pto.vrsqrt

pto.vrsqrt is part of the Unary Vector Instructions instruction set.

Summary

%result holds reciprocal-square-root values per active lane.

Mechanism

pto.vrsqrt computes the lane-wise reciprocal square root: dst[i] = 1 / sqrt(src[i]). This is commonly used in normalizing vectors. Inactive lanes leave the destination unchanged.

Syntax

PTO Assembly Form

vrsqrt %result, %input, %mask

AS Level 1 (SSA)

%result = pto.vrsqrt %input, %mask : !pto.vreg<NxT>, !pto.mask<G> -> !pto.vreg<NxT>

Documented A5 types or forms: f16, f32.

Inputs

Operand Type Description
%input !pto.vreg<NxT> Source vector register; read at each active lane i
%mask !pto.mask<G> Predicate mask; lanes where mask bit is 1 (true) are active

Expected Outputs

Result Type Description
%result !pto.vreg<NxT> Lane-wise reciprocal square root: dst[i] = 1 / sqrt(src[i]) on active lanes; inactive lanes are unmodified

Side Effects

This operation has no architectural side effect beyond producing its SSA results. It does not implicitly reserve buffers, signal events, or establish memory fences unless the form says so.

Constraints

Constraints

Only floating-point element types are legal. Active inputs containing +0 or -0 follow the target's divide-style exceptional behavior.

Exceptions

Exceptions

  • The verifier rejects illegal operand shapes, unsupported element types, and attribute combinations that are not valid for the selected instruction set or target profile.
  • Target-defined numeric exceptional behavior, such as divide-by-zero or out-of-domain inputs, remains subject to the selected backend profile unless this page narrows it further.
  • Any additional illegality stated in the constraints section is also part of the contract.

Target-Profile Restrictions

Target-Profile Restrictions
  • Documented A5 coverage: f16, f32.
  • A5 is the most detailed concrete profile in the current manual; CPU simulation and A2/A3-class targets may support narrower subsets or emulate the behavior while preserving the visible PTO contract.
  • Code that depends on an instruction-set-specific type list, distribution mode, or fused form should treat that dependency as target-profile-specific unless the PTO manual states cross-target portability explicitly.

Performance

A5 Latency

Element Type Latency (cycles) A5 RV
f32 13 RV_VMULS (scalar-mul path)
f16 13 RV_VMULS (scalar-mul path)

A2/A3 Throughput

Metric Value Constant
Startup latency 14 A2A3_STARTUP_BINARY
Completion latency 20 (FP) A2A3_COMPL_FP_MUL
Per-repeat throughput 1 A2A3_RPT_1
Pipeline interval 18 A2A3_INTERVAL

Examples

for (int i = 0; i < N; i++)
    dst[i] = 1.0f / sqrtf(src[i]);