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Correctness bugs in instrset_detect.cpp #91
Description
- instrset_detect.cpp:102 — AVX-512 check can crash on some systems. It reads CPUID leaf 0xD, which only says what the processor supports, not what the OS has enabled; it should use
xgetbv(0). On an OS or VM that hasn't turned on AVX-512 state, the check still passes and the first AVX-512 instruction faults. - instrset_detect.cpp:103 — AVX-512 mask is incomplete. The mask
0x60skips bit 7 (Hi16_ZMM, which covers registers zmm16–31); it should be0xE0. If bits 5–6 are enabled but bit 7 isn't, code using zmm16–31 faults. - instrset_detect.cpp:106 — Undefined behavior before C++20.
1 << 31shifts into the sign bit of a signed int;1u << 31fixes it.
Activity
AgnerF commentedon Sep 29, 2026
Are these bug reports AI generated? The code actually calls xgetbv(0).
HolyWu commentedon Sep 30, 2026
Yes, they are.
AVX512 needs three more XCR0 bits, and line 95 never looks at them:
An OS can enable YMM state but not those. Older kernels and some hypervisors do this. Then line 95 passes and AVX2 is correctly reported. But line 103 reads CPUID 0xD, which reports which XCR0 bits the CPU can enable, not which the OS did enable. So line 103 passes anyway. AVX512 code then hits #UD.
Fix: replace line 102-103 with if ((xgetbv(0) & 0xE0) != 0xE0) return iset;. That also covers bit 7, which line 103's 0x60 mask skips.