NDK clang編譯器的一個bug

willhua發表於2020-05-10
編譯

NDK clang編譯器的一個bug

問題程式碼

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];
}

編譯測試

首先，查閱了https://static.docs.arm.com/ihi0073/c/IHI0073C_arm_neon_intrinsics_ref.pdf，對於vld1q_f32_x4這個指令，v7/A32/A64都是支援的。

不同編譯器版本結果：首先，對於所有的版本，如果使用#else塊的程式碼，都是可以編譯成功的，對於使用#if 01塊的程式碼，結果如下：

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

不僅僅vld1q_f32_x4，對於vld1_u8_x2;vst1q_f32_x4等類似指令都存在這樣的問題。

效能對比

測試程式碼：

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;
}

測試了使用三種NDK版本編譯arm64-v8a測試，同時使用r19c編譯了armeabi-v7a，分別使用#if和#else分之，發現耗時都是在3.55ms左右，無明顯差別。

類似問題：https://github.com/mattgodbolt/compiler-explorer/issues/1906

地址對齊

雖然使用r19c的版本編譯armeabi-v7a成功，或者使用不優化的r20c也一樣，但是執行時發生了crash。原因是執行vldN(q)_type_xN指令時，地址不對齊導致的crash。

而對於arm64-v8a版本，把所有傳給vldN(q)_type_xN的地址列印出來，同樣發現也有0x7350800001這樣的地址，而且地址末位為0到E的都有，但是卻沒有報錯。也即，對於該指令只有armeabi-v7a有地址對齊要求，而arm64-v8a卻沒有？

同時，常規的vldN(q)_type指令則沒有地址對齊的要求，所以最好不要使用vldN(q)_type_xN。

在程式碼中因為地址對齊而導致的crash日誌：

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)

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