GNU C本地向量：如何广播标量，如x86的_mm_set1_epi16 [英] GNU C native vectors: how to broadcast a scalar, like x86's _mm_set1_epi16

问题描述

如何编写可移植的 GNU C内置载体版本这不依赖于x86 set1的内部函数吗？

$ $ $ $ $ $ $ $ $ typedef uint16_t v8su __attribute __（（vector_size（16））） ;

v8su set1_u16_x86（uint16_t标量）{
return（v8su）_mm_set1_epi16（scalar）; //转换为gcc
所需的元素}

当然， p

$ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $ $' S，S};
}

我不想编写一个用于播放单曲的AVX2版本字节！

即使只有gcc-only或clang-only的回答也很有趣，对于要分配给变量，而不仅仅是作为二元运算符的操作数使用（这适用于gcc，见下文）。

---

如果我想使用广播标量作为二元运算符的一个操作数，它可以与gcc一起工作（，如手册​​中所记录的那样），但不会与叮当声：

  v8su vecdiv10（v8su v）{返回v / 10; } //不能用clang编译
  

在clang中，如果我只针对x86，使用本地向量语法让编译器为我生成模乘乘反常数和指令，我可以这样写：

  v8su vecdiv_set1（v8su v）{
 return v /（v8su）_mm_set1_epi16（10）; // gcc需要演员
} 
  

但是我必须改变内在的，如果我扩展向量（< _mm256_set1_epi16 ），而不是通过改变为 vector_size（32）来将整个代码转换为AVX2一个地方（对于不需要洗牌的纯垂直SIMD）。它也击败了原生载体的部分目的，因为它不能编译ARM或任何非x86目标。

clang，不考虑与 __ m128i {aka __vector（2）long long int}兼容的 v8us {aka __vector（8）short unsigned int} 。

顺便说一下，所有这些都可以编译成gcc和clang的良好汇编（在Godbolt上查看）。 这只是一个优雅写法的问题，它的可读语法不会重复标量N次。例如 v / 10 足够紧凑，甚至不需要将它放在自己的函数中。

国际刑事法院是一项奖金，但不是必需的。对于ICC来说，GNU C原生载体显然是事后考虑的，甚至是这样简单的东西不能有效地编译。 set1_u16 编译为8个标量存储和一个向量加载，而不是MOVD / VPBROADCASTW（启用 -xHOST >，因为它不承认 -march = haswell ，但Godbolt在支持AVX2的服务器上运行）。纯粹铸造 _mm _ intrinsics的结果是可以的，但该部门调用了一个SVML函数！

解决方案

使用两个观察结果可以找到GCC和Clang的通用广播解决方案 /clang.llvm.org/docs/LanguageExtensions.html#vectors-and-extended-vectorsrel =nofollow noreferrer> Clang的OpenCL向量扩展和GCC的向量扩展支持标量 - 向量操作。

x - 0 = x （但 x + 0 由于签名为零而不起作用））。

这是一个针对四个浮点数向量的解决方案。

<$ p $ （__clang__）
typedef float v4sf __attribute __（（ext_vector_type（4）））;
#else
typedef float v4sf __attribute__（（vector_size（16）））;
#endif

v4sf broadcast4f（float x）{
return x - （v4sf）{};
}

https://godbolt.org/g/PXr3Xb

相同的通用解决方案可用于不同的载体。这是一个8个无符号短裤的例子。

  #if defined（__clang__）
 typedef unsigned short v8su __attribute __（（ext_vector_type（8）））; 
 #else 
 typedef unsigned short v8su __attribute __（（vector_size（16）））; 
 #endif 
 
 v8su broadcast8us（short x）{
 return x  - （v8su）{}; 
 
 $ / code> 

ICC（17）支持GCC向量扩展的一个子集，但不支持无论是矢量+标量还是矢量*标量但内部函数对于广播仍然是必需的。 MSVC不支持任何向量
的扩展名。

How do I write a portable GNU C builtin vectors version of this, which doesn't depend on the x86 set1 intrinsic?

typedef uint16_t v8su __attribute__((vector_size(16)));

v8su set1_u16_x86(uint16_t scalar) {
    return (v8su)_mm_set1_epi16(scalar);   // cast needed for gcc
}

Surely there must be a better way than

v8su set1_u16(uint16_t s) {
    return (v8su){s,s,s,s,  s,s,s,s};
}

I don't want to write an AVX2 version of that for broadcasting a single byte!

Even a gcc-only or clang-only answer to this part would be interesting, for cases where you want to assign to a variable instead of only using as an operand to a binary operator (which works well with gcc, see below).

---

If I want to use a broadcast-scalar as one operand of a binary operator, this works with gcc (as documented in the manual), but not with clang:

v8su vecdiv10(v8su v) { return v / 10; }   // doesn't compile with clang

With clang, if I'm targeting only x86 and just using native vector syntax to get the compiler to generate modular multiplicative inverse constants and instructions for me, I can write:

v8su vecdiv_set1(v8su v) {
    return v / (v8su)_mm_set1_epi16(10);   // gcc needs the cast
}

But then I have to change the intrinsic if I widen the vector (to _mm256_set1_epi16), instead of converting the whole code to AVX2 by changing to vector_size(32) in one place (for pure-vertical SIMD that doesn't need shuffling). It also defeats part of the purpose of native vectors, since that won't compile for ARM or any non-x86 target.

The ugly cast is required because gcc, unlike clang, doesn't consider v8us {aka __vector(8) short unsigned int} compatible with __m128i {aka __vector(2) long long int}.

BTW, all of this compiles to good asm with gcc and clang (see it on Godbolt). This is just a question of how to write elegantly, with readable syntax that doesn't repeat the scalar N times. e.g. v / 10 is compact enough that there's no need to even put it in its own function.

Compiling efficiently with ICC is a bonus, but not required. GNU C native vectors are clearly an afterthought for ICC, and even simple stuff like this doesn't compile efficiently. set1_u16 compiles to 8 scalar stores and a vector load, instead of MOVD / VPBROADCASTW (with -xHOST enabled, because it doesn't recognize -march=haswell, but Godbolt runs on a server with AVX2 support). Purely casting the results of _mm_ intrinsics is ok, but the division calls an SVML function!

解决方案

A generic broadcast solution can be found for GCC and Clang using two observations

1. Clang's OpenCL vector extensions and GCC's vector extensions support scalar - vector operations.
2. x - 0 = x (but x + 0 does not work due to signed zero).

Here is a solution for a vector of four floats.

#if defined (__clang__)
typedef float v4sf __attribute__((ext_vector_type(4)));
#else
typedef float v4sf __attribute__ ((vector_size (16)));
#endif

v4sf broadcast4f(float x) {
  return x - (v4sf){};
}

https://godbolt.org/g/PXr3Xb

The same generic solution can be used for different vectors. Here is an example for a vector of eight unsigned shorts.

#if defined (__clang__)
typedef unsigned short v8su __attribute__((ext_vector_type(8)));
#else
typedef unsigned short v8su __attribute__((vector_size(16)));
#endif

v8su broadcast8us(short x) {
  return x - (v8su){};
}

ICC (17) supports a subset of the GCC vector extensions but does not support either vector + scalar or vector*scalar yet so intrinsics are still necessary for broadcasts. MSVC does not support any vector extensions.

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