UCOMISD

UCOMISD is the quiet version of COMISD: it compares the low doubles of two operands and sets ZF, PF and CF the same way, all three for an unordered result, but raises the invalid-operation exception only when an input is a signaling NaN, not a quiet one. COMISD’s page explains how the flags map to condition codes, why compilers swap the operands for “less than”, and the one case, equality, where Clang bypasses both instructions for a mask built with CMPEQSD or CMPNEQSD.

On Zen 5, in a masked case with DAZ set, a quiet NaN compared against a signaling NaN, read from memory, set ZF, PF and CF (unordered) and IE, since one input was signaling; in a masked, clean case, comparing a non-NaN value against itself gave “equal” with PF clear, ordered as any repeated value must be, unlike the self-compare of a NaN that isnan() relies on.

What compiled code shows most#

GCC 14 and Clang 23 at -O2 use UCOMISD for isless(), isgreater() and the other quiet macros, and for isunordered(), which both compile to ucomisd xmm0, xmm1 and SETP. isnan(x) is ucomisd xmm0, xmm0 and SETP in both, since only a NaN is unordered with itself. GCC also uses UCOMISD for == and !=, testing ZF and PF after it as COMISD’s page describes; Clang instead builds a mask with CMPEQSD or CMPNEQSD for those two, unless -ffp-exception-behavior=strict is given, when it falls back to UCOMISD like GCC.

UCOMISD writes no XMM register, and its memory operand is eight bytes at any alignment. VUCOMISD is the VEX and EVEX form (the EVEX one can suppress all exceptions with {sae}), and UCOMISS is the version for singles.

Forms

Data flow

Where the elements of the sources go, element 0 on the right; results that depend on a control value are drawn for an example. How to read.

Operation

What the forms do, in pseudocode that our tests run against results recorded on real processors. How to read.

x = ucomi(xmm1.f64[0], src2.f64[0])  # 0 >, 1 <, 2 =, 3 unordered
zf = x in (2, 3)
pf = x == 3
cf = x in (1, 3)
of = sf = af = 0
# only an SNaN raises IE

Operands

Flags

Exceptions

The exception class is XED's name for the set of exception conditions that a form follows. How to read.

All forms

SSE_TYPE_3

Attributes

What else XED records about the forms. How to read.

All forms

follows MXCSR

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to UCOMISD, each with the form it compiles to and the Clang options it needs. The forms come from compiling every intrinsic and decoding the result with Intel XED; all intrinsics A–Z explains the method.

CPUID and processors

Each form belongs to an XED ISA set. A form can be used when the processor reports every CPUID bit of one of its ISA set's alternatives.

More SSE2 instructions

• POR
• PSADBW
• PSHUFD
• PSHUFHW
• PSHUFLW
• PSLLD
• PSLLDQ
• PSLLQ
• PSLLW
• PSRAD
• PSRAW
• PSRLD
• PSRLDQ
• PSRLQ
• PSRLW
• PSUBB
• PSUBD
• PSUBQ
• PSUBSB
• PSUBSW
• PSUBUSB
• PSUBUSW
• PSUBW
• PUNPCKHBW
• PUNPCKHDQ
• PUNPCKHQDQ
• PUNPCKHWD
• PUNPCKLBW
• PUNPCKLDQ
• PUNPCKLQDQ
• PUNPCKLWD
• PXOR
• SHUFPD
• SQRTPD
• SQRTSD
• SUBPD
• SUBSD
• UNPCKHPD
• UNPCKLPD
• XORPD

All 124 SSE2 instructions