Re: [PATCH] arm64: save movk instructions in mov_q when the lower 16|32 bits are all zero

Jisheng Zhang
2022-07-26T13:53:52Z

On Tue, Jul 19, 2022 at 07:13:41PM +0100, Will Deacon wrote:
> On Sat, Jul 09, 2022 at 04:48:30PM +0800, Jisheng Zhang 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>
> > ---
> > arch/arm64/include/asm/assembler.h | 4 ++++
> > 1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> > index 8c5a61aeaf8e..09f408424cae 100644
> > --- a/arch/arm64/include/asm/assembler.h
> > +++ b/arch/arm64/include/asm/assembler.h
> > @@ -568,9 +568,13 @@ alternative_endif
> > movz \reg, :abs_g3:\val
> > movk \reg, :abs_g2_nc:\val
> > .endif
> > + .if ((((\val) >> 16) & 0xffff) != 0)
> > movk \reg, :abs_g1_nc:\val
> > .endif
> > + .endif
> > + .if (((\val) & 0xffff) != 0)
> > movk \reg, :abs_g0_nc:\val
> > + .endif
>
> Please provide some numbers showing that this is worthwhile.
>

No, I have no performance numbers, but here are my opnion
about this patch: the two checks doesn't add maintaince effort, its
readability is good, if the two checks can save two movk instructions,
it's worthwhile to add the checks.

Thanks