What is the main difference from the ordinary form?
The M form returns canonical NaN if either input is NaN; the ordinary form returns the other non-NaN operand when exactly one input is NaN.
FMAXM.D
RISC-V FMAXM.D Instruction Details
Instruction ManualR-typeReturn the maximum of two double-precision floating-point operands.
Instruction Syntax
fmaxm.d rd, rs1, rs2
Operand Breakdown
rd: destination floating-point register receiving the formatted result.
rs1/rs2: source floating-point registers; arithmetic/rounding forms with rm use rm or dynamic frm for rounding mode.
ZfaDFloating-Point Arithmetic
Instruction Behavior
FMAXM.D is the Zfa double-precision IEEE 754-2019 maximum operation: if either input is NaN, the result is canonical NaN; otherwise it returns the numerically larger operand and handles -0.0 versus +0.0 as specified.
Quick Understanding & Search Notes
FMAXM.D's key search point is NaN handling. The M form is the Zfa version and returns canonical NaN if either input is NaN.
-0.0 and +0.0 are ordered: -0.0 is less than +0.0.
The instruction does not use rm; it is a selection operation, not rounded arithmetic.
Common Usage Scenarios
Floating-Point Compare
Understand this scenario with real code like «fmaxm.d f10, f11, f12 ; IEEE 754-2019 max (double)».
Numerical Computing
Understand this scenario with real code like «fmaxm.d f10, f11, f12 ; IEEE 754-2019 max (double)».
Pre-Use Checklist
Syntax Check
- Confirm the current instruction format is R-type.
- Confirm the operand order matches the example.
Semantic Check
- Ensure the destination register usage is compatible with the calling convention.
- Confirm this is not the lower-level form of a pseudo-instruction expansion.
Pitfalls / Common Confusions
Unlike ordinary FMIN/FMAX, FMINM/FMAXM returns canonical NaN if either input is NaN.
A signaling NaN sets NV; quiet-NaN handling differs between ordinary and M forms.
-0.0 is considered less than +0.0.
FAQ
Does FMAXM.D round?
No. It selects between two operands and has no rm rounding field.