On Tue, May 17, 2022 at 6:44 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This is the i386 backend specific piece of my revised patch for
> PR middle-end/98865, where Richard Biener has suggested that I perform
> the desired transformation during RTL expansion where the backend can
> control whether it is profitable to convert a multiplication into a
> bit-wise AND and a negation. This works well for x86_64, but alas
> exposes a latent bug with -m32, where a DImode multiplication incorrectly
> appears to be cheaper than negdi2+anddi3(!?). The fix to ix86_rtx_costs
> is to report that a DImode (multi-word) multiplication actually requires
> three SImode multiplications and two SImode additions. This also corrects
> the cost of TImode multiplication on TARGET_64BIT.
>
> 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. This change avoids the need for a !ia32 target selector
> for the upcoming new test case gcc.target/i386/pr98865.c.
> Ok for mainline?
>
>
> 2022-05-17 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/ChangeLog
> * config/i386/i386.cc (ix86_rtx_costs) [MULT]: When mode size
> is wider than word_mode, a multiplication costs three word_mode
> multiplications and two word_mode additions.
LGTM.
Thanks,
Uros.
>
> Thanks in advance,
> Roger
> --
>
