[Bug target/105965] New: x86: single-element vectors don't have scalar FMA insns used anymore

["jbeulich at suse dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105965

            Bug ID: 105965
           Summary: x86: single-element vectors don't have scalar FMA
                    insns used anymore
           Product: gcc
           Version: 12.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jbeulich at suse dot com
  Target Milestone: ---

While this used to work fine up to gcc8, gcc9 and newer use
vmuls[sdh]+vadds[sdh] instead. No similar issue exists when operating on
scalars, or when operating on multi-element vectors not matching any available
register size (so my guess of "target" as the component may not be correct).

This has regressed the test harness of the Xen Project's insn emulator [1],
which no longer exercises any scalar FMA insns because of the compiler not
emitting any. (Note that using intrinsics is not really an option, as the
primary goal is to test insns with memory operands. Yet the intrinsics don't
lend themselves to such because of using 128-bit parameter types.)

The issue is uniform for FMA, FMA4, AVX512F, and AVX512-FP16. It can be easily
seen by compiling

T test(T x, T y, T z) {
        return x * y + z;
}

#define TEST(n) \
typedef T __attribute__((vector_size(n * sizeof(T)))) v##n##_t; \
v##n##_t test##n(v##n##_t x, v##n##_t y, v##n##_t z) { \
        return x * y + z; \
}

TEST(1)
TEST(2)
TEST(4)
TEST(8)
TEST(16)
TEST(32)
TEST(64)

with e.g. "-mfpmath=sse -O3 -c -mfma -DT=float", but obvious other option
combinations similarly demonstrate the issue.

[1] https://xenbits.xen.org/gitweb/?p=xen.git;a=tree;f=tools/tests/x86_emulator