Infinite range find algorithm: update transition comment for memchr optimization for ARM64

[AlexGuteniev]

Towards #2866

Reportedly, VSO-1488938 is not reproducible.

[AlexGuteniev]

Is memchr for infinite size even broken on ARM64?

[CaseyCarter]

Is memchr for infinite size even broken on ARM64?

The existence of VSO-1538014 suggests as much, but I'm not certain that it's been verified. Can anyone with ready access to an ARM64 box run:

#include <cstring>
#include <Windows.h>

int main() {
    void* p = ::VirtualAlloc(nullptr, 65536, MEM_RESERVE, PAGE_NOACCESS);
    void* p2 = ::VirtualAlloc(p, 4096, MEM_COMMIT, PAGE_READWRITE);
    char* p3 = (char*)p2 + 4093;
    p3[0] = '1';
    p3[1] = '2';
    p3[2] = '3';
    std::memchr(p3, '2', 1000);
}

and report back whether the program succeeds or AVs?

[strega-nil-ms]

@CaseyCarter

PS C:\..\vctools\asan\llvm> cl /nologo test.cpp
test.cpp
PS C:\..\vctools\asan\llvm> .\test.exe
PS C:\..\vctools\asan\llvm> echo $LASTEXITCODE
-1073741819

that looks like an AV to me.

[StephanTLavavej]

As I mentioned in Discord just now, I wonder what happens with x64/ARM64EC mixing, but I think we can go ahead here.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

Thanks for extending the reach of this optimization! 🐇 🚀 🎉