x86-64.net

Instruction

VFMSUB231SS

Fused multiply-subtract of the low singles: second times third minus first, into the first; the rest is unchanged.

Extensions
Forms
4 (2 VEX, 2 EVEX)
Data
Intel XED v2026.08.23

VFMSUB231SS computes op2 × op3 - op1 for the low single, rounded once: the product minus the destination’s old value, s = x × y - s. Singles 1 to 3 keep the destination’s values and bits 511:128 are cleared. VFMSUB231PS covers the subtracting family and the error of a product, and VFMADD231SS what the scalar forms change: the upper singles, masking of the low single and a rounding mode on xmm registers. VFMSUB231SD is the double form.

A chain through the subtrahend

In our builds at -O2 with -march=x86-64-v3 or -v4, the loop s = x[i] * y[i] - s over floats becomes VFMSUB231SS in Clang 23, two steps and then VFMSUB132SS steps that move the value between two registers. GCC 14 keeps VMULSS and VSUBSS there, also with -march=znver5, and emits VFMSUB231SS, y[i] from memory, only with --param=avoid-fma-max-bits=0 or -mtune=icelake-server: in its default tuning it leaves a chain through the addend unfused. _mm_mask3_fmsub_ss(a, b, c, k), which keeps c where bit 0 of k is clear, is vfmsub231ss xmm2{k1}, xmm0, xmm1 in both compilers.

Recorded on Zen 5

  • 10^30 × 10^30 - (-10^30) overflows: vfmsub231ss xmm12, xmm6, xmm6 under MXCSR 3FC0H (round down) and vfmsub231ss xmm14{k2}{z}, xmm8, dword ptr [mem] under FFC0H (toward zero) both returned the largest finite single, 7F7FFFFFH, and set OE and PE.
  • vfmsub231ss xmm7{k3}, xmm12, xmm7, {rd-sae} computed 2^-149 × 1 - 1, just above -1, as -1, rounding down, without the fault that precision, unmasked in 6600H, would otherwise raise.

AMD’s latency spreadsheet for Zen 5 gives the VEX and the EVEX rows alike a latency of 4 cycles, two per cycle on pipes FP0 and FP1.

Forms

Every encoding of VFMSUB231SS that Intel XED knows, one row per XED instruction form (iform). How to read the notation.
InstructionEncodingRequires
VFMSUB231SS xmm, xmm, m32 VFMSUB231SS_XMMdq_XMMd_MEMdVEX.LIG.66.0F38.W0 BB /rFMA + AVX
VFMSUB231SS xmm, xmm, xmm VFMSUB231SS_XMMdq_XMMd_XMMdVEX.LIG.66.0F38.W0 BB /rFMA + AVX
VFMSUB231SS xmm {k}{z}, xmm, xmm, {er} VFMSUB231SS_XMMf32_MASKmskw_XMMf32_XMMf32_AVX512EVEX.LIG.66.0F38.W0 BB /rAVX10.1 or AVX512F
VFMSUB231SS xmm {k}{z}, xmm, m32 VFMSUB231SS_XMMf32_MASKmskw_XMMf32_MEMf32_AVX512EVEX.LIG.66.0F38.W0 BB /rAVX10.1 or AVX512F

Latency and throughput

What the scheduling models of LLVM 23.1.2 give for each form, not measurements: latency / reciprocal throughput in cycles, the ports under them. A row is a model, or models that agree; a dash, a form that its processors lack. Memory forms: latency from the address, ports without the load and store units. Which processors each model covers.
Modelxmm, xmm, m32
xmm {k}{z}, xmm, m32		xmm, xmm, xmm
xmm {k}{z}, xmm, xmm, {er}no mask, with a mask, with {er}		xmm {k}{z}, xmm, xmm, {er}zeroing
Haswell, Broadwell10 / 0.5p015 / 0.5p01
Skylake9 / 0.5p014 / 0.5p01
Skylake-SP, Ice Lake9 / 0.5p014 / 0.5p014 / 0.5p01
Alder Lake11 / 0.5p014 / 0.5p01
Sapphire Rapids11 / 0.5p014 / 0.5p014 / 0.5p01
Lunar Lake10 / 2V0 V14 / 2V0 V1
Piledriver10 / 1.5FPFMA FPU0 FPU15 / 1.5FPFMA FPU0 FPU1 FPU2 FPU3
Zen, Zen 212 / 0.5FPU0 FPU15 / 0.5FPU0 FPU1
Zen 311 / 0.5FP0 FP14 / 0.5FP0 FP1
Zen 411 / 1FP0 FP14 / 11FP0 FP12 / 12FP0 FP1
  1. Zen 4 model: AMD's spreadsheets give reciprocal throughput 0.5 for Zen 4 and Zen 5 (AMD 57647, row 1296; AMD 58455, row 1371).
  2. Zen 4 model: AMD's spreadsheets give latency 4 for Zen 4 and Zen 5 (AMD 57647, row 1297; AMD 58455, row 652).

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.

Scalar: only the low single is computed; the other singles come from xmm1.
VFMSUB231SS: the low elementResult single 0 is computed from single 0 of xmm1, xmm2 and xmm3; singles 3 to 1 are those of xmm1. Result singles 3 to 0: a3, a2, a1, r0.0123xmm1a1k1r0a1a2a3xmm10123xmm2b0123xmm3or m32c

The write mask k1 applies to the low single: with bit 0 clear it keeps its old value, or is zeroed with {z}.

Result single 0 is computed from single 0 of xmm1, xmm2 and xmm3; singles 3 to 1 are those of xmm1. Result singles 3 to 0: a3, a2, a1, r0.

