ARM GCC错误？使用vldr链而不是一个vldmia ...

Question

考虑以下NEON优化功能:

void mat44_multiply_neon(float32x4x4_t& result, const float32x4x4_t& a, const float32x4x4_t& b) {
    // Make sure "a" is mapped to registers in the d0-d15 range,
    // as requested by NEON multiply operations below:
    register float32x4_t a0 asm("q0") = a.val[0];
    register float32x4_t a1 asm("q1") = a.val[1];
    register float32x4_t a2 asm("q2") = a.val[2];
    register float32x4_t a3 asm("q3") = a.val[3];
    asm volatile (
    "\n\t# multiply two matrices...\n\t"
    "# result (%q0,%q1,%q2,%q3)  = first column of B (%q4) * first row of A (q0-q3)\n\t"
    "vmul.f32 %q0, %q4, %e8[0]\n\t"
    "vmul.f32 %q1, %q4, %e9[0]\n\t"
    "vmul.f32 %q2, %q4, %e10[0]\n\t"
    "vmul.f32 %q3, %q4, %e11[0]\n\t"
    "# result (%q0,%q1,%q2,%q3) += second column of B (%q5) * second row of A (q0-q3)\n\t"
    "vmla.f32 %q0, %q5, %e8[1]\n\t"
    "vmla.f32 %q1, %q5, %e9[1]\n\t"
    "vmla.f32 %q2, %q5, %e10[1]\n\t"
    "vmla.f32 %q3, %q5, %e11[1]\n\t"
    "# result (%q0,%q1,%q2,%q3) += third column of B (%q6) * third row of A (q0-q3)\n\t"
    "vmla.f32 %q0, %q6, %f8[0]\n\t"
    "vmla.f32 %q1, %q6, %f9[0]\n\t"
    "vmla.f32 %q2, %q6, %f10[0]\n\t"
    "vmla.f32 %q3, %q6, %f11[0]\n\t"
    "# result (%q0,%q1,%q2,%q3) += last column of B (%q7) * last row of A (q0-q3)\n\t"
    "vmla.f32 %q0, %q7, %f8[1]\n\t"
    "vmla.f32 %q1, %q7, %f9[1]\n\t"
    "vmla.f32 %q2, %q7, %f10[1]\n\t"
    "vmla.f32 %q3, %q7, %f11[1]\n\t\n\t"
    : "=&w"  (result.val[0]), "=&w"  (result.val[1]), "=&w"  (result.val[2]), "=&w" (result.val[3])
    : "w"   (b.val[0]),      "w"   (b.val[1]),      "w"   (b.val[2]),      "w"   (b.val[3]),
      "w"   (a0),            "w"   (a1),            "w"   (a2),            "w"   (a3)
    :
    );
}

为什么GCC 4.5会产生这种憎恶,因为加载了第一个矩阵:

vldmia  r1, {d0-d1}
vldr    d2, [r1, #16]
vldr    d3, [r1, #24]
vldr    d4, [r1, #32]
vldr    d5, [r1, #40]
vldr    d6, [r1, #48]
vldr    d7, [r1, #56]

......而不仅仅是:

vldmia  r1, {q0-q3}

...？

我使用的选项:

arm-none-eabi-gcc-4.5.1 -x c++ -march=armv7-a -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -O3 -ffast-math -fgcse-las -funsafe-loop-optimizations -fsee -fomit-frame-pointer -fstrict-aliasing -ftree-vectorize

请注意,使用iPhoneOS提供的编译器会产生相同的结果:

/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc-4.2 -x c++ -arch armv7 -mcpu=cortex-a8 -mfpu=neon -mfloat-abi=softfp -O3 -ffast-math -fgcse-las -funsafe-loop-optimizations -fsee -fomit-frame-pointer -fstrict-aliasing -ftree-vectorize

Answer 1

简单回答:

GCC编译器目前不擅长生成ARM代码.如果你看起来接近其他代码,你会发现GCC几乎从不安排注册,它可以使用多个寄存器加载/存储,除了硬编码的地方,如函数prolog/epilog和内联memcpy.

当涉及使用霓虹灯指令时,代码变得更糟.这与NEON单元的工作方式有关:您可以将寄存器对视为四字或双字.这是(据我所知)GCC支持的体系结构中寄存器使用的独特功能.因此,代码生成器不会在所有实例中生成最佳代码.

顺便说一下:虽然我很喜欢它:GCC不知道在Cortex-A8上使用'free'桶形移位器功能会对寄存器调度产生重要影响,而GCC大多数错误.