Re: [PATCH] Make the default rtx_costs of MULT/DIV variants consistent.

[Jeff Law <jeffreyalaw@gmail.com>]

On 5/30/2022 7:46 AM, Roger Sayle wrote:
> GCC's middle-end provides a default cost model for RTL expressions, for
> backends that don't specify their own instruction timings, that can be
> summarized as multiplications are COSTS_N_INSNS(4), divisions are
> COSTS_N_INSNS(7) and all other operations are COSTS_N_INSNS(1).
> This patch tweaks the above definition so that fused-multiply-add
> (FMA) and high-part multiplications cost the same as regular
> multiplications,
> or more importantly aren't (by default) considered less expensive.  Likewise
> the saturating forms of multiplication and division cost the same as the
> regular variants.  These values can always be changed by the target, but
> the goal is to avoid RTL expansion substituting a suitable operation with
> its saturating equivalent because it (accidentally) looks much cheaper.
> For example, PR 89845 is about implementing division/modulus via highpart
> multiply, which may accidentally look extremely cheap.
>
> I believe there should be no code generation changes for this patch,
> but of course I'm happy to address any adverse changes on rare targets.
> This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
> and make -k check, both with and without --target_board=unix{-m32},
> with no new failures.  Ok for mainline?
>
>
> 2022-05-30  Roger Sayle  <roger@nextmovesoftware.com>
>
> gcc/ChangeLog
>          * rtlanal.cc (rtx_cost) <MULT>: Treat FMA, SS_MULT, US_MULT,
>          SMUL_HIGHPART and UMUL_HIGHPART as having the same cost as MULT.
>          <DIV>: Likewise, SS_DIV and US_DIV have the same default as DIV.
OK.

Jeff