Re: [RFC 1/2] RISC-V: Add support for _Bfloat16.

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/19/23 02:44, Jin Ma wrote:
> gcc/ChangeLog:
> 
> 	* config/riscv/iterators.md (HFBF): New.
> 	* config/riscv/riscv-builtins.cc (riscv_init_builtin_types):
> 	Initialize data type_Bfloat16.
> 	* config/riscv/riscv-modes.def (FLOAT_MODE): New.
> 	(ADJUST_FLOAT_FORMAT): New.
> 	* config/riscv/riscv.cc (riscv_mangle_type): Support for BFmode.
> 	(riscv_scalar_mode_supported_p): Ditto.
> 	(riscv_libgcc_floating_mode_supported_p): Ditto.
> 	(riscv_block_arith_comp_libfuncs_for_mode): New.
> 	(riscv_init_libfuncs): Opening and closing some libfuncs for BFmode.
> 	* config/riscv/riscv.md (mode" ): Add BF.
> 	(truncdfbf2): New.
> 	(movhf): Support for BFmode.
> 	(mov<mode>): Ditto.
> 	(*mov<mode>_softfloat):  Ditto.
> 	(fix_truncbf<GPR:mode>2): New.
> 	(fixuns_truncbf<GPR:mode>2): New.
> 	(float<mode>bf2): New.
> 	(floatuns<mode>bf2): New.
> 
> libgcc/ChangeLog:
> 
> 	* config/riscv/sfp-machine.h (_FP_NANFRAC_B): New.
> 	(_FP_NANSIGN_B): New.
> 	* config/riscv/t-softfp32: Add support for BF libfuncs.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/riscv/bf16_arithmetic.c: New test.
> 	* gcc.target/riscv/bf16_call.c: New test.
> 	* gcc.target/riscv/bf16_comparisons.c: New test.
> 	* gcc.target/riscv/bf16_convert-1.c: New test.
> 	* gcc.target/riscv/bf16_convert-2.c: New test.
> 	* gcc.target/riscv/bf16_convert_run.c: New test.
So this can't go in the tree until the extension has moved into a frozen 
state.  Hopefully that'll happen before we close stage1 development in Nov.



> diff --git a/gcc/config/riscv/riscv.md b/gcc/config/riscv/riscv.md
> index e00b8ee3579..5048628c784 100644
> --- a/gcc/config/riscv/riscv.md
> +++ b/gcc/config/riscv/riscv.md
> @@ -1631,6 +1631,20 @@ (define_insn "truncdfhf2"
>     [(set_attr "type" "fcvt")
>      (set_attr "mode" "HF")])
>   
> +;; The conversion of DF to BF needs to be done with SF if there is a
> +;; chance to generate at least one instruction, otherwise just using
> +;; libfunc __truncdfbf2.
> +(define_expand "truncdfbf2"
> +  [(set (match_operand:BF     0 "register_operand" "=f")
> +       (float_truncate:BF
> +           (match_operand:DF 1 "register_operand" " f")))]
> +  "TARGET_DOUBLE_FLOAT || TARGET_ZDINX"
> +  {
> +    convert_move (operands[0],
> +		  convert_modes (SFmode, DFmode, operands[1], 0), 0);
> +    DONE;
> +  })
So for conversions to/from BFmode, doesn't generic code take care of 
this for us?  Search for convert_mode_scalar in expr.cc. That code will 
utilize SFmode as an intermediate step just like your expander.   Is 
there some reason that generic code is insufficient?

Similarly for the the other conversions.

Otherwise it looks pretty good.

Jeff