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Twitter troll task to remove if-else from grading function. Compile with clang/gcc -Wpedantic -Wall -Wextra -O3 -msse -mpopcnt
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#include <immintrin.h> | |
typedef unsigned int u32; | |
typedef unsigned char u8; | |
char check_grade(u8 score) { | |
score = score > 100 ? 100 : score; | |
__m128i scores = _mm_set1_epi8(score); | |
__m128i brackets = _mm_set_epi8(89, 79, 69, 59, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F, 0x7F); | |
__m128i res_mask = _mm_cmpgt_epi8(scores, brackets); | |
u32 res = _mm_movemask_epi8(res_mask); | |
int index = _mm_popcnt_u32(res); | |
return "FDCBA"[index]; | |
} | |
void check_grade4(char out_grades[restrict 4], u8 score[restrict 4]) { | |
__m128i scores = _mm_set_epi8( | |
score[0], score[0], score[0], score[0], | |
score[1], score[1], score[1], score[1], | |
score[2], score[2], score[2], score[2], | |
score[3], score[3], score[3], score[3] | |
); | |
__m128i max_scores = _mm_set1_epi8(100); | |
scores = _mm_min_epu8(scores, max_scores); | |
__m128i brackets = _mm_set_epi8( | |
89, 79, 69, 59, | |
89, 79, 69, 59, | |
89, 79, 69, 59, | |
89, 79, 69, 59 | |
); | |
__m128i res_masks = _mm_cmpgt_epi8(scores, brackets); | |
u32 res = _mm_movemask_epi8(res_masks); | |
static const char * const LUT = "FDCBA"; | |
int index0 = _mm_popcnt_u32(res & 0xF000); | |
int index1 = _mm_popcnt_u32(res & 0x0F00); | |
int index2 = _mm_popcnt_u32(res & 0x00F0); | |
int index3 = _mm_popcnt_u32(res & 0x000F); | |
out_grades[0] = LUT[index0]; | |
out_grades[1] = LUT[index1]; | |
out_grades[2] = LUT[index2]; | |
out_grades[3] = LUT[index3]; | |
} | |
// Test | |
#include <stdio.h> | |
#define ALIGNED(bytes) __attribute__ ((aligned (bytes))) | |
int main(void) { | |
enum {SIZE = 128}; | |
ALIGNED(16) u8 scores[SIZE]; | |
ALIGNED(16) char grades[SIZE]; | |
ALIGNED(16) char grades4[SIZE]; | |
for (u32 i = 0; i < SIZE; ++i) { | |
scores[i] = i; | |
} | |
for (u32 i = 0; i < SIZE; ++i) { | |
grades[i] = check_grade(scores[i]); | |
} | |
for (u32 i = 0; i < SIZE; i += 4) { | |
check_grade4(grades4 + i, scores + i); | |
} | |
puts("Score | check_grade | check_grade4'"); | |
puts("-----------------------------------"); | |
for (u32 i = 0; i < SIZE; ++i) { | |
printf("%-3u | '%c' | '%c'\n", i, grades[i], grades4[i]); | |
fflush(stdout); | |
if (grades[i] != grades4[i]) | |
{ | |
__builtin_trap(); | |
} | |
} | |
return 0; | |
} |
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