Expansion of narrowing math built-ins into power instructions

[Joseph Myers]

On Mon, 12 Aug 2019, Segher Boessenkool wrote:

> (define_insn "add_truncdfsf3"
>   [(set (match_operand:SF 0 "gpc_reg_operand" "=f,wa")
> 	(float_truncate:SF
> 	  (plus:DF (match_operand:DF 1 "gpc_reg_operand" "%d,wa")
> 		   (match_operand:DF 2 "gpc_reg_operand" "d,wa"))))]

That sort of pattern is incorrect for a fused operation such as fadd, 
because combine could match it for code that is supposed to do separate 
addition and narrowing conversion.  The RTL needs to be something that 
does *not* match the combination of separate operations (just as fma has 
its own RTL, and a separate pass is responsible for converting separate 
operations to fused ones in the -ffp-contract=fast case where it's 
permitted).

-- 
Joseph S. Myers
joseph@codesourcery.com