Messages in this thread | | | Date | Tue, 19 Jul 2022 19:13:41 +0100 | From | Will Deacon <> | Subject | Re: [PATCH] arm64: save movk instructions in mov_q when the lower 16|32 bits are all zero |
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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.
Will
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