[x264-devel] SIGBUS ("Bus error") on ARMv7 for video encoding with libx264

[Maximilian Strauch]

Hi everyone,

I  tried to encode a video using FFmpeg on my ODROID-XU4 (ARMv7) and got a „Bus error.“. I already created an bug report in the FFmpeg bug tracker [1], but they directed my to here.

I attached three files to this mail:
 - cpuinfo.txt — only showing the details of the CPU
 - gdb.txt — the debugging output of GNU debugger
 - log.txt — the FFmepg verbose output log

Using the web I figured out that there is some kind of misaligned memory access going on which is then catcher by the CPU because it ist forbidden on ARMv7 (??). I think the most important part from the GDB output is:

{{
0x0084880c in x264_mbtree_propagate_list_internal_neon ()
(gdb) bt
#0  0x0084880c in x264_mbtree_propagate_list_internal_neon ()
#1  0x0083fb4e in x264_mbtree_propagate_list_neon ()
#2  0x007fdd54 in x264_macroblock_tree_propagate.constprop ()
#3  0xa320798c in ?? ()
Backtrace stopped: previous frame identical to this frame (corrupt stack?)
(gdb) disass $pc-32,$pc+32
Dump of assembler code from 0x8487ec to 0x84882c:
   0x008487ec <x264_mbtree_propagate_list_internal_neon+20>:	vmov.i32	q11, #4	; 0x00000004
   0x008487f0 <x264_mbtree_propagate_list_internal_neon+24>:	vmov.i8	q3, #32	; 0x20
   0x008487f4 <x264_mbtree_propagate_list_internal_neon+28>:	vdup.16	q8, d5[0]
   0x008487f8 <x264_mbtree_propagate_list_internal_neon+32>:	vzip.16	q0, q8
   0x008487fc <x264_mbtree_propagate_list_internal_neon+36>:	ldr	r12, [sp, #8]
   0x00848800 <x264_mbtree_propagate_list_internal_neon+40>:	subs	r12, r12, #8
   0x00848804 <x264_mbtree_propagate_list_internal_neon+44>:	vld1.16	{d28-d29}, [r1 :128]!
   0x00848808 <x264_mbtree_propagate_list_internal_neon+48>:	vld1.16	{d30-d31}, [r2 :128]!
=> 0x0084880c <x264_mbtree_propagate_list_internal_neon+52>:	vld1.16	{d16-d19}, [r0 :128]!
   0x00848810 <x264_mbtree_propagate_list_internal_neon+56>:	vand	q15, q15, q10
   0x00848814 <x264_mbtree_propagate_list_internal_neon+60>:	vceq.i16	q1, q15, q10
   0x00848818 <x264_mbtree_propagate_list_internal_neon+64>:	vmull.u16	q12, d28, d4
   0x0084881c <x264_mbtree_propagate_list_internal_neon+68>:	vmull.u16	q13, d29, d4
   0x00848820 <x264_mbtree_propagate_list_internal_neon+72>:	vrshrn.i32	d30, q12, #6
   0x00848824 <x264_mbtree_propagate_list_internal_neon+76>:	vrshrn.i32	d31, q13, #6
   0x00848828 <x264_mbtree_propagate_list_internal_neon+80>:	vbsl	q1, q15, q14
}}

At a place in the web [2] I found that the error might be triggered by " ld1.16 {d28-d29}, [r1 :128]!" which "requires that the base address [be] is at least 128-bit aligned“ and that removing this „:128“ part will solve the issue but maybe destroy the algorithm. So I ask you for help, please!

Regards,
Max

[1] https://trac.ffmpeg.org/ticket/5109
[2] https://community.arm.com/thread/9335

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