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：

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)