9

使用 arm-elf-gcc-4.5 -O3 -march=armv7-a -mthumb -mfpu=neon -mfloat-abi=softfp 编译此程序时:

#include <arm_neon.h>
extern float32x4_t cross(const float32x4_t& v1, const float32x4_t& v2) {
    float32x4x2_t
        xxyyzz1(vzipq_f32(v1, v1)),
        xxyyzz2(vzipq_f32(v2, v2));

    float32x2_t
        xx1(vget_low_f32(xxyyzz1.val[0])),
        yy1(vget_high_f32(xxyyzz1.val[0])),
        zz1(vget_low_f32(xxyyzz1.val[1])),
        xx2(vget_low_f32(xxyyzz2.val[0])),
        yy2(vget_high_f32(xxyyzz2.val[0])),
        zz2(vget_low_f32(xxyyzz2.val[1]));

    float32x2_t
        x(vmls_f32(vmul_f32(yy1, zz2), zz1, yy2)),
        y(vmls_f32(vmul_f32(zz1, xx2), xx1, zz2)),
        z(vmls_f32(vmul_f32(xx1, yy2), yy1, xx2));

    return vcombine_f32(vuzp_f32(x, y).val[0], z);
}

......这就是我得到的。注意标有@<<< 的两条无用指令

_Z5crossRK19__simd128_float32_tS1_:
    vldmia  r0, {d16-d17}
    vldmia  r1, {d22-d23}
    vmov    q10, q8  @ v4sf
    vmov    q9, q11  @ v4sf
    vzip.32 q8, q10
    vzip.32 q11, q9
    vmov    d24, d17
    vmov    d21, d22
    vmov    d22, d23
    vmul.f32    d17, d24, d18
    vmul.f32    d19, d20, d21
    vmls.f32    d19, d16, d18
    vmls.f32    d17, d20, d22
    vmul.f32    d16, d16, d22
    vuzp.32 d17, d19
    vmls.f32    d16, d24, d21
    sub sp, sp, #80 @<<<
    vswp    d17, d16
    vmov    r0, r1, d16  @ v4sf
    vmov    r2, r3, d17
    add sp, sp, #80 @<<<
    bx

堆栈永远不会被访问,但堆栈指针会递减,然后以相同的量递增。为什么?

如果我修改原始代码以在序言的末尾和结尾的开头包含 asm 注释,如下所示:

#include <arm_neon.h>
extern float32x4_t cross(const float32x4_t& v1, const float32x4_t& v2) {
    asm volatile("# End of prologue");
    float32x4x2_t
        xxyyzz1(vzipq_f32(v1, v1)),
        xxyyzz2(vzipq_f32(v2, v2));

    float32x2_t
        xx1(vget_low_f32(xxyyzz1.val[0])),
        yy1(vget_high_f32(xxyyzz1.val[0])),
        zz1(vget_low_f32(xxyyzz1.val[1])),
        xx2(vget_low_f32(xxyyzz2.val[0])),
        yy2(vget_high_f32(xxyyzz2.val[0])),
        zz2(vget_low_f32(xxyyzz2.val[1]));

    float32x2_t
        x(vmls_f32(vmul_f32(yy1, zz2), zz1, yy2)),
        y(vmls_f32(vmul_f32(zz1, xx2), xx1, zz2)),
        z(vmls_f32(vmul_f32(xx1, yy2), yy1, xx2));

    float32x4_t res(vcombine_f32(vuzp_f32(x, y).val[0], z));
    asm volatile("# Start of epilogue");
    return res;
}

然后我得到稍微不同的版本:

_Z5crossRK19__simd128_float32_tS1_:
    sub sp, sp, #80
    # End of prologue
    vldmia  r0, {d16-d17}
    vldmia  r1, {d22-d23}
    vmov    q10, q8  @ v4sf
    vmov    q9, q11  @ v4sf
    vzip.32 q8, q10
    vzip.32 q11, q9
    vmov    d24, d17
    vmov    d21, d22
    vmov    d22, d23
    vmul.f32    d17, d24, d18
    vmul.f32    d19, d20, d21
    vmls.f32    d19, d16, d18
    vmls.f32    d17, d20, d22
    vmul.f32    d16, d16, d22
    vuzp.32 d17, d19
    vmls.f32    d16, d24, d21
    vswp    d17, d16
    # Start of epilogue
    vmov    r0, r1, d16  @ v4sf
    vmov    r2, r3, d17
    add sp, sp, #80
    bx  lr

堆栈指针递减/递增显然是序言/结尾的一部分,即使不使用堆栈也会发生。这是为了符合某些标准,还是 gcc 优化错误?

