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FMA and FMA3

Architecture referenceFeex

Dem still dey call am FMA, FMA3

FMA dey calculate floating-point multiply and add with only one final rounding. FMA3 dey talk about di three-operand x86 fused multiply-add instruction family; dem dey list am separate from AVX2.

Wetin you need know

FMA dey calculate floating-point multiply and add with only one final rounding. FMA3 dey talk about di three-operand x86 fused multiply-add instruction family; dem dey list am separate from AVX2.

How e dey work

Fused operation dey calculate product and sum as one single math operation before e round go di destination format. Dis fit save small value wey for loss if dem round di multiplication first. Di 132, 213 and 231 instruction form dem dey choose which operand go give di product, di addend, and di destination wey dem go overwrite. Scalar and packed form dem dey behave differently across lane dem.

Where e dey useful

Polynomial calculation, matrix multiplication and digital filter dem fit use FMA take join arithmetic operation dem. Di fused result fit differ from program wey deliberately use two separate operation dem. So compiler contraction setting dem and numerical reproducibility requirement dem matter just as instruction availability dey matter.

How to check support and enablement

For di VEX FMA3 instruction dem, check CPUID.01H:ECX bit 12 together with di complete AVX OS-state check dem. If you dey use EVEX form dem, check di extra AVX-512 subset dem and state wey dose form dem need. Inspect compiler flag dem wen calculation result change after rebuild.

Di program must contain correct implementation and select am at runtime. Hardware enumeration, OS-enabled register state and di binary compiled baseline na separate check dem. Virtual machine fit deliberately show fewer feature dem pass e host, and container no dey add instruction to di host CPU.

Wetin di feature no dey guarantee

FMA no mean say every floating-point calculation go get extra precision, and e no dey guarantee same result across different compiler setting dem. FMA4 na different instruction family. Neither AVX nor AVX2 alone fit prove say FMA3 dey supported.

Compatibility and performance inside real life

Measure di full workload with representative data. Wider vector dem fit reduce instruction count while memory traffic, synchronization or conversion cost dem remain di same. Instruction latency, throughput and power behavior depend on di exact microarchitecture and operating condition dem.

Practical verification checklist

  1. Record di exact processor or platform model plus di application, OS and runtime version dem wey dey involve. Make you keep di exact word dem wey di error or requirement use.
  2. Check di manufacturer reference for dat exact model, den inspect di real capability and enabled state inside di environment where di program dey run.
  3. Compare every single requirement of di binary or backend wey you choose. Name wey resemble am, alias, or broad technology label no reach.
  4. Confirm say di application select di implementation wey dem intend. Use di diagnostics wey dem document and supported fallback if requirement dem no complete.
  5. Make only one supported change at a time, den run di same workload again. Keep di original configuration and recovery way ready.

If dem list di feature but software no fit use am

Start from di exact component wey dey fail. Hardware database dey describe capability wey dem record; di operating system or hypervisor fit show different real environment. If illegal-instruction error happen, you still need to check di exact instruction wey cause di fault and di binary build, because corrupt file or anoda instruction wey no dey supported fit give di same kind problem.

Install di application, driver or runtime version wey get support for dat exact platform, and use di vendor documented baseline or fallback build if e need am. Software update no fit create instruction wey di processor no get. Firmware and security-policy change dem suppose follow di system manufacturer procedure and recovery plan, no be just any common optimization advice.

Sources and scope of dis explanation

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

Dis reference dem establish architectural behavior and software requirement dem. Dem no dey independently verify every feature value for any linked hardware model page. For model support, use dat page manufacturer evidence and check di active system configuration. Dis article no supply benchmark, certification or independent review date.

Sources and verification

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

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