不断加载到彩车SSE寄存器 [英] Load constant floats into SSE registers

问题描述

我试图找出一种有效的方式来加载编译时间常数花车到SSE（2/3）寄存器。我试着做一些简单的code这样的，

I'm trying to figure out an efficient way to load compile time constant floats into SSE(2/3) registers. I've tried doing simple code like this,

const __m128 x = { 1.0f, 2.0f, 3.0f, 4.0f }; 

不过，从内存中产生4 MOVSS说明！

but that generates 4 movss instructions from memory!

movss       xmm0,dword ptr [__real@3f800000 (14048E534h)] 
movss       xmm1,dword ptr [__real@40000000 (14048E530h)] 
movaps      xmm6,xmm12 
shufps      xmm6,xmm12,0C6h 
movss       dword ptr [rsp],xmm0 
movss       xmm0,dword ptr [__real@40400000 (14048E52Ch)] 
movss       dword ptr [rsp+4],xmm1 
movss       xmm1,dword ptr [__real@40a00000 (14048E528h)] 

其中加载标量进出内存

...（？！？！）

which load the scalars in and out of memory... (?!?!)

虽然这样做。

float Align(16) myfloat4[4] = { 1.0f, 2.0f, 3.0f, 4.0f, }; // out in global scope

生成。

movaps      xmm5,xmmword ptr [::myarray4 (140512050h)]

在理想情况下，这将是很好，如果我有常数他们将是一个办法不拉平触摸记忆，只是立即样式指令去做（如编译成指令本身的常量）。

Ideally, it would be nice if I have constants their would be a way not to even touch memory and just do it with immediate style instructions (e.g. the constants compiled into the instruction itself).

感谢

推荐答案

如果你想用它强制一个负载，你可以尝试（GCC）：

If you want to force it to a single load, you could try (gcc):

__attribute__((aligned(16))) float vec[4] = { 1.0f, 1.1f, 1.2f, 1.3f };
__m128 v = _mm_load_ps(vec); // edit by sor: removed the "&" cause its already an address

如果您有Visual C ++中，使用 __ declspec（调整（16））要求适当的约束。

If you have Visual C++, use __declspec(align(16)) to request the proper constraint.

在我的系统，这个（编译的gcc -m32 -msse -O2 ;在所有的杂波code，但仍然没有优化保留了单 MOVAPS 到底）创建以下组件code（GCC / AT＆amp; T公司的语法）：

On my system, this (compiled with gcc -m32 -msse -O2; no optimization at all clutters the code but still retains the single movaps in the end) creates the following assembly code (gcc / AT&T syntax):

    andl    $-16, %esp
    subl    $16, %esp
    movl    $0x3f800000, (%esp)
    movl    $0x3f8ccccd, 4(%esp)
    movl    $0x3f99999a, 8(%esp)
    movl    $0x3fa66666, 12(%esp)
    movaps  (%esp), %xmm0

请注意，它对齐分配stackspace并把常量在那里之前stackpointer。离开 __属性__（（对齐））出可能，这取决于你的编译器，创建不正确code，它并没有这样做，所以要小心，并检查拆卸。

Note that it aligns the stackpointer before allocating stackspace and putting the constants in there. Leaving the __attribute__((aligned)) out may, depending on your compiler, create incorrect code that doesn't do this, so beware, and check the disassembly.

此外：结果
既然你已经要求的如何把常量到code 的，只是尝试上面用静态预选赛的浮动阵列。这将创建下列组件：

Additionally:
Since you've been asking for how to put constants into the code, simply try the above with a static qualifier for the float array. That creates the following assembly:

    movaps  vec.7330, %xmm0
    ...
vec.7330:
    .long   1065353216
    .long   1066192077
    .long   1067030938
    .long   1067869798

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