From: palmer@sifive.com (Palmer Dabbelt)
To: linux-riscv@lists.infradead.org
Subject: [GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1
Date: Fri, 26 Oct 2018 12:32:47 -0700 (PDT) [thread overview]
Message-ID: <mhng-5d10d061-df2c-4077-9ff3-4e3845f85293@palmer-si-x1c4> (raw)
In-Reply-To: <CAHk-=wimJ+g2O7DND0XZ54XZSrY_aiVWsiyyAXL9T1+=BG9Ptg@mail.gmail.com>
On Thu, 25 Oct 2018 18:12:46 PDT (-0700), Linus Torvalds wrote:
> On Wed, Oct 24, 2018 at 1:42 PM Palmer Dabbelt <palmer@sifive.com> wrote:
>>
>> I'm pretty sure this is our largest patch set since the original kernel
>> contribution, and it's certainly the one with the most contributors.
>> While I don't have anything else I know I'm going to submit for the
>> merge window, I would be somewhat surprised if I didn't screw anything
>> up.
>
> So I'm not entirely sure this is a screw-up, but it does have some
> signs of being one:
>
>> lib: Add umoddi3 and udivmoddi4 of GCC library routines
>
> We have *actively* tried to avoid needing umoddi3 and friends, and
> have so far succceeded in doing that across many different
> architectures, and many many years.
>
> And when we _have_ added it, we've added it as architecture-specific
> optimized routines when people really need it.
>
> The reason to avoid a full 64-by-64 divide is that
>
> (a) it is really expensive
>
> (b) code that needs it is almost always actually wrong
>
> Almost always, the 64-bit divide turns out to be entirely pointless,
> and the code can be written as a (much cheaper) 64-by-32 divide
> instead using the functions in <asm/div.h> instead.
>
> Does RISC-V _really_ need that disgusting full 64-bit divide, or was
> the addition of that simply a "we didn't know better, and didn't
> realize just how expensive and wrong it is"?
>
> I've pulled this, but I really _really_ hope that I will get a second
> pull request that gets rid of this again, and fixes whatever RISC-V
> code that triggered this to just do not do an insane full 64-bit
> divide on a 32-bit architecture.
Thanks. We'll go figure out how to remove it.
WARNING: multiple messages have this Message-ID (diff)
From: Palmer Dabbelt <palmer@sifive.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Greg KH <gregkh@linuxfoundation.org>, linux-riscv@lists.infradead.org
Subject: Re: [GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1
Date: Fri, 26 Oct 2018 12:32:47 -0700 (PDT) [thread overview]
Message-ID: <mhng-5d10d061-df2c-4077-9ff3-4e3845f85293@palmer-si-x1c4> (raw)
Message-ID: <20181026193247.q0Vi8ik1fC_ePKa1H3TupuG4Ti57u8h9ts6Sft4YiH0@z> (raw)
In-Reply-To: <CAHk-=wimJ+g2O7DND0XZ54XZSrY_aiVWsiyyAXL9T1+=BG9Ptg@mail.gmail.com>
On Thu, 25 Oct 2018 18:12:46 PDT (-0700), Linus Torvalds wrote:
> On Wed, Oct 24, 2018 at 1:42 PM Palmer Dabbelt <palmer@sifive.com> wrote:
>>
>> I'm pretty sure this is our largest patch set since the original kernel
>> contribution, and it's certainly the one with the most contributors.
>> While I don't have anything else I know I'm going to submit for the
>> merge window, I would be somewhat surprised if I didn't screw anything
>> up.
>
> So I'm not entirely sure this is a screw-up, but it does have some
> signs of being one:
>
>> lib: Add umoddi3 and udivmoddi4 of GCC library routines
>
> We have *actively* tried to avoid needing umoddi3 and friends, and
> have so far succceeded in doing that across many different
> architectures, and many many years.
>
> And when we _have_ added it, we've added it as architecture-specific
> optimized routines when people really need it.
>
> The reason to avoid a full 64-by-64 divide is that
>
> (a) it is really expensive
>
> (b) code that needs it is almost always actually wrong
>
> Almost always, the 64-bit divide turns out to be entirely pointless,
> and the code can be written as a (much cheaper) 64-by-32 divide
> instead using the functions in <asm/div.h> instead.
>
> Does RISC-V _really_ need that disgusting full 64-bit divide, or was
> the addition of that simply a "we didn't know better, and didn't
> realize just how expensive and wrong it is"?
>
> I've pulled this, but I really _really_ hope that I will get a second
> pull request that gets rid of this again, and fixes whatever RISC-V
> code that triggered this to just do not do an insane full 64-bit
> divide on a 32-bit architecture.
Thanks. We'll go figure out how to remove it.
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next prev parent reply other threads:[~2018-10-26 19:32 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-24 20:42 [GIT PULL] RISC-V Patches for the 4.20 Merge Window, Part 1 Palmer Dabbelt
2018-10-24 20:42 ` Palmer Dabbelt
2018-10-26 1:12 ` Linus Torvalds
2018-10-26 1:12 ` Linus Torvalds
2018-10-26 1:35 ` Paul Walmsley
2018-10-26 1:35 ` Paul Walmsley
2018-10-26 1:51 ` Linus Torvalds
2018-10-26 1:51 ` Linus Torvalds
2018-10-26 16:13 ` Zong Li
2018-10-26 16:13 ` Zong Li
2018-10-26 16:19 ` Zong Li
2018-10-26 16:19 ` Zong Li
2018-10-26 19:32 ` Palmer Dabbelt [this message]
2018-10-26 19:32 ` Palmer Dabbelt
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