Messages in this thread | | | Date | Wed, 27 Jul 2022 09:35:11 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: save movk instructions in mov_q when the lower 16|32 bits are all zero |
| |
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
| |