[Bug target/109158] New: arm: errors when mixing __attribute__((pcs("aapcs-vfp"))) with +nofp

[Bug target/109158] New: arm: errors when mixing __attribute__((pcs("aapcs-vfp"))) with +nofp

[stammark at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109158

            Bug ID: 109158
           Summary: arm: errors when mixing
                    __attribute__((pcs("aapcs-vfp"))) with +nofp
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: minor
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stammark at gcc dot gnu.org
  Target Milestone: ---

I've detected a couple of minor issues when using
__attribute__((pcs("aapcs-vfp"))) in no-fp architectures, resulting in wrong
code or an ICE.


reproducer code:

__attribute__((pcs("aapcs-vfp"))) a(__fp16);
void foo() {
  a(0.0);
}

Issue 1: when compiled as `-march=armv8.1-m.main+mve+nofp -mfloat-abi=softfp
-mfp16-format=ieee` we emit an invalid FP instruction `vmov.f16` to put the
immediate into `s0` before returning from the function.

I believe this is a simple case of the `*mov<mode>_vfp_<mode>16`assuming that
all variants are valid with any `TARGET_VFP_FP16INST || TARGET_HAVE_MVE` when
actually the `vmov.f16` cases are only valid with TARGET_VFP_FP16INST, but not
with MVE.


Issue 2: when compiled as `-march=armv8.1-m.main+nofp -mfloat-abi=softfp` (i.e.
no MVE, no FP at all) we ICE with `maximum number of generated reload insns per
insn achieved` -- this class of error also happens with `float` and `double`
types.

IMO this is an invalid configuration and we should be emitting a user error
instead of the ICE, similar to what we do if the user requests -mfloat-abi=hard
without any MVE or FP present:

```      else if (TARGET_HARD_FLOAT_ABI)
        {
          arm_pcs_default = ARM_PCS_AAPCS_VFP;
          if (!bitmap_bit_p (arm_active_target.isa, isa_bit_vfpv2)
              && !bitmap_bit_p (arm_active_target.isa, isa_bit_mve))
            error ("%<-mfloat-abi=hard%>: selected architecture lacks an FPU");
        }```