NDK clang编译器的一个bug
问题代码
float32_t Sum_float(float32_t *data, const int count)
{
float32x4_t res = vdupq_n_f32(0.0f);
for(int i = 0; i < (count & (~15)); i += 16)
{
#if 01
float32x4x4_t v0 = vld1q_f32_x4(data + i);
float32x4_t v00 = v0.val[0];
float32x4_t v01 = v0.val[1];
float32x4_t v02 = v0.val[2];
float32x4_t v03 = v0.val[3];
#else
float32x4_t v00 = vld1q_f32(data + i);
float32x4_t v01 = vld1q_f32(data + i + 4);
float32x4_t v02 = vld1q_f32(data + i + 8);
float32x4_t v03 = vld1q_f32(data + i + 12);
#endif
v00 = vaddq_f32(v00, v02);
v01 = vaddq_f32(v01, v03);
res = vaddq_f32(res, vaddq_f32(v00, v01));
}
float32x2_t res1 = vadd_f32(vget_low_f32(res), vget_high_f32(res));
float32_t v0[2];
vst1_f32(v0, res1);
v0[0] += v0[1];
for(int i = count & (~15); i < count; ++i){
v0[0] += data[i];
}
return v0[0];
}
编译测试
首先，查阅了https://static.docs.arm.com/ihi0073/c/IHI0073C_arm_neon_intrinsics_ref.pdf，对于vld1q_f32_x4这个指令，v7/A32/A64都是支持的。
不同编译器版本结果：首先，对于所有的版本，如果使用#else块的代码，都是可以编译成功的，对于使用#if 01块的代码，结果如下：
| armeabi-v7a with o1 | armeabi=v7a with o0 | arm64-v8a | |
|---|---|---|---|
| r20c | clang++: error: clang frontend command failed due to signal (use -v to see invocation) | ok | ok |
| r19c | ok | ok | ok |
| r15c | error: use of undeclared identifier 'vld1q_f32_x4' | error: use of undeclared identifier 'vld1q_f32_x4' | ok |
不仅仅vld1q_f32_x4，对于vld1_u8_x2;vst1q_f32_x4等类似指令都存在这样的问题。
性能对比
测试代码：
int main()
{
const size_t len = 1024*1024 * 16;
float32_t *data = new float32_t[len];
for(size_t i = 0; i < len; ++i) {
data[i] = std::rand() / 100.0;
}
clock_t t0 = std::clock();
float32_t sum = Sum_float(data, len);
printf("sum=%f , time cost=%f \n", sum, 1000.0 * (double)(std::clock() - t0) / CLOCKS_PER_SEC);
return 0;
}
测试了使用三种NDK版本编译arm64-v8a测试，同时使用r19c编译了armeabi-v7a，分别使用#if和#else分之，发现耗时都是在3.55ms左右，无明显差别。
类似问题：https://github.com/mattgodbolt/compiler-explorer/issues/1906
地址对齐
虽然使用r19c的版本编译armeabi-v7a成功，或者使用不优化的r20c也一样，但是执行时发生了crash。原因是执行vldN(q)_type_xN指令时，地址不对齐导致的crash。
而对于arm64-v8a版本，把所有传给vldN(q)_type_xN的地址打印出来，同样发现也有0x7350800001这样的地址，而且地址末位为0到E的都有，但是却没有报错。也即，对于该指令只有armeabi-v7a有地址对齐要求，而arm64-v8a却没有？
同时，常规的vldN(q)_type指令则没有地址对齐的要求，所以最好不要使用vldN(q)_type_xN。
在代码中因为地址对齐而导致的crash日志：
libc : Fatal signal 7 (SIGBUS), code 1 (BUS_ADRALN), fault addr 0xf0900001 in tid 27659 (ClarityOpt), pid 27659 (ClarityOpt)
crash_dump32: obtaining output fd from tombstoned, type: kDebuggerdTombstone
crash_dump32: performing dump of process 27659 (target tid = 27659)
DEBUG : Process name is /data/local/tmp/ClarityOpt, not key_process
DEBUG : *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
DEBUG : Build fingerprint: 'OPPO/PCCM00/OP4A7A:10/QKQ1.191222.002/1584699103:user/release-keys'
DEBUG : Revision: '0'
DEBUG : ABI: 'arm'
DEBUG : Timestamp: 2020-05-09 15:15:16+0800
DEBUG : pid: 27659, tid: 27659, name: ClarityOpt >>> /data/local/tmp/ClarityOpt <<<
DEBUG : uid: 0
crash_dump32: type=1400 audit(0.0:27044): avc: denied { read } for name="ClarityOpt" dev="sda11" ino=30524 scontext=u:r:crash_dump:s0 tcontext=u:object_r:shell_data_file:s0 tclass=file permissive=1
crash_dump32: type=1400 audit(0.0:27045): avc: denied { open } for path="/data/local/tmp/ClarityOpt" dev="sda11" ino=30524 scontext=u:r:crash_dump:s0 tcontext=u:object_r:shell_data_file:s0 tclass=file permissive=1
crash_dump32: type=1400 audit(0.0:27046): avc: denied { getattr } for path="/data/local/tmp/ClarityOpt" dev="sda11" ino=30524 scontext=u:r:crash_dump:s0 tcontext=u:object_r:shell_data_file:s0 tclass=file permissive=1
crash_dump32: type=1400 audit(0.0:27047): avc: denied { map } for path="/data/local/tmp/ClarityOpt" dev="sda11" ino=30524 scontext=u:r:crash_dump:s0 tcontext=u:object_r:shell_data_file:s0 tclass=file permissive=1
DEBUG : signal 7 (SIGBUS), code 1 (BUS_ADRALN), fault addr 0xf0900001
DEBUG : r0 00000043 r1 00000000 r2 a9a5ac6f r3 00000003
DEBUG : r4 f0900001 r5 ffcb0a00 r6 ffcb0a40 r7 ffcb0b60
DEBUG : r8 f0900007 r9 00000001 r10 f0900000 r11 f0900000
DEBUG : ip ffcb0500 sp ffcb09f0 lr 00000004 pc 021d265e
DEBUG :
DEBUG : backtrace:
DEBUG : #00 pc 0000365e /data/local/tmp/ClarityOpt (BuildId: fb1d8b990741386becb60ff1c8b10583efb05f70)
DEBUG : #01 pc 00004271 /data/local/tmp/ClarityOpt (BuildId: fb1d8b990741386becb60ff1c8b10583efb05f70)
DEBUG : #02 pc 00004c9f /data/local/tmp/ClarityOpt (BuildId: fb1d8b990741386becb60ff1c8b10583efb05f70)
DEBUG : #03 pc 00004dd3 /data/local/tmp/ClarityOpt (BuildId: fb1d8b990741386becb60ff1c8b10583efb05f70)
DEBUG : #04 pc 000513bb /apex/com.android.runtime/lib/bionic/libc.so (__libc_init+66) (BuildId: 8e41d0dce7911ae25a51deb63aa9720c)
DEBUG : #05 pc 00002a98 /data/local/tmp/ClarityOpt (BuildId: fb1d8b990741386becb60ff1c8b10583efb05f70)