Sure.

https://godbolt.org/z/8857KzTno 

Failed to match this instruction:
(set (reg:VNx2DF 134 [ vect__31.47 ])
    (fma:VNx2DF (neg:VNx2DF (float_extend:VNx2DF (reg:VNx2SF 136 [ vect__28.44 ])))
        (reg:VNx2DF 150 [ vect__8.12 ])
        (reg:VNx2DF 171 [ vect__29.45 ])))



juzhe.zhong@rivai.ai
 
From: Jeff Law
Date: 2023-06-29 02:16
To: Juzhe-Zhong; gcc-patches
CC: kito.cheng; kito.cheng; palmer; palmer; rdapp.gcc
Subject: Re: [PATCH] RISC-V: Support vfwnmacc/vfwmsac/vfwnmsac combine lowering
 
 
On 6/28/23 05:55, Juzhe-Zhong wrote:
> Similar to vfwmacc. Add combine patterns as follows:
> 
> For vfwnmsac:
> 1. (set (reg) (fma (neg (float_extend (reg))) (float_extend (reg))) (reg) )))
> 2. (set (reg) (fma (neg (float_extend (reg))) (reg) (reg) )))
> 
> For vfwmsac:
> 1. (set (reg) (fma (float_extend (reg)) (float_extend (reg))) (neg (reg)) )))
> 2. (set (reg) (fma (float_extend (reg)) (reg) (neg (reg)) )))
> 
> For vfwnmacc:
> 1. (set (reg) (fma (neg (float_extend (reg))) (float_extend (reg))) (neg (reg)) )))
> 2. (set (reg) (fma (neg (float_extend (reg))) (reg) (neg (reg)) )))
> 
> gcc/ChangeLog:
> 
>          * config/riscv/autovec-opt.md (*double_widen_fnma<mode>): New pattern.
>          (*single_widen_fnma<mode>): Ditto.
>          (*double_widen_fms<mode>): Ditto.
>          (*single_widen_fms<mode>): Ditto.
>          (*double_widen_fnms<mode>): Ditto.
>          (*single_widen_fnms<mode>): Ditto.
> 
 
> +
> +;; This helps to match ext + fnma.
> +(define_insn_and_split "*single_widen_fnma<mode>"
> +  [(set (match_operand:VWEXTF 0 "register_operand")
> + (fma:VWEXTF
> +   (neg:VWEXTF
> +     (float_extend:VWEXTF
> +       (match_operand:<V_DOUBLE_TRUNC> 2 "register_operand")))
> +   (match_operand:VWEXTF 3 "register_operand")
> +   (match_operand:VWEXTF 1 "register_operand")))]
I'd like to understand this better.  It looks like it's meant to be a 
bridge to another pattern.  However, it looks like it would be a 4->1 
pattern without needing a bridge.  So I'd like to know why that code 
isn't working.
 
Can you send the before/after combine dumps which show this bridge 
pattern being used?
 
The same concern exists with the other bridge patterns, but I don't 
think I need to see the before/after for each of them.
 
 
 
Thanks,
Jeff