ucomisd

Unordered Compare Scalar Double-Precision

UCOMISD xmm1, xmm2/m64

Compares low double and sets EFLAGS.

Details

Performs an unordered comparison of two double-precision floating-point values and sets EFLAGS (ZF, PF, CF) based on the result. If either operand is a signaling NaN, the comparison is unordered; quiet NaNs also produce an unordered result. EFLAGS reflect unordered (ZF=PF=CF=1), less, greater, or equal conditions.

Pseudocode Operation

cmp_result ← unordered_compare(dest.low64_double, src.low64_double); ZF ← (cmp_result == equal); PF ← (cmp_result == unordered); CF ← (cmp_result == less || cmp_result == unordered); AF ← 0; OF ← 0; SF ← 0;

Example

UCOMISD xmm1, xmm2/m64

Encoding

Binary Layout

Format SSE2

Opcode 66 0F 2E

Extension SSE2

Operands

dest
128-bit XMM SIMD register
src
128-bit XMM SIMD register or Memory operand

Reference (Intel® SDM)

Instruction Forms

Opcode	Instruction	Op/En	64/32-bit Mode	CPUID	Description
66 0F 2E /r	UCOMISD xmm1, xmm2/m64	A	V/V	SSE2	Compare low double precision floating-point values in xmm1 and xmm2/mem64 and set the EFLAGS flags accordingly.
VEX.LIG.66.0F.WIG 2E /r	VUCOMISD xmm1, xmm2/m64	A	V/V	AVX	Compare low double precision floating-point values in xmm1 and xmm2/mem64 and set the EFLAGS flags accordingly.
EVEX.LLIG.66.0F.W1 2E /r	VUCOMISD xmm1, xmm2/m64{sae}	B	V/V	AVX512F OR AVX10.1	Compare low double precision floating-point values in xmm1 and xmm2/m64 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

Performs an unordered compare of the double precision floating-point values in the low quadwords 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 (unordered, greater than, less than, or equal). The OF, SF, and AF flags in the EFLAGS register are set to 0. The unordered 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 64 bit memory location. The UCOMISD instruction differs from the COMISD instruction in that it signals a SIMD floating-point invalid operation exception (#I) only when a source operand is an SNaN. The COMISD instruction signals an invalid operation exception only if a source operand is either an SNaN or a QNaN. 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 VCOMISD is encoded with VEX.L=0. Encoding VCOMISD with VEX.L=1 may encounter unpredictable behavior across different processor generations.

Operation

(V)UCOMISD (All Versions)
RESULT := UnorderedCompare(DEST[63:0] <> SRC[63:0]) {
(* 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; }





UCOMISD—Unordered Compare Scalar Double Precision Floating-Point Values and Set EFLAGS                                               Vol. 2B 4-732

Intel C/C++ Compiler Intrinsic Equivalent

VUCOMISD int _mm_comi_round_sd(__m128d a, __m128d b, int imm, int sae);
UCOMISD int _mm_ucomieq_sd(__m128d a, __m128d b)
UCOMISD int _mm_ucomilt_sd(__m128d a, __m128d b)
UCOMISD int _mm_ucomile_sd(__m128d a, __m128d b)
UCOMISD int _mm_ucomigt_sd(__m128d a, __m128d b)
UCOMISD int _mm_ucomige_sd(__m128d a, __m128d b)
UCOMISD int _mm_ucomineq_sd(__m128d a, __m128d b)

Exceptions

SIMD Floating-Point Exceptions

Invalid (if SNaN operands), Denormal.

Other Exceptions

VEX-encoded instructions, see Table 2-20, “Type 3 Class Exception Conditions,” additionally: #UD If VEX.vvvv != 1111B. EVEX-encoded instructions, see Table 2-50, “Type E3NF Class Exception Conditions.” UCOMISD—Unordered Compare Scalar Double Precision Floating-Point Values and Set EFLAGS Vol. 2B 4-733