编辑:编译器是 arm-elf-gcc-4.5 (GCC) 4.5.0,配置为:/opt/local/var/macports/build/_opt_local_var_macports_sources_rsync.macports.org_release_ports_cross_arm-elf-gcc/work/gcc-4.5.0/配置 --prefix=/opt/local --infodir=/opt/local/share/info --mandir=/opt/local/share/man --target=arm-elf --program-prefix=arm-elf- --program-suffix=-4.5 --without-included-gettext --enable-obsolete --with-newlib --disable-__cxa_atexit --enable-multilib --enable-biendian --disable-libgfortran --with-gxx -include-dir=/opt/local/arm-elf/include/c++/4.5.0/ --enable-languages=c,c++,objc --build=x86_64-apple-darwin10 --enable-fpu

编辑:我设法使用以下 C 源代码查明问题。仅当使用向量类型的数组作为临时对象时才会发生这种情况,例如声明为的 float32x4x2_t struct { float32x4_t val[2]; },即使这些临时对象是寄存器。我相信这是一个错误,所以我报告了它

#include <arm_neon.h>

// This one is ok
extern float32x4_t add(float32x4_t* v1, float32x4_t* v2) {
    return vaddq_f32(*v1, *v2);
#if 0
    produced assembly:

add:
    vldmia  r0, {d16-d17}
    vldmia  r1, {d18-d19}
    vadd.f32    q8, q8, q9
    vmov    r0, r1, d16
    vmov    r2, r3, d17
    bx  lr

#endif
}

// This one uses float32x4x2_t temporaries and has the bug
extern float32x4_t cross(float32x4_t* v1, float32x4_t* v2) {
    float32x4x2_t
        xxyyzz1=vzipq_f32(*v1, *v1),
        xxyyzz2=vzipq_f32(*v2, *v2);

    float32x2_t
        xx1=vget_low_f32(xxyyzz1.val[0]),
        yy1=vget_high_f32(xxyyzz1.val[0]),
        zz1=vget_low_f32(xxyyzz1.val[1]),
        xx2=vget_low_f32(xxyyzz2.val[0]),
        yy2=vget_high_f32(xxyyzz2.val[0]),
        zz2=vget_low_f32(xxyyzz2.val[1]);

    float32x2_t
        x=vmls_f32(vmul_f32(yy1, zz2), zz1, yy2),
        y=vmls_f32(vmul_f32(zz1, xx2), xx1, zz2),
        z=vmls_f32(vmul_f32(xx1, yy2), yy1, xx2);

    return vcombine_f32(vuzp_f32(x, y).val[0], z);
#if 0
    produced assembly:

cross:
    vldmia  r0, {d18-d19}
    vldmia  r1, {d16-d17}
    vmov    q10, q9
    vmov    q11, q8
    vzip.32 q9, q10
    vzip.32 q8, q11
    vmov    d24, d19
    vmov    d21, d16
    vmov    d16, d17
    vmul.f32    d19, d20, d21
    vmul.f32    d17, d24, d22
    vmls.f32    d17, d20, d16
    vmls.f32    d19, d18, d22
    vmul.f32    d16, d18, d16
    vuzp.32 d17, d19
    vmls.f32    d16, d24, d21
    sub sp, sp, #48           @ here
    vswp    d17, d16
    vmov    r0, r1, d16
    vmov    r2, r3, d17
    add sp, sp, #48           @ and here
    bx  lr

#endif
}
4

1 回答 1

5

原来这是一个错误,所以关闭它。

于 2011-05-10T03:44:55.527 回答