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FMA and FMA3

Architecture referenceFeex

Also called FMA, FMA3

FMA computes a floating-point multiply and add with one final rounding. FMA3 refers to the three-operand x86 fused multiply-add instruction family; it is separately enumerated from AVX2.

What you need to know

FMA computes a floating-point multiply and add with one final rounding. FMA3 refers to the three-operand x86 fused multiply-add instruction family; it is separately enumerated from AVX2.

How it works

A fused operation evaluates a product and sum as one arithmetic operation before rounding to the destination format. This can preserve a contribution that would be lost if the multiplication were rounded first. The 132, 213 and 231 instruction forms select which operands provide the product, addend and overwritten destination. Scalar and packed forms have different lane behavior.

Where it is useful

Polynomial evaluation, matrix multiplication and digital filters can use FMA to combine arithmetic operations. The fused result can differ from a program that deliberately uses two separate operations. Compiler contraction settings and numerical reproducibility requirements therefore matter as well as instruction availability.

How to check support and enablement

For the VEX FMA3 instructions, check CPUID.01H:ECX bit 12 together with the complete AVX OS-state checks. If using EVEX forms, check the additional AVX-512 subsets and state required by those forms. Inspect compiler flags when numerical results change after a rebuild.

The program must both contain an appropriate implementation and select it at runtime. Hardware enumeration, OS-enabled register state and the binary's compiled baseline are separate checks. A virtual machine can deliberately expose fewer features than its host, and a container does not add instructions to the host CPU.

What the feature does not establish

FMA does not mean every floating-point expression is evaluated with extra precision, and it does not guarantee identical results across compiler settings. FMA4 is a different instruction family. Neither AVX nor AVX2 alone proves support for FMA3.

Compatibility and performance in practice

Measure the complete workload with representative data. Wider vectors can reduce instruction count while leaving memory traffic, synchronization or conversion costs unchanged. Instruction latency, throughput and power behavior depend on the exact microarchitecture and operating conditions.

A practical verification checklist

  1. Record the exact processor or platform model and the application, OS and runtime versions involved. Preserve the wording of the actual error or requirement.
  2. Check the manufacturer reference for that exact model, then inspect the effective capability and enabled state in the environment where the program runs.
  3. Compare every requirement of the selected binary or backend. A similarly named feature, alias or broad technology label is not enough.
  4. Confirm the application selected the intended implementation. Use its documented diagnostics and a supported fallback when requirements are not met.
  5. Make one supported change at a time and repeat the same workload. Keep the original configuration and recovery path available.

If the feature is listed but software cannot use it

Start with the exact failing component. A hardware database describes recorded capability; the operating system or hypervisor can expose a different effective environment. An illegal-instruction error also needs investigation of the actual faulting instruction and binary build, because corruption or an unrelated unsupported instruction can produce a similar symptom.

Install the application, driver or runtime version supported for the exact platform, and use the vendor's documented baseline or fallback build if needed. A software update cannot create an instruction that the processor lacks. Firmware and security-policy changes should follow the system manufacturer's procedure and a recovery plan rather than a generic optimization recipe.

Sources and scope of this explanation

Intel 64 and IA-32 Architectures Software Developer's Manual · Linux kernel: using XSTATE features in user space

These references establish architectural behavior and software requirements. They do not independently verify every feature value on a linked hardware model page. For model support, use that page's manufacturer evidence and check the effective system configuration. This article does not supply a benchmark, certification or independent review date.

Sources and verification

Editorial ownership: Feex. No independent review date is recorded. How we research.

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