[LLVMdev] [cfe-dev] Bug in ARM Thumb inline asm? (original) (raw)

James Molloy james at jamesmolloy.co.uk
Tue Feb 10 14:39:47 PST 2015

Hi Richard,

My belief is that we don't fully support this syntax. I believe, and I'm sure Renato on CC will tell me if I'm wrong as he implemented it, that we only support this for non-allocatable registers, such as SP or FP.

Cheers,

James On Tue, 10 Feb 2015 at 21:29, Richard Pennington <rich at 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: ... offt 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; _asmsyscall("r"(r7), "0"(r0), "r"(r1), "r"(r2), "r"(r3), "r"(r4)); } And then this macro: _#define _asmsyscall(...) do { _ _asm volatile ( "svc 0" _ _: "=r"(r0) : VAARGS : "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 <_syscallret> 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

cfe-dev mailing list cfe-dev at cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150210/d8c1fbdf/attachment.html>

More information about the llvm-dev mailing list