(original) (raw)

On Tue, 10 Feb 2015 at 21:29, Richard Pennington <rich@pennware.com> wrote:

I'm porting the musl C library to ARM Thumb. It looks like inline asm is

failing in some cases. Here's one:



The lseek system call looks like this:

...

off_t result;

return syscall(SYS__llseek, fd, offset>>32, offset, &result,

whence) ? -1 : result;

...



Which eventually goes through this macro:



static inline long __syscall5(long n, long a, long b, long c, long d,

long e)

{

register long r7 __asm__("r7") = n;

register long r0 __asm__("r0") = a;

register long r1 __asm__("r1") = b;

register long r2 __asm__("r2") = c;

register long r3 __asm__("r3") = d;

register long r4 __asm__("r4") = e;

__asm_syscall("r"(r7), "0"(r0), "r"(r1), "r"(r2), "r"(r3),

"r"(r4));

}



And then this macro:

#define __asm_syscall(...) do { \

__asm__ __volatile__ ( "svc 0" \

: "=r"(r0) : __VA_ARGS__ : "memory"); \

return r0; \

} while (0)



Gives:

Disassembly of section .text:



00000000 :

0: b590 push {r4, r7, lr}

2: af01 add r7, sp, #4

4: b083 sub sp, #12

6: 278c movs r7, #140 ; 0x8c

8: 46ec mov ip, sp

a: 4619 mov r1, r3

c: 68bc ldr r4, [r7, #8] ; XXX r7 is clobbered here.

e: 4663 mov r3, ip

10: df00 svc 0

12: f7ff fffe bl 0 <__syscall_ret>

16: 9a00 ldr r2, [sp, #0]

18: 9901 ldr r1, [sp, #4]

1a: 2800 cmp r0, #0

1c: bf1c itt ne

1e: f04f 32ff movne.w r2, #4294967295 ; 0xffffffff

22: f04f 31ff movne.w r1, #4294967295 ; 0xffffffff

26: 4610 mov r0, r2

28: b003 add sp, #12

2a: bd90 pop {r4, r7, pc}



The question is, does the line



register long r7 __asm__("r7") = n;



make any guarantee about the value of r7 in the asm block?



Adding the -fomit-frame-pointer flag fixes it, but is the bug in the

code or in the compiler?



-Rich

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