Messages in this thread Patch in this message | | | From | Ard Biesheuvel <> | Date | Wed, 27 Jul 2022 08:15:11 -0700 | 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, 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.
> --- > 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 > .endm > > /* > -- > 2.34.1 > > > _______________________________________________ > linux-arm-kernel mailing list > linux-arm-kernel@lists.infradead.org > http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
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