UCOMISS

UCOMISS is the quiet version of COMISS: it compares the low singles 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. A quiet NaN, which is what arithmetic produces from an invalid operation, passes through without a trace. It is the compare that compiled code shows most:

• Clang 23 at -O2 uses it for every float comparison in C, < and > included, and GCC 14 for == and !=, for isless(), isgreater() and the other quiet macros, and for isunordered().
• isnan(x) becomes ucomiss xmm0, xmm0 and SETP in both compilers, since only a NaN is unordered with itself.
• The six _mm_ucomi*_ss intrinsics compile to it, with the parity test that keeps an equality false for a NaN.

COMISS explains how the flags map to condition codes and why compilers swap the operands for “less than”.

What the recorded cases show#

On Zen 5, a quiet NaN compared with another quiet NaN set ZF, PF and CF and left MXCSR unchanged, while a signaling NaN set MXCSR.IE in a VUCOMISS case. A denormal input sets the denormal flag, DE, even though nothing is computed from it: comparing the smallest normal single with a negative denormal gave “greater” and set DE. With MXCSR.DAZ set, denormal inputs count as zeros of the same sign instead.

UCOMISS writes no XMM register, and its memory operand is four bytes at any alignment. VUCOMISS is the VEX and EVEX form (the EVEX one can suppress all exceptions with {sae}), UCOMISD the version for doubles, and AVX10.2’s VUCOMXSS the quiet partner of VCOMXSS.

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.f32[0], src2.f32[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 UCOMISS, 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 SSE instructions

• MOVLHPS
• MOVLPS
• MOVMSKPS
• MOVNTPS
• MOVNTQ
• MOVSS
• MOVUPS
• MULPS
• MULSS
• ORPS
• PAVGB
• PAVGW
• PEXTRW
• PINSRW
• PMAXSW
• PMAXUB
• PMINSW
• PMINUB
• PMOVMSKB
• PMULHUW
• PREFETCHNTA
• PREFETCHT0
• PREFETCHT1
• PREFETCHT2
• PSADBW
• PSHUFW
• RCPPS
• RCPSS
• RSQRTPS
• RSQRTSS
• SFENCE
• SHUFPS
• SQRTPS
• SQRTSS
• STMXCSR
• SUBPS
• SUBSS
• UNPCKHPS
• UNPCKLPS
• XORPS

All 65 SSE instructions