Re: [RFC, tentative patch] Adjust cost for conversion expression

[Jeff Law <law@redhat.com>]

On 11/24/2016 03:58 AM, Pitchumani Sivanupandi wrote:
> GCC inlines small functions if the code size after expansion is not
> excedded.
> For test case (inline.c, avr-gcc -Os -S inline.c) code size become
> higher if
> 'func2' is inlined. It happens because the CONVERT_EXPR/ NOP_EXPR are
> considered
> as zero cost expression.
>
> Few conversions will cost additional instructions. For targets like AVR
> it will cost considerably as it's register size is just one byte.
>
> Attached the tentative patch that changes the CONVERT_EXPR/ NOP_EXPR cost
> to 1 if the LHS is bigger than RHS and target's word_mode.
>
> Is this Ok?
>
> Would it be reasonable if cost evaluated as below instead of constant 1?
>   if (LHS PRECISION > RHS PRECISION)
>     cost = LHS_PRECISION / word_mode - 1
>   else
>     cost = 0
>
> Built GCC for native with bootstrap enabled. No issues.
My high level worry here is that this introduces a target dependency 
into the early part of the tree optimization pipeline (the test against 
word_mode).

I'd be more open to giving a cost to any widening conversion.    That 
kind of change might have a wide reaching impact though.

jeff