Created
November 17, 2025 17:34
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AVX load instructions can be made to ignore segfaults on unused data
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| #include <immintrin.h> | |
| #include <stdio.h> | |
| #include <stdlib.h> | |
| #include <stdint.h> | |
| #include <sys/mman.h> | |
| // Compile with clang++ -g -mavx512bw -mavx512f -o mask-test mask-test.cc | |
| int main() { | |
| char* addr = (char*)mmap(0, 8192, PROT_READ | PROT_WRITE, MAP_ANON | MAP_PRIVATE, -1, 0); | |
| int result = mprotect(addr + 4096, 4096, PROT_NONE); | |
| if (result != 0) abort(); | |
| __mmask32 mask = 0xFFFFFFFFULL; | |
| for (int i = 0; i < 64; i += 2) { | |
| // This address appears to go over the end of the page into the | |
| // protected page when i != 0 and we load a 512 bit (64 byte) value. | |
| char* bad = addr + 4096 - 64 + i; | |
| // Load 512 bits into register, but use the mask to zero some of the | |
| // 16 bit words. | |
| __m512i input = _mm512_maskz_loadu_epi16(mask, bad); | |
| // Make sure the optimizer doesn't remove the load instruction. | |
| uint16_t values[32]; | |
| _mm512_store_si512((__m512i*)values, input); | |
| printf("Values[31] = 0x%04x\n", values[31]); | |
| // For the next iteration we add a bit of masking which will both zero the | |
| // value in the register, but also suppresses faults! | |
| mask >>= 1; | |
| } | |
| } |
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