VFMSUBSS instruction (FMA4)

Forms

Every encoding of VFMSUBSS that Intel XED knows, one row per XED instruction form (iform). How to read the notation.

Instruction	Encoding	Requires
`VFMSUBSS xmm, xmm, m32, xmm` VFMSUBSS_XMMdq_XMMd_MEMd_XMMd	`VEX.LIG.66.0F3A.W0 6E /r /is4`	FMA4
`VFMSUBSS xmm, xmm, xmm, xmm` VFMSUBSS_XMMdq_XMMd_XMMd_XMMd	`VEX.LIG.66.0F3A.W0 6E /r /is4` `VEX.LIG.66.0F3A.W1 6E /r /is4`	FMA4
`VFMSUBSS xmm, xmm, xmm, m32` VFMSUBSS_XMMdq_XMMd_XMMd_MEMd	`VEX.LIG.66.0F3A.W1 6E /r /is4`	FMA4

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 are zeroed.

Result single 0 is computed from single 0 of xmm2, xmm3 and xmm4; singles 3 to 1 are 0. Result singles 3 to 0: 0, 0, 0, r0.

Operation

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

features = cpuid_leaf(0x80000001, 0)[2]
if not features >> 16 & 1:
    fault(UD)                    # CPUID: no FMA4
a = xmm2.f32[0]
b = src3.f32[0]
c = src4.f32[0]
xmm1.f32[0] = fmsub(a, b, c)     # a * b - c, rounded once
for i in range(1, 4):
    xmm1.d[i] = 0
# bits 127:32 cleared, as AMD's manual says
# xmm1 is zeroed from bit 128 up

Run on AMD Zen 5 (EPYC 9645): 3 cases. Its cases ran where CPUID reported no FMA4, and every one raised an exception.

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.

Operand	Kind	Action
`VFMSUBSS xmm, xmm, xmm/m32, xmm`
`xmm`	explicit	write
`xmm`	explicit	read
`xmm/m32`	explicit	read
`xmm`	explicit	read
`VFMSUBSS xmm, xmm, xmm, m32`
`xmm`	explicit	write
`xmm`	explicit	read
`xmm`	explicit	read
`m32`	explicit	read

Flags

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

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 VFMSUBSS, 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.

Intrinsic	Compiles to
`_mm_msub_ss` `__m128 _mm_msub_ss(__m128 __A, __m128 __B, __m128 __C)`	`VFMSUBSS xmm, xmm, xmm, xmm` VEX.LIG -mfma4

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 set	CPUID	Processors
`FMA4` FMA4	FMA4 `CPUID.80000001H:ECX[16]` FMA4	Bulldozer, Piledriver

VFMSUBSS