Add mul_sub() fused multiply-add operation #492
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Implements fused multiply-add (FMA) operations for SimdFloat, the rust-lang#1 most critical missing feature in portable-simd based on analysis of PyTorch's SIMD implementation. Methods: - mul_add(a, b) - computes (self * a) + b with single rounding - mul_sub(a, b) - computes (self * a) - b with single rounding Benefits: - Improved accuracy: single rounding error vs two separate roundings - Better performance: 2 operations in 1 instruction on modern CPUs - Universal hardware support: FMA3 (x86), NEON vfma (ARM), RISC-V F extension Implementation: - Delegates to core::intrinsics::simd::simd_fma LLVM intrinsic - Zero-cost abstraction with #[inline] - mul_sub implemented as mul_add(a, -b) Testing (14 tests): - 3 accuracy tests proving FMA superiority: * Catastrophic cancellation: (1+ε)(1-ε) - 1 * Discriminant calculation: b² - 4ac (quadratic formula) * Polynomial evaluation with Horner's method - Basic operations (f32x4, f64x4, mul_add, mul_sub) - Special values (infinity, NaN, MAX, MIN, subnormals) - Size variations (f32x2, f32x8) - Negative values Example demonstrates: - Basic FMA usage - Polynomial evaluation (Horner's method) - Dot product accumulation - Accuracy comparison Use cases: - Neural networks (dot products, matrix multiply) - Scientific computing (polynomial evaluation, numerical stability) - Graphics (lighting calculations, transformations) - Physics simulations (force calculations, integration) 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>
ARM NEON uses flush-to-zero (FTZ) for subnormal values in SIMD operations. Updated test to accept either the correct subnormal result or zero.
GrigoryEvko
changed the title
StdFloat already provides mul_add. This PR now only adds mul_sub.
Author
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Maybe move mul_add from std_float::StdFloat to core_simd/src/simd/num/float.rs as I accidentaly tried? |
Both mul_add and mul_sub now live in StdFloat for consistency.
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This PR adds and
mul_sub()methods to theSimdFloattrait. This operation compute(self * a) - brespectively, using a single fused multiply-add instruction where available.