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  <roger@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.

Thanks in advance,
Roger
--