x86doc › UCOMISS - Unordered Compare Scalar Single Precision Floating-Point Values and Set EFLAGS

UCOMISS—Unordered Compare Scalar Single Precision Floating-Point Values and Set EFLAGS

Opcode/Instruction	Op /En	64/32 bit Mode Support	CPUID Feature Flag	Description
NP 0F 2E /r UCOMISS xmm1, xmm2/m32	A	V/V	SSE	Compare low single precision floating-point values in xmm1 and xmm2/mem32 and set the EFLAGS flags accordingly.
VEX.LIG.0F.WIG 2E /r VUCOMISS xmm1, xmm2/m32	A	V/V	AVX	Compare low single precision floating-point values in xmm1 and xmm2/mem32 and set the EFLAGS flags accordingly.
EVEX.LLIG.0F.W0 2E /r VUCOMISS xmm1, xmm2/m32{sae}	B	V/V	AVX512F	Compare low single precision floating-point values in xmm1 and xmm2/mem32 and set the EFLAGS flags accordingly.

Instruction Operand Encoding

Op/En	Tuple Type	Operand 1	Operand 2	Operand 3	Operand 4
A	N/A	ModRM:reg (r)	ModRM:r/m (r)	N/A	N/A
B	Tuple1 Scalar	ModRM:reg (w)	ModRM:r/m (r)	N/A	N/A

Description

Compares the single precision floating-point values in the low doublewords of operand 1 (first operand) and operand 2 (second operand), and sets the ZF, PF, and CF flags in the EFLAGS register according to the result (unor-dered, greater than, less than, or equal). The OF, SF, and AF flags in the EFLAGS register are set to 0. The unor-dered result is returned if either source operand is a NaN (QNaN or SNaN).

Operand 1 is an XMM register; operand 2 can be an XMM register or a 32 bit memory location.

The UCOMISS instruction differs from the COMISS instruction in that it signals a SIMD floating-point invalid opera-tion exception (#I) only if a source operand is an SNaN. The COMISS instruction signals an invalid operation excep-tion when a source operand is either a QNaN or SNaN.

The EFLAGS register is not updated if an unmasked SIMD floating-point exception is generated.

Note: VEX.vvvv and EVEX.vvvv are reserved and must be 1111b, otherwise instructions will #UD.

Software should ensure VCOMISS is encoded with VEX.L=0. Encoding VCOMISS with VEX.L=1 may encounter unpredictable behavior across different processor generations.

Operation

(V)UCOMISS (All Versions)

RESULT := UnorderedCompare(DEST[31:0] <> SRC[31:0]) {
(* Set EFLAGS *) CASE (RESULT) OF
    UNORDERED: ZF,PF,CF := 111;
    GREATER_THAN: ZF,PF,CF := 000;
    LESS_THAN: ZF,PF,CF := 001;
    EQUAL: ZF,PF,CF := 100;
ESAC;
OF, AF, SF := 0; }

Intel C/C++ Compiler Intrinsic Equivalent

VUCOMISS

int _mm_comi_round_ss(__m128 a, __m128 b, int imm, int sae);

UCOMISS

int _mm_ucomieq_ss(__m128 a, __m128 b);

UCOMISS

int _mm_ucomilt_ss(__m128 a, __m128 b);

UCOMISS

int _mm_ucomile_ss(__m128 a, __m128 b);

UCOMISS

int _mm_ucomigt_ss(__m128 a, __m128 b);

UCOMISS

int _mm_ucomige_ss(__m128 a, __m128 b);

UCOMISS

int _mm_ucomineq_ss(__m128 a, __m128 b);

SIMD Floating-Point Exceptions

Invalid (if SNaN Operands), Denormal.

Other Exceptions

VEX-encoded instructions, see Table 2-20, “Type 3 Class Exception Conditions,” additionally:

If VEX.vvvv != 1111B.

EVEX-encoded instructions, see Table 2-48, “Type E3NF Class Exception Conditions.”