Operation

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

if k1[0]:  # 1 without a write mask
    a = xmm1.f32[0]
    b = xmm2.f32[0]
    c = src3.f32[0]
    xmm1.f32[0] = fmsub(b, c, a)
elif zeroing:
    xmm1.f32[0] = 0
# else: xmm1.f32[0] keeps its value
# xmm1.d[1] to [3] keep their values
# {er}: the form's rounding, not MXCSR.RC, and no flags
# xmm1 is zeroed from bit 128 up

Run on AMD Zen 5 (EPYC 9645): 6 cases.

Operands

What each form reads and writes, as Intel XED records it: the operands of its syntax, in order, and the implicit ones that the syntax does not name. How to read.
OperandKindAction
VFMSUB231SS xmm, xmm, xmm/m32
xmmexplicitread, write
xmmexplicitread
xmm/m32explicitread
VFMSUB231SS xmm {k}{z}, xmm, xmm/m32, {er}
xmmexplicitread, write (masked)
{k}explicitread (write mask)
xmmexplicitread
xmm/m32explicitread

Flags

None: XED records no flag that the forms test or change.

Exceptions

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

VEX forms
avx-type-3
EVEX forms
AVX512-E3

Attributes

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

All forms
follows MXCSR
EVEX forms
write masking
EVEX memory forms
memory fault suppression

Intrinsics

The C intrinsics of Clang 23.1.2 that compile to VFMSUB231SS, 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.

IntrinsicCompiles to
_mm_mask3_fmsub_round_ss __m128 _mm_mask3_fmsub_round_ss(__m128 W, __m128 X, __m128 Y, __mmask8 U, const int R)

VFMSUB231SS xmm {k}, xmm, xmm, {er} EVEX.LIG -mavx512f

_mm_mask3_fmsub_ss __m128 _mm_mask3_fmsub_ss(__m128 __W, __m128 __X, __m128 __Y, __mmask8 __U)

VFMSUB231SS xmm {k}, xmm, xmm EVEX.LIG -mavx512f

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.

CPUID requirements and processors of the XED ISA sets above. Alternatives are separated by “or”; processors follow XED’s chip model, whose AMD processors end with Zen 2, and the CPU data of this site for Zen 3 to Zen 6.
ISA setCPUIDProcessors
FMA

FMA + AVX
CPUID.01H:ECX[12] FMA
CPUID.01H:ECX[28] AVX

XED and Intel’s SDM differ: XED also asks for the AVX bit, CPUID.01H:ECX[28], besides FMA, CPUID.01H:ECX[12]. The opcode tables of revision 093 of Intel’s SDM name FMA alone for every VEX form, but its Volume 1 tells software to check for AVX first and then for FMA, so in effect the two agree.

Sources: Intel XED v2026.08.23, datafiles/hswavx/cpuid.xed.txt; Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2 (325383-093, September 2026), VFMADD132PD/VFMADD213PD/VFMADD231PD and the other FMA instructions: the opcode tables, from page 5-207; Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 1 (253665-093, September 2026), section 14.5.3, Detection of FMA, page 14-24.

Haswell, Broadwell, Skylake, Comet Lake, Cannon Lake, Ice Lake (client), Tiger Lake, Alder Lake, Arrow Lake, Panther Lake, Nova Lake, Skylake-SP, Cascade Lake, Cooper Lake, Ice Lake (server), Sapphire Rapids, Emerald Rapids, Granite Rapids, Diamond Rapids, Sierra Forest, Clearwater Forest, Knights Landing, Knights Mill, Piledriver, Zen, Zen+, Zen 2, Zen 3, Zen 4, Zen 5, Zen 6
AVX512F_SCALAR

AVX10.1
CPUID.(EAX=07H,ECX=1):EDX[19] AVX10
CPUID.(EAX=24H,ECX=0):EBX[7:0] ≥ 1 AVX10 version

or

AVX512F
CPUID.(EAX=07H,ECX=0):EBX[16] AVX512F

Cannon Lake, Ice Lake (client), Tiger Lake, Nova Lake, Skylake-SP, Cascade Lake, Cooper Lake, Ice Lake (server), Sapphire Rapids, Emerald Rapids, Granite Rapids, Diamond Rapids, Knights Landing, Knights Mill, Zen 4, Zen 5, Zen 6

More FMA instructions

All 60 FMA instructions

More AVX10.1 instructions

All 686 AVX10.1 instructions

Sources

  1. Intel XED v2026.08.23 (commit 0bcb6237345c): forms, encodings, operands, flags, exception classes, attributes, ISA sets, CPUID bits and chip model.
  2. The CPU data of x86-64.net: Zen 3 to Zen 6, from GCC’s -march feature sets and AMD’s Processor Programming References, which the page of each processor cites.
  3. Clang's x86 intrinsic headers, LLVM 23.1.2 (commit 85ac56026243): the intrinsics, their signatures and doc comments, and the Clang of that release, which compiled each of them.
  4. LLVM 23.1.2's x86 scheduling models (commit 85ac56026243): the latency, reciprocal throughput and ports of each form, as llvm-mca reports them.
  5. Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 2 (325383-093, September 2026): Volume 2C, page 5-282 (VFMSUB132SS/VFMSUB213SS/VFMSUB231SS)
  6. Intel Advanced Vector Extensions 10.2 Architecture Specification, revision 7.0 (361050-007, April 2026)
  7. Software Optimization Guide for the AMD Zen5 Microarchitecture (58455, rev. 1.00, August 2024), Wayback Machine copy of AMD's ZIP
  8. Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 1 (253665-093, September 2026)

The tables are derived from Intel XED, Copyright Intel Corporation, licensed under the Apache License 2.0; x86-64.net converted and reformatted them. The intrinsics and their signatures come from Clang and the timings from LLVM's scheduling models, both part of the LLVM Project, licensed under the Apache License 2.0 with LLVM Exceptions. The text is our own.