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PROGRAMMING WITH AVX
The following pseudocode illustrates this recommended application AVX detection process:
Example 13-1. Detection of AVX Instruction
INT supports_AVX()
{ mov
eax, 1
cpuid
and
ecx, 018000000H
cmp
ecx, 018000000H; check both OSXSAVE and AVX feature flags
jne
not_supported
; processor supports AVX instructions and XGETBV is enabled by OS
mov
ecx, 0; specify 0 for XCR0 register
XGETBV
; result in EDX:EAX
and
eax, 06H
cmp
eax, 06H; check OS has enabled both XMM and YMM state support
jne
not_supported
mov
eax, 1
jmp
done
NOT_SUPPORTED:
mov
eax, 0
done:
Note: It is unwise for an application to rely exclusively on CPUID.1:ECX.AVX[bit 28] or at all on
CPUID.1:ECX.XSAVE[bit 26]: These indicate hardware support but not operating system support. If YMM state
management is not enabled by an operating systems, AVX instructions will #UD regardless of
CPUID.1:ECX.AVX[bit 28]. “CPUID.1:ECX.XSAVE[bit 26] = 1” does not guarantee the OS actually uses the XSAVE
process for state management.
13.5.1
Detection of VEX-Encoded AES and VPCLMULQDQ
VAESDEC/VAESDECLAST/VAESENC/VAESENCLAST/VAESIMC/VAESKEYGENASSIST instructions operate on YMM
states. The detection sequence must combine checking for CPUID.1:ECX.AES[bit 25] = 1 and the sequence for
detection application support for AVX.
Example 13-2. Detection of VEX-Encoded AESNI Instructions
INT supports_VAESNI()
{ mov
eax, 1
cpuid
and
ecx, 01A000000H
cmp
ecx, 01A000000H; check OSXSAVE AVX and AESNI feature flags
jne
not_supported
; processor supports AVX and VEX-encoded AESNI and XGETBV is enabled by OS
mov
ecx, 0; specify 0 for XCR0 register
XGETBV
; result in EDX:EAX
and
eax, 06H
cmp
eax, 06H; check OS has enabled both XMM and YMM state support
jne
not_supported
mov
eax, 1
jmp
done
NOT_SUPPORTED:
mov
eax, 0
done:
13-18 Vol. 1