Commit aa9db616774e24af7ab2fbcddd5711057b8a901e
2019-04-15T17:55:47
x86 SIMD: Check for CPUID leaf 07H before using According to Intel's manual [1], "If a value entered for CPUID.EAX is higher than the maximum input value for basic or extended function for that processor then the data for the highest basic information leaf is returned." Right now, libjpeg-turbo doesn't first check that leaf 07H is supported before attempting to use it, so the ostensible AVX2 bit (Bit 05) of the CPUID result might actually be Bit 05 from a lower leaf. That bit might be set, even if the CPU doesn't support AVX2. This commit modifies the x86 and x86-64 SIMD feature detection code so that it first checks whether CPUID leaf 07H is supported before attempting to use it to check for AVX2 instruction support. DRC: This commit should fix https://bugzilla.mozilla.org/show_bug.cgi?id=1520760 However, I have not personally been able to reproduce that issue, despite using a Nehalem (pre-AVX2) CPU on which the maximum CPUID leaf has been limited via a BIOS setting. Closes #348 [1] "Intel® 64 and IA-32 Architectures Software Developer's Manual, Volume 2 (2A, 2B, 2C & 2D): Instruction Set Reference, A-Z", https://software.intel.com/sites/default/files/managed/a4/60/325383-sdm-vol-2abcd.pdf, page 3-192.