[ARM] Support fp16/bf16 using t constraint

[lenary]

Summary

fp16 and bf16 values can be used in GCC's inline assembly using the "t"
constraint, which means "VFP floating-point registers s0-s31" - fp16 and
bf16 values are stored in S registers too.

This change ensures that LLVM is compatible with GCC for programs that
use fp16 and the 't' constraint.

Fixes #57753

[tmatheson]

LGTM

[DavidSpickett]

LGTM.

Does this "t" constraint or equivalent exist for AArch64 and does something need to be fixed there too?

[tmatheson]

In D134553#3814461, @DavidSpickett wrote:

Does this "t" constraint or equivalent exist for AArch64 and does something need to be fixed there too?

There’s no ’t’ for aarch64:
https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints

llvm/lib/Target/ARM/ARMISelLowering.cpp

case 'x':
       if (VT.getSizeInBits() == 64)
         return RCPair(0U, &ARM::DPR_8RegClass);
       if (VT.getSizeInBits() == 128)
         return RCPair(0U, &ARM::QPR_8RegClass);
       break;
     case 't':
       if (VT == MVT::Other)
         break;
+      if (VT == MVT::f16 || VT == MVT::bf16)
+        return RCPair(0U, &ARM::HPRRegClass);
       if (VT == MVT::f32 || VT == MVT::i32)
         return RCPair(0U, &ARM::SPRRegClass);
       if (VT.getSizeInBits() == 64)
         return RCPair(0U, &ARM::DPR_VFP2RegClass);
       if (VT.getSizeInBits() == 128)
         return RCPair(0U, &ARM::QPR_VFP2RegClass);
       break;
     }

llvm/test/CodeGen/ARM/inlineasm-fp-half.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=arm -mattr=+armv8.2-a,+fp-armv8,+fullfp16,+bf16,-neon %s -o - | FileCheck %s
+; RUN: llc -mtriple=thumb -mattr=+armv8.2-a,+fp-armv8,+fullfp16,+bf16,-neon %s -o - | FileCheck %s
+
+
+define arm_aapcscc half @f(half %x) nounwind {
+; CHECK-LABEL: f:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:    vmov.f16 s0, r0
+; CHECK-NEXT:    @APP
+; CHECK-NEXT:    vsqrt.f16 s0, s0
+; CHECK-NEXT:    @NO_APP
+; CHECK-NEXT:    vmov r0, s0
+; CHECK-NEXT:    bx lr
+entry:
+  %0 = tail call half asm "vsqrt.f16 $0, $1", "=t,t"(half %x)
+  ret half %0
+}
+
+define arm_aapcscc bfloat @h(bfloat %x) nounwind {
+; CHECK-LABEL: h:
+; CHECK:       @ %bb.0: @ %entry
+; CHECK-NEXT:    vmov.f16 s0, r0
+; CHECK-NEXT:    @APP
+; CHECK-NEXT:    vmov.f32 s0, s0
+; CHECK-NEXT:    @NO_APP
+; CHECK-NEXT:    vmov.f16 r0, s0
+; CHECK-NEXT:    bx lr
+entry:
+  %0 = tail call bfloat asm "vmov.f32 $0, $1", "=t,t"(bfloat %x)
+  ret bfloat %0
+}