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

[Will Deacon <will@kernel.org>]

On Tue, Jul 26, 2022 at 09:44:40PM +0800, Jisheng Zhang wrote:
> 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.

Not unless you can measure a performance increase, no. The code is always
going to be more readable without this stuff added so we shouldn't clutter
our low-level assembly macros with nested conditionals just for fun.

Will

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