From: Jisheng Zhang <jszhang@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will@kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] arm64: save movk instructions in mov_q when the lower 16|32 bits are all zero
Date: Thu, 28 Jul 2022 23:17:38 +0800 [thread overview]
Message-ID: <YuKokvBjDxATePpH@xhacker> (raw)
In-Reply-To: <YuKh2pmJC6/17Riy@xhacker>
On Thu, Jul 28, 2022 at 10:49:02PM +0800, Jisheng Zhang wrote:
> On Wed, Jul 27, 2022 at 08:15:11AM -0700, Ard Biesheuvel wrote:
> > On Sat, 9 Jul 2022 at 01:58, Jisheng Zhang <jszhang@kernel.org> wrote:
> > >
> > > Currently mov_q is used to move a constant into a 64-bit register,
> > > when the lower 16 or 32bits of the constant are all zero, the mov_q
> > > emits one or two useless movk instructions. If the mov_q macro is used
> > > in hot code path, we want to save the movk instructions as much as
> > > possible. For example, when CONFIG_ARM64_MTE is 'Y' and
> > > CONFIG_KASAN_HW_TAGS is 'N', the following code in __cpu_setup()
> > > routine is the pontential optimization target:
> > >
> > > /* set the TCR_EL1 bits */
> > > mov_q x10, TCR_MTE_FLAGS
> > >
> > > Before the patch:
> > > mov x10, #0x10000000000000
> > > movk x10, #0x40, lsl #32
> > > movk x10, #0x0, lsl #16
> > > movk x10, #0x0
> > >
> > > After the patch:
> > > mov x10, #0x10000000000000
> > > movk x10, #0x40, lsl #32
> > >
> > > Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
> >
> > This is broken for constants that have 0xffff in the top 16 bits, as
> > in that case, we will emit a MOVN/MOVK/MOVK sequence, and omitting the
> > MOVKs will set the corresponding field to 0xffff not 0x0.
>
> Thanks so much for this hint. I think you are right about the 0xffff in
> top 16bits case.
>
the patch breaks below usage case:
mov_q x0, 0xffffffff00000000
I think the reason is mov_q starts from high bits, if we change the
macro to start from LSB, then that could solve the breakage. But this
needs a rewrite of mov_q
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next prev parent reply other threads:[~2022-07-28 15:27 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-07-09 8:48 [PATCH] arm64: save movk instructions in mov_q when the lower 16|32 bits are all zero Jisheng Zhang
2022-07-19 18:13 ` Will Deacon
2022-07-26 13:44 ` Jisheng Zhang
2022-07-27 8:35 ` Will Deacon
2022-07-27 15:15 ` Ard Biesheuvel
2022-07-28 14:48 ` Jisheng Zhang
2022-07-28 15:17 ` Jisheng Zhang [this message]
2022-07-28 15:40 ` Ard Biesheuvel
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