In Thumb-2 code, it is not possible to use registers 8-15 in most instructions, while this is possible in ARM code. Hence, the following assembly gives a runtime Illegal Instruction error:
.syntax unified
.fpu vfp
.thumb
.text
.globl main
main:
str r12,[sp,#-4] @ r12 is too high, source register is 3 bits wide
However, I do not get a warning at compile time, even though I use -Wall:
pi@rasppi:~/ctests$ arm-linux-gnueabihf-as -Wall -o high.o high.s
ARM GAS high.s page 1
1 .syntax unified
2 .fpu vfp
3 .thumb
4 .text
5 .globl main
6 main:
7 0000 4DF804CC str r12,[sp,#-4]
pi@rasppi:~/ctests$ arm-linux-gnueabihf-gcc -Wall -o high high.o
pi@rasppi:~/ctests$ ./high
Illegal instruction
pi@rasppi:~/ctests$ file high
high: ELF 32-bit LSB executable, ARM, EABI5 version 1 (SYSV), dynamically linked, interpreter /lib/ld-linux-armhf.so.3, for GNU/Linux 2.6.32, BuildID[sha1]=c9d90a7d6386bf97a18f9da87a7b2ce422402659, not stripped
Is there any tool that can be used to check for illegal instructions at compile time?
One problem is that you have Thumb code at the symbol main, but haven't annotated that as a Thumb symbol. Thus the linker doesn't set the lsb of the symbol address, so at runtime the call gets there in ARM state and subsequently chokes on a Thumb encoding. To solve that, you need to add the .thumb_func annotation:
...
.globl main
.thumb_func
main:
...
With that in place, it's highly likely to still crash with either an undefined instruction or a segfault when you run off the end of that code into whatever code/data/unmapped page follows. You need to actually return from main.
Your initial assumption is incorrect, because if it were a problem with register usage as you suspect, it wouldn't even assemble. The "tool that can be used to check for illegal instructions at compile time" is the assembler.