[llvm-dev] [LLD] Writing thunks before the corresponding section
Bruce Hoult via llvm-dev
llvm-dev at lists.llvm.org
Wed Sep 7 13:50:39 PDT 2016
On Wed, Sep 7, 2016 at 7:55 PM, Peter Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hello Simon,
> Yes it is okay to write ARM thunks before an InputSection. There is a
> similar "inline state change" thunk in ARM that does BX PC, NOP to
> change state and fall through.
Maybe it's a little bit evil, but I've found that SUB PC,PC,#3 works just
fine to change to Thumb state without any NOP needed on all
current-generation CPUs I've tried it on, and in particular Raspberry Pi 2
(Cortex A7), Pi 3 (Cortex A53) and Odroid XU4 (Cortex A15).
Unfortunately I never though to try this ten years ago on the ARM7TDMI
e.g. (assumes Linux EABI kernel)
.equ SYSCALL_EXIT, 1
.equ SYSCALL_WRITE, 4
.equ STDOUT, 1
hello: .asciz "Hello asm!\n"
It is worth mentioning that disassembly of ARM and Thumb Thunks may
> look a bit strange if they are moved from after the InputSection. This
> is because they lack a mapping symbol ($a or $t) that tells the
> disassembler what instruction set to disassemble. I've got adding
> mapping symbol for linker generated InputSections on my list of things
> to do.
This disassembles fine when built in the standard way so there's clearly no
fundamental problem with disassembling past inline thunks:
$ as asm_test.s -o asm_test.o
$ ld asm_test.o -o asm_test
$ objdump -d asm_test
asm_test: file format elf32-littlearm
Disassembly of section .text:
10054: e24ff003 sub pc, pc, #3
10058: 2001 movs r0, #1
1005a: a103 add r1, pc, #12 ; (adr r1, 10068 <hello>)
1005c: 220b movs r2, #11
1005e: 2704 movs r7, #4
10060: df00 svc 0
10062: 2701 movs r7, #1
10064: df00 svc 0
10066: 46c0 nop ; (mov r8, r8)
10068: 6c6c6548 .word 0x6c6c6548
1006c: 7361206f .word 0x7361206f
10070: 000a216d .word 0x000a216d
NB that first e24ff003 is an ARM instruction, *not* Thumb2.
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