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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug middle-end/103641] [11/12 regression] Severe compile time
 regression in SLP vectorize step
Date: Fri, 10 Dec 2021 13:17:09 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103641

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |roger at nextmovesoftware =
dot com

--- Comment #18 from Roger Sayle <roger at nextmovesoftware dot com> ---
Hm.  PR 87256 was much easier to diagnose, as the cost of multiplication on
hppa64 was 20-30 ALU instructions, so a single pathological multiplier would
combinatorially challenge synth_mult.  The new costs on AArch64 have a vect=
or
multiplication cost of 4, which is very reasonable.  But I did see somewher=
e in
Andre's patch that had an ALU cost of COSTS_N_INSNS(0), which would fool
synth_mult that it's allowed/beneficial to use an infinite number of ALU
instructions to avoid a single multiplication.  Likewise, the very cheap (f=
ree)
shifts on ARM.  But this is just guess work; a reduced testcase that exhibi=
ts
the problem would be helpful.=