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  1. JDK
  2. JDK-8307446

RISC-V: Improve performance of floating point to integer conversion

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    • Enhancement
    • Resolution: Fixed
    • P4
    • 21
    • 21
    • hotspot
    • None
    • 21
    • b22
    • riscv
    • linux

    Backports

      Description

        Currently, risc-v port converts floating point to integer using `FCVT_SAFE` in macroAssembler_riscv.cpp.

        The main issue here is Java spec returns 0 when the floating point number is NaN [1].
        But for RISC-V ISA, instructions converting a floating-point value to an integer value (`FCVT.W.S`/`FCVT.L.S`/`FCVT.W.D`/`FCVT.L.D`) return the largest/smallest value when the floating point number is NaN [2].
        That requires additional logic to handle the case when the src of conversion is NaN, as the following code did:

        ```
        #define FCVT_SAFE(FLOATCVT, FLOATEQ) \
        void MacroAssembler:: FLOATCVT##_safe(Register dst, FloatRegister src, Register tmp) { \
          Label L_Okay; \
          fscsr(zr); \
          FLOATCVT(dst, src); \
          frcsr(tmp); \
          andi(tmp, tmp, 0x1E); \
          beqz(tmp, L_Okay); \
          FLOATEQ(tmp, src, src); \
          bnez(tmp, L_Okay); \
          mv(dst, zr); \
          bind(L_Okay); \
        }

        FCVT_SAFE(fcvt_w_s, feq_s)
        FCVT_SAFE(fcvt_l_s, feq_s)
        FCVT_SAFE(fcvt_w_d, feq_d)
        FCVT_SAFE(fcvt_l_d, feq_d)
        ```

        We can improve the logic of NaN checking with the `fclass` instruction just as JDK-8297359 did.

        1. https://docs.oracle.com/javase/specs/jls/se20/html/jls-5.html#jls-5.1.3
        2. https://github.com/riscv/riscv-isa-manual/blob/63aeaada9b2fee7ca15e5c6b6a28f3b710fb7e58/src/f-st-ext.adoc?plain=1#L365-L386

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                fjiang Feilong Jiang
                fjiang Feilong Jiang
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                  Created:
                  Updated:
                  Resolved: