VZEROUPPER—Zero Upper Bits of YMM and ZMM Registers

Opcode/Instruction Op /En 64/32 bit Mode Support CPUID Feature Flag Description

VEX.128.0F.WIG 77

VZEROUPPER

 ZO V/V AVX Zero bits in positions 128 and higher of some YMM and ZMM registers.

Instruction Operand Encoding

Op/En Operand 1 Operand 2 Operand 3 Operand 4
ZO N/A N/A N/A N/A

Description

In 64-bit mode, the instruction zeroes the bits in positions 128 and higher in YMM0-YMM15 and ZMM0-ZMM15. Outside 64-bit mode, it zeroes those bits only in YMM0-YMM7 and ZMM0-ZMM7. VZEROUPPER does not modify the lower 128 bits of these registers and it does not modify ZMM16-ZMM31.

This instruction is recommended when transitioning between AVX and legacy SSE code; it will eliminate perfor-mance penalties caused by false dependencies.

Note: VEX.vvvv is reserved and must be 1111b otherwise instructions will #UD. In Compatibility and legacy 32-bit mode only the lower 8 registers are modified.

Operation

simd_reg_file[][] is a two dimensional array representing the SIMD register file containing all the overlapping xmm, ymm, and zmm registers present in that implementation. The major dimension is the register number: 0 for xmm0, ymm0, and zmm0; 1 for xmm1, ymm1, and zmm1; etc. The minor dimension size is the width of the implemented SIMD state measured in bits.

VZEROUPPER

IF (64-bit mode)
    limit :=15
ELSE
    limit := 7
FOR i in 0 .. limit:
    simd_reg_file[i][MAXVL-1:128] := 0

Intel C/C++ Compiler Intrinsic Equivalent

VZEROUPPER:

_mm256_zeroupper()

SIMD Floating-Point Exceptions

None.

Other Exceptions

See Table 2-25, “Type 8 Class Exception Conditions.”