代码之家 › 专栏 › 技术社区 › Pavel P

如何清除霓虹灯第一条非零车道以外的所有车道?

neon intrinsics arm c++

Pavel P · 技术社区 · 7 年前

我在一个uint32x4的霓虹灯寄存器里有一个面具。在这个掩码中,至少设置了4个整数中的1个(例如,0xffffffff),但是,在寄存器中可能设置了多个项。如何确保只设置一个?

在C伪代码中:

uint32x4_t clearmask(uint32x4_t m)
{
         if (m[0]) { m[1] = m[2] = m[3] = 0; }
    else if (m[1]) { m[2] = m[3] = 0; }
    else if (m[2]) { m[3] = 0; }
    return m;
}

基本上,我想清除除一条固定车道以外的所有车道。明显 straightforward implementation in neon 可以是:

uint32x4_t cleanmask(uint32x4_t m)
{
    uint32x4_t mx;
    mx = vdupq_lane_u32(vget_low_u32(vmvnq_u32(m)), 0);
    mx = vsetq_lane_u32(0xffffffff, mx, 0);
    m = vandq_u32(m, mx);

    mx = vdupq_lane_u32(vget_low_u32(vmvnq_u32(m)), 1);
    mx = vsetq_lane_u32(0xffffffff, mx, 1);
    m = vandq_u32(m, mx);

    mx = vdupq_lane_u32(vget_high_u32(vmvnq_u32(m)), 0);
    mx = vsetq_lane_u32(0xffffffff, mx, 2);
    m = vandq_u32(m, mx);

    return m;
}

如何在Arm Neon中更有效地实现这一点?

2 回复 | 直到 7 年前

Peter Cordes 7 年前

Very simple

vceq.u32    q1, q0, #0
vmov.i8     d7, #0xff
vext.8      q2, q3, q1, #12

vand        q0, q0, q2
vand        d1, d1, d2
vand        d1, d1, d4

aarch64

cmeq    v1.4s, v0.4s, #0
movi    v31.16b, #0xff

ext     v2.16b, v31.16b, v1.16b, #12
ext     v3.16b, v31.16b, v1.16b, #8
ext     v4.16b, v31.16b, v1.16b, #4

and     v0.16b, v0.16b, v2.16b
and     v0.16b, v0.16b, v3.16b
and     v0.16b, v0.16b, v4.16b

ext / vext

v0 = [  d   c   b   a ]

v2 = [ !c  !b  !a  -1 ]
v3 = [ !b  !a  -1  -1 ]
v4 = [ !a  -1  -1  -1 ]

d

c a b

mvn

Peter Cordes 7 年前

vdupq_lane_u32(vget_low_u32(m), 1); vdup.32 q9, d16[1]

uint32x4_t cleanmask_xor(uint32x4_t m)
{
    //                 {  a    b    c   d }
    uint32x4_t maska = {  0, ~0U, ~0U, ~0U};
    uint32x4_t maskb = {~0U,   0, ~0U, ~0U};
    uint32x4_t maskc = {~0U, ~0U,   0, ~0U};

    uint32x4_t tmp = vdupq_lane_u32(vget_low_u32(m), 0);
    uint32x4_t aflip = tmp ^ maska;
    m &= aflip;  // if a was non-zero, the rest are zero

    tmp = vdupq_lane_u32(vget_low_u32(m), 1);
    uint32x4_t bflip = tmp ^ maskb;
    m &= bflip;  // if b was non-zero, the rest are zero

    tmp = vdupq_lane_u32(vget_high_u32(m), 0);
    uint32x4_t cflip = tmp ^ maskc;
    m &= cflip;  // if b was non-zero, the rest are zero

    return m;
}

Godbolt

/* design notes
  [ a   b   c   d ]
  [ a  ~a  ~a  ~a ] 

&:[ a   0   0   0 ]
or[ 0   b   c   d ]

= [ e   f   g   h  ]
  [ ~f  f   ~f  ~f ]  // not b, because f can be zero when b isn't

= [ i   j   k   l ]
  ...
*/

vmov.32 d1[0], r3 -1 -1U veor vmvn

#if 1
    // clang sets up the address of each constant separately
    //                 {  a    b    c   d }
    uint32x4_t maska = {  0, ~0U, ~0U, ~0U};
    uint32x4_t maskb = {~0U,   0, ~0U, ~0U};
    uint32x4_t maskc = {~0U, ~0U,   0, ~0U};
#else
    static const uint32_t maskbuf[] = 
      { -1U, -1U, 0, -1U, -1U, -1U};
    // unaligned loads.
    // or load one + shuffle?
#endif

推荐文章

AstralHex · 矩阵乘法代码工作不正常

5 月前

Baba Dan Constantin · SSE4.1在矩阵4x4乘法上比SSE3慢?

6 月前

Giogre · 为包含许多数值字段的简单“struct”重载比较运算符

6 月前

einpoklum · 定义一个并不真正提供now()函数的std::chrono Clock是“合法的”吗?

6 月前

Fishie · 作为类成员的智能指针是否仍然自动释放?[关闭]

6 月前

Die4Toast · 递归调用成员箭头运算符->

6 月前

Angle.Bracket · 如何用C++将UTF-8文件名写入MS Windows控制台?

6 月前

Anka HanÄ±m · 关于结构和动态数组地址的问题

6 月前

Adam Barnes · 我如何定义一个基于constexpr函数返回值进行限制的概念?

6 月前

user2138149 · 为什么我不能获取包含多个元素的结构体中某些元素的地址?[副本]

6 月前