pto.vexpdif

pto.vexpdif is part of the SFU And DSA Instructions instruction set.

Summary

Fused exp(x - max) for numerically stable softmax.

Mechanism

pto.vexpdif is a specialized pto.v* operation. It exposes fused, widening, or domain-specific hardware behavior through one stable virtual mnemonic so the instruction set can be reasoned about at the ISA level.

Syntax

PTO Assembly Form

vexpdif %result, %input, %max

AS Level 1 (SSA)

%result = pto.vexpdif %input, %max : !pto.vreg<NxT>, !pto.vreg<NxT> -> !pto.vreg<NxT>

Documented A5 types or forms: f16, f32.

Inputs

%input is the source vector and %max is the broadcasted subtraction term.

Expected Outputs

%result is the fused exp(input - max) vector.

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

Floating-point element types only.

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

Timing Disclosure

The current public VPTO timing material for PTO micro instructions remains limited. For pto.vexpdif, those public sources describe the instruction semantics, operand legality, and pipeline placement, but they do not publish a numeric latency or steady-state throughput.

Metric Status Source Basis
A5 latency Not publicly published Current public VPTO timing material
Steady-state throughput Not publicly published Current public VPTO timing material

If software scheduling or performance modeling depends on the exact cost of pto.vexpdif, treat that cost as target-profile-specific and measure it on the concrete backend rather than inferring a manual constant.

Examples

for (int i = 0; i < N; i++)
    dst[i] = expf(src[i] - max[i]);

Use case: Softmax numerator computation with numerical stability.