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Date: Mon, 12 Aug 2019 21:52:00 -0000
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Joseph Myers <joseph@codesourcery.com>
Cc: Tejas Joshi <tejasjoshi9673@gmail.com>, gcc@gcc.gnu.org,        Martin Jambor <mjambor@suse.cz>, hubicka@ucw.cz
Subject: Re: Expansion of narrowing math built-ins into power instructions
Message-ID: <20190812215224.GC31406@gate.crashing.org>
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On Mon, Aug 12, 2019 at 09:20:18PM +0000, Joseph Myers wrote:
> 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).

Ugh, we allow disabling contraction, I forgot.  Rats.


Segher