regression] Pointless warning about missed vector optimization

["krebbel at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105175

            Bug ID: 105175
           Summary: [12 Regression] Pointless warning about missed vector
                    optimization
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: krebbel at gcc dot gnu.org
  Target Milestone: ---

For this code snippet extracted from Qemu source:

enum { QEMU_MIGRATION_COOKIE_PERSISTENT = 1 };
struct {
  unsigned flags;
  unsigned flagsMandatory
} qemuMigrationCookieGetPersistent_mig;
qemuMigrationCookieGetPersistent() {
  qemuMigrationCookieGetPersistent_mig.flags &=
      QEMU_MIGRATION_COOKIE_PERSISTENT;
  qemuMigrationCookieGetPersistent_mig.flagsMandatory &=
      QEMU_MIGRATION_COOKIE_PERSISTENT;
}

cc1 -O3 -mno-sse t.c -Wvector-operation-performance

gives me:

t.c: In function ‘qemuMigrationCookieGetPersistent’:
t.c:7:46: warning: vector operation will be expanded with a single scalar
operation [-Wvector-operation-performance]
    7 |   qemuMigrationCookieGetPersistent_mig.flags &=

The generated code actually looks quite decent. Both integer AND operations are
merged into a 64 bit AND since
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=f31da42e047e8018ca6ad9809273bc7efb6ffcaf

This appears to be a nice optimization to me. However, in tree-vect-generic.cc
we then complain about this being implemented with just a scalar instruction.
Apart from this being pretty confusing for the programmer who never requested
anything to be vectorized I also don't see why it is a bad thing to implement a
vector operation with a scalar operation as long as it is able to cover the
entire vector with that.

With GCC 12 we have auto-vectorization enabled already with -O2, so I expect
this warning to surface much more frequently now. In particular on targets like
s390 where older distros still have to build everything without hardware vector
support this might be annoying. Also I'm not sure whether this warning ever
points at an actual problem. To me it looks like we should just drop it
altogether.