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Re: divide 64-bit by constant for 32-bit target machines
- From: Michael Hope <michael dot hope at linaro dot org>
- To: Dinar Temirbulatov <dtemirbulatov at gmail dot com>
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Tue, 24 Apr 2012 13:48:26 +1200
- Subject: Re: divide 64-bit by constant for 32-bit target machines
- References: <CAMnfPmPi9U9E=ntqYx1vdsC3E+6XZ7isTk4vdXLDD02xT_nemail@example.com>
On 21 April 2012 00:57, Dinar Temirbulatov <firstname.lastname@example.org> wrote:
> Here is the patch that adds support for divide 64-bit by constant for
> 32-bit target machines, this patch was tested on arm-7a with no new
> regressions, also I am not sure on how to avoid for example i686
> targets since div operation there is fast compared to over targets and
> it showed better performance with ?libc/sysdeps/wordsize-32/divdi3.c
> __udivdi3 vs my implementation on the compiler side, it is not clear
> for me by looking at the udiv_cost[speed][SImode] value.
Hi Dinar. I'm afraid it gives the wrong results for some dividends:
* 82625484914982912 / 2023346444509052928: gives 4096, should be zero
* 18317463604061229328 / 2023346444509052928: gives 4109, should be 9
* 12097415865295708879 / 4545815675034402816: gives 130, should be 2
* 18195490362097456014 / 6999635335417036800: gives 10, should be 2
The expanded version is very large. Perhaps it should only turn on at
-O2 and always turn off at -Os?
The speed increase is quite impressive - I'm seeing between 2.7 and
20x faster on a Cortex-A9 for things like x / 3.