intrinsics

问题 Using C#'s Vector<T> , how can we most efficiently vectorize the operation of finding the index of a particular element in a set? As constraints, the set will always be a Span<T> of an integer primitive, and it will contain at most 1 matching element. I have come up with a solution that seems alright, but I'm curious if we can do better. Here is the approach: Create a Vector<T> consisting only of the target element, in each slot. Use Vector.Equals() between the input set vector and the vector

问题 There's only 1 circumstance where __builtin_clz gives the wrong answer. I'm curious what's causing that behavior. When I use the literal value 0 I always get 32 as expected. But 0 as a variable yields 31. Why does the method of storing the value 0 matter? I've taken an architecture class but don't understand the diffed assembly. It looks like when given the literal value 0, the assembly somehow always has the correct answer of 32 hard coded even without optimizations. And the method for

问题 I have been using the following "trick" in C code with SSE2 for single precision floats for a while now: static inline __m128 SSEI_m128shift(__m128 data) { return (__m128)_mm_srli_si128(_mm_castps_si128(data), 4); } For data like [1.0, 2.0, 3.0, 4.0] , it results in [2.0, 3.0, 4.0, 0.0] , i.e. it does a left shift by one position and fills the data structure with a zero. If I remember correctly, the above inline function compiles down to a single instruction (with gcc at least). I am somehow

问题 Closed. This question does not meet Stack Overflow guidelines. It is not currently accepting answers. Want to improve this question? Update the question so it's on-topic for Stack Overflow. Closed 5 years ago . Improve this question I'm looking into using these to improve the performance of some code but good documentation seems hard to find for the functions defined in the *mmintrin.h headers, can anybody provide me with pointers to good info on these? EDIT: particularly interested in a very

问题 I'm reading What Every Programmer Should Know About Memory pdf by Ulrich Drepper. At the beginning of part 6 theres's a code fragment: #include <emmintrin.h> void setbytes(char *p, int c) { __m128i i = _mm_set_epi8(c, c, c, c, c, c, c, c, c, c, c, c, c, c, c, c); _mm_stream_si128((__m128i *)&p[0], i); _mm_stream_si128((__m128i *)&p[16], i); _mm_stream_si128((__m128i *)&p[32], i); _mm_stream_si128((__m128i *)&p[48], i); } With such a comment right below it: Assuming the pointer p is

问题 from http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/classfile/vmSymbols.hpp, I can see the intrinsic method declare like: do_intrinsic(_getByte, sun_misc_Unsafe, getByte_name, getByte_signature, F_RN) \ but how to find the actually implementation(assembly code I think) of the method _getByte ? 回答1: but how to find the actually implementation(assembly code I think) of the method _getByte By looking for vmIntrinsics::_getByte in your IDE or simply by grepping

问题 from http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/classfile/vmSymbols.hpp, I can see the intrinsic method declare like: do_intrinsic(_getByte, sun_misc_Unsafe, getByte_name, getByte_signature, F_RN) \ but how to find the actually implementation(assembly code I think) of the method _getByte ? 回答1: but how to find the actually implementation(assembly code I think) of the method _getByte By looking for vmIntrinsics::_getByte in your IDE or simply by grepping

问题 I'm trying to subtract two images(grayscaled) by using Neon intrinsics as an exercise, I don't know what is the best way to subtract two vectors using the C intrinsics. void subtractTwoImagesNeonOnePass( uint8_t *src, uint8_t*dest, uint8_t*result, int srcWidth) { for (int i = 0; i<srcWidth; i++) { // load 8 pixels uint8x8x3_t srcPixels = vld3_u8 (src); uint8x8x3_t dstPixels = vld3_u8 (src); // subtract them uint8x8x3_t subPixels = vsub_u8(srcPixels, dstPixels); // store the result vst1_u8

问题 The instruction exists ( vbroadcastss zmm/m32 ) but there seems to be no intrinsic to generate it. I can code it as static inline __m512 mybroadcast(float *x) { __m512 v; asm inline ( "vbroadcastss %1,%0 " : "=v" (v) : "m" (*x) ); return v; } Is there a way to do this without inline asm? 回答1: I think _mm512_set1_ps is what you want. https://software.intel.com/sites/landingpage/IntrinsicsGuide/#text=_mm512_set1_ps&expand=5236,4980 来源： https://stackoverflow.com/questions/59128802/is-there-an

问题 I'm developing an iOS app that needs to convert images from RGB -> BGRA fairly quickly. I would like to use NEON intrinsics if possible. Is there a faster way than simply assigning the components? void neonPermuteRGBtoBGRA(unsigned char* src, unsigned char* dst, int numPix) { numPix /= 8; //process 8 pixels at a time uint8x8_t alpha = vdup_n_u8 (0xff); for (int i=0; i<numPix; i++) { uint8x8x3_t rgb = vld3_u8 (src); uint8x8x4_t bgra; bgra.val[0] = rgb.val[2]; //these lines are slow bgra.val[1]