On 17 October 2017 at 12:27, Liuwenliang (Lamb) <liuwenliang@xxxxxxxxxx> wrote:
> On 10/17/2017 12:40 AM, Abbott Liu wrote:
>> Ard Biesheuvel [ard.biesheuvel@xxxxxxxxxx] wrote
>>This is unnecessary:
>>
>>ldr r1, =TASK_SIZE
>>
>>will be converted to a mov instruction by the assembler if the value of TASK_SIZE fits its 12-bit immediate field.
>>
>>So please remove the whole #ifdef, and just use ldr r1, =xxx
>
> Thanks for your review.
>
> The assembler on my computer don't convert ldr r1,=xxx into mov instruction.
What I said was
'if the value of TASK_SIZE fits its 12-bit immediate field'
and your value of TASK_SIZE is 0xb6e00000, which cannot be decomposed
in the right way.
If you build with KASAN disabled, it will generate a mov instruction instead.
> Here is the objdump for vmlinux:
>
> c0a3b100 <__irq_svc>:
> c0a3b100: e24dd04c sub sp, sp, #76 ; 0x4c
> c0a3b104: e31d0004 tst sp, #4
> c0a3b108: 024dd004 subeq sp, sp, #4
> c0a3b10c: e88d1ffe stm sp, {r1, r2, r3, r4, r5, r6, r7, r8, r9, sl, fp, ip}
> c0a3b110: e8900038 ldm r0, {r3, r4, r5}
> c0a3b114: e28d7030 add r7, sp, #48 ; 0x30
> c0a3b118: e3e06000 mvn r6, #0
> c0a3b11c: e28d204c add r2, sp, #76 ; 0x4c
> c0a3b120: 02822004 addeq r2, r2, #4
> c0a3b124: e52d3004 push {r3} ; (str r3, [sp, #-4]!)
> c0a3b128: e1a0300e mov r3, lr
> c0a3b12c: e887007c stm r7, {r2, r3, r4, r5, r6}
> c0a3b130: e1a0972d lsr r9, sp, #14
> c0a3b134: e1a09709 lsl r9, r9, #14
> c0a3b138: e5990008 ldr r0, [r9, #8]
> ---c0a3b13c: e59f1054 ldr r1, [pc, #84] ; c0a3b198 <__irq_svc+0x98> //ldr r1, =TASK_SIZE
> c0a3b140: e5891008 str r1, [r9, #8]
> c0a3b144: e58d004c str r0, [sp, #76] ; 0x4c
> c0a3b148: ee130f10 mrc 15, 0, r0, cr3, cr0, {0}
> c0a3b14c: e58d0048 str r0, [sp, #72] ; 0x48
> c0a3b150: e3a00051 mov r0, #81 ; 0x51
> c0a3b154: ee030f10 mcr 15, 0, r0, cr3, cr0, {0}
> ---c0a3b158: e59f103c ldr r1, [pc, #60] ; c0a3b19c <__irq_svc+0x9c> //orginal irq_svc also used same instruction
> c0a3b15c: e1a0000d mov r0, sp
> c0a3b160: e28fe000 add lr, pc, #0
> c0a3b164: e591f000 ldr pc, [r1]
> c0a3b168: e5998004 ldr r8, [r9, #4]
> c0a3b16c: e5990000 ldr r0, [r9]
> c0a3b170: e3380000 teq r8, #0
> c0a3b174: 13a00000 movne r0, #0
> c0a3b178: e3100002 tst r0, #2
> c0a3b17c: 1b000007 blne c0a3b1a0 <svc_preempt>
> c0a3b180: e59d104c ldr r1, [sp, #76] ; 0x4c
> c0a3b184: e59d0048 ldr r0, [sp, #72] ; 0x48
> c0a3b188: ee030f10 mcr 15, 0, r0, cr3, cr0, {0}
> c0a3b18c: e5891008 str r1, [r9, #8]
> c0a3b190: e16ff005 msr SPSR_fsxc, r5
> c0a3b194: e8ddffff ldm sp, {r0, r1, r2, r3, r4, r5, r6, r7, r8, r9, sl, fp, ip, sp, lr, pc}^
> ---c0a3b198: b6e00000 .word 0xb6e00000 //TASK_SIZE:0xb6e00000
> c0a3b19c: c0ccccf0 .word 0xc0ccccf0
>
>
>
> Even "ldr r1, =TASK_SIZE" won't be converted to a mov instruction by some assembler, I also think it is better
> to remove the whole #ifdef because the influence of performance by ldr is very limited.
>
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