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Re: [AArch64] Give some new costs for Cortex-A57 floating-point operations


On Fri, Jun 10, 2016 at 09:29:46AM +0100, James Greenhalgh wrote:
> On Fri, Jun 03, 2016 at 09:35:50AM +0100, James Greenhalgh wrote:
> > 
> > Hi,
> > 
> > This patch rebases the floating-point cost table for Cortex-A57 to be
> > relative to the cost of a floating-point move. This in response to this
> > feedback from Richard Sandiford [2] on Ramana's patch to calls.c [1] from
> > 2014:
> > 
> >   I think this is really a bug in the backend.  The backend is assigning a
> >   cost of COSTS_N_INSNS (3) to a floating-point constant not because the
> >   constant itself is expensive -- it's actually as cheap as a register
> >   in this context -- but because the backend considers floating-point
> >   moves to be 3 times more expensive than cheap integer moves.
> > 
> > The argument is that a move in mode X should be treated with cost
> > COSTS_N_INSNS (1), and other instructions should have a cost relative to
> > that move. For example, in this patch we say that instructions building a
> > floating-point constant are the same cost as a floating-point register to
> > register move. Fixing this fixes the issue Ramana was seeing, in a way
> > consistent with what other back-ends do.
> > 
> > This patch gives a small improvement to Spec2000FP on a Cortex-A57
> > platform.
> > 
> > Bootstrapped on aarch64-none-linux-gnu with no issues.
> > 
> > OK?
> 
> *ping*

*ping^*

https://gcc.gnu.org/ml/gcc-patches/2016-06/msg00251.html

Thanks,
James

> > 2016-06-03  James Greenhalgh  <james.greenhalgh@arm.com>
> > 
> > 	* config/arm/aarch-cost-tables.h (cortexa57_extra_costs): Make FP
> > 	costs relative to the cost of a register move.
> > 
> 


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