[RISCV] Add isel for bitcasting between bfloat and half types (#158828)

There is no RISCV isel for bitcast between f16 and bf16 which will
trigger "cannot select" fatal error.

Co-authored-by: Ying Wang <[email protected]>

Author: yingwang-bj

llvm/lib/Target/RISCV/RISCVInstrInfoZfbfmin.td

@@ -70,3 +70,10 @@ def : Pat<(bf16 (fpround FPR64:$rs1)),
 def : Pat<(fpextend (bf16 FPR16:$rs1)),
           (FCVT_D_S (FCVT_S_BF16 FPR16:$rs1, FRM_DYN), FRM_RNE)>;
 }
+
+let Predicates = [HasStdExtZfhmin, HasStdExtZfbfmin] in {
+// bf16 -> f16
+def : Pat<(f16 (bitconvert (bf16 FPR16:$src))), (f16 FPR16:$src)>;
+// f16 -> bf16
+def : Pat<(bf16 (bitconvert (f16 FPR16:$src))), (bf16 FPR16:$src)>;
+}

llvm/test/CodeGen/RISCV/bfloat-convert-half.ll

@@ -0,0 +1,29 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 6
+; RUN: sed 's/iXLen/i32/g' %s | llc -mtriple=riscv32 -mattr=+zfhmin,+zfbfmin \
+; RUN:   -verify-machineinstrs | FileCheck %s --check-prefixes=RV32
+; RUN: sed 's/iXLen/i64/g' %s | llc -mtriple=riscv64 -mattr=+zfhmin,+zfbfmin \
+; RUN:   -verify-machineinstrs | FileCheck %s --check-prefixes=RV64
+
+define bfloat @fcvt_bf16_h(half %a) nounwind {
+; RV32-LABEL: fcvt_bf16_h:
+; RV32:       # %bb.0:
+; RV32-NEXT:    ret
+;
+; RV64-LABEL: fcvt_bf16_h:
+; RV64:       # %bb.0:
+; RV64-NEXT:    ret
+  %r = bitcast half %a to bfloat
+  ret bfloat %r
+}
+
+define half @test_h_bf16(bfloat %a) nounwind {
+; RV32-LABEL: test_h_bf16:
+; RV32:       # %bb.0:
+; RV32-NEXT:    ret
+;
+; RV64-LABEL: test_h_bf16:
+; RV64:       # %bb.0:
+; RV64-NEXT:    ret
+  %r = bitcast bfloat %a to half
+  ret half %r
+}