Trailing-Ones Anticipation for Reducing the Latency of the Rounding Incrementer in FP FMA Units
Toru Koizumi, Ryota Shioya, Takuya Yamauchi, Tomoya Adachi, Ken Namura, Junichiro Makino · 2025
Floating-point fused multiply-add (FMA) operations are fundamental in many fields such as scientific computing, graphics processing, and machine learning. In conventional floating-point FMA designs, the internal steps of multiplication and addition, which are performed using integer arithmetic, are followed by a post-processing stage. It has been known that the post-processing stage contributes to approximately 60% of the total latency of the double-precision FMA operation. We propose a novel trailing-ones anticipation technique that predicts trailing-ones bits of the mantissa before rounding, in parallel with the post-processing stage. With this technique, the rounding incrementer can be implemented using a single XOR operation, thereby reducing the total latency. We evaluated the latency using Synopsys Design Compiler for synthesis, confirming that the proposed technique reduced total latency by 4 %.