When using a mask register with AVX-512 load and stores, is a fault raised for invalid accesses to masked out elements?

问题

When I do a writemasked AVX-512 store, like so:

vmovdqu8 [rsi] {k1}, zmm0

Will the instruction fault if some portion of the memory accessed at [rsi, rsi + 63] is not mapped but the writemask is zero for all those locations (i.e., the data is not actually modified due to the mask).

Another way of asking it is if these AVX-512 masked stores have a similar fault suppression ability to vmaskmov introduced in AVX.

回答1:

No fault is raised if masked out elements touch invalid memory.

Here's some Windows test code to prove that masking does indeed suppress memory faults.

#include <immintrin.h>
#include <iostream>
#include <Windows.h>
using namespace std; 


int main(){
    const size_t PAGE = 4096;

    //  Map 2 pages.
    char* ptr = (char*)VirtualAlloc(
        nullptr, 2*PAGE,
        MEM_COMMIT,
        PAGE_READWRITE
    );

    //  Store 64 bytes across page boundary.
    cout << "Store across page boundary." << endl;
    _mm512_storeu_si512(ptr + PAGE - 32, _mm512_set1_epi8(-1));

    //  Unmap top page.
    cout << "Unmap top page." << endl;
    VirtualFree(ptr + PAGE, PAGE, MEM_DECOMMIT);

    //  Write on boundary masking out the part that touches the top (unmapped page).
    //  Does not crash because bad accesses are masked out.
    cout << "Store across page boundary, but mask out bytes that are on unmapped page." << endl;
    _mm512_mask_storeu_epi8(ptr + PAGE - 32, 0x00000000ffffffff, _mm512_set1_epi8(-1));

    //  Store 64 bytes across page boundary.
    //  Crashes because of bad access.
    cout << "Store across page boundary." << endl;
    _mm512_storeu_si512(ptr + PAGE - 32, _mm512_set1_epi8(-1));

    cout << "Release bottom page." << endl;
    VirtualFree(ptr, 0, MEM_RELEASE);

    system("pause");
}

Output:

Store across page boundary.
Unmap top page.
Store across page boundary, but mask out bytes that are on unmapped page.
Store across page boundary.
**Access violation**

This test works as follows:

Map 2 adjacent pages.
Do an AVX512 store across the page boundary to prove that both pages are mapped.
Unmap the upper page.
Do the same AVX512 store, but mask out the bytes that are on the upper page. It does not crash.
Repeat the 1st AVX512 store (without masking). It crashes, thus proving that the upper page has been unmapped and the masking suppressed the crash.

来源：https://stackoverflow.com/questions/54497141/when-using-a-mask-register-with-avx-512-load-and-stores-is-a-fault-raised-for-i

标签

x86

avx

avx512