Vector Instruction Set: Compare And Select

pto.v* compare and select instruction sets are defined here. Predicate production and selection behavior are specified explicitly because later vector and control operations depend on those exact mask semantics.

Category: Comparison and conditional selection operations Pipeline: PIPE_V (Vector Core)

Operations that compare vectors and conditionally select elements.

Common Operand Model

  • %src0 and %src1 are source vector operands.
  • %scalar is the scalar operand for scalar-comparison instruction sets.
  • %seed is the incoming predicate that limits which lanes participate in the compare.
  • %result is either a predicate mask (vcmp, vcmps) or a vector register (vsel, vselr, vselrv2).

Comparison Operations

pto.vcmp

  • syntax: %result = pto.vcmp %src0, %src1, %seed, "CMP_MODE" : !pto.vreg<NxT>, !pto.vreg<NxT>, !pto.mask<G> -> !pto.mask<G>
  • semantics: Element-wise comparison, output predicate mask.
for (int i = 0; i < N; i++)
    if (seed[i])
        dst[i] = (src0[i] CMP src1[i]) ? 1 : 0;

Compare modes:

Mode Operation
eq Equal (==)
ne Not equal (!=)
lt Less than (<)
le Less than or equal (<=)
gt Greater than (>)
ge Greater than or equal (>=)

Example: 

%all_active = pto.pset_b32 "PAT_ALL" : !pto.mask<G>
%lt_mask = pto.vcmp %a, %b, %all_active, "lt" : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.mask<b32>
// lt_mask[i] = 1 if a[i] < b[i]

  • inputs: %src0, %src1, and %seed; CMP_MODE selects the comparison predicate.
  • outputs: %result is the generated predicate mask.
  • constraints and limitations: Only lanes enabled by %seed participate. Integer and floating-point comparisons follow their own element-type-specific comparison rules.

pto.vcmps

  • syntax: %result = pto.vcmps %src, %scalar, %seed, "CMP_MODE" : !pto.vreg<NxT>, T, !pto.mask<G> -> !pto.mask<G>
  • semantics: Compare vector against scalar.
for (int i = 0; i < N; i++)
    if (seed[i])
        dst[i] = (src[i] CMP scalar) ? 1 : 0;

Example: 

%positive_mask = pto.vcmps %values, %c0_f32, %all_active, "gt"
    : !pto.vreg<64xf32>, f32, !pto.mask<b32> -> !pto.mask<b32>
// positive_mask[i] = 1 if values[i] > 0

  • inputs: %src is the vector source, %scalar is the scalar comparison value, and %seed is the incoming predicate.
  • outputs: %result is the generated predicate mask.
  • constraints and limitations: For 32-bit scalar forms, the scalar source MUST satisfy the backend's legal scalar-source constraints for this instruction set.

Selection Operations

pto.vsel

  • syntax: %result = pto.vsel %src0, %src1, %mask : !pto.vreg<NxT>, !pto.vreg<NxT>, !pto.mask<G> -> !pto.vreg<NxT>
  • semantics: Per-lane select based on mask.
for (int i = 0; i < N; i++)
    dst[i] = mask[i] ? src0[i] : src1[i];

Example — Conditional assignment: 

// dst = mask ? true_vals : false_vals
%result = pto.vsel %true_vals, %false_vals, %condition
    : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.vreg<64xf32>

  • inputs: %src0 is the true-path vector, %src1 is the false-path vector, and %mask selects between them.
  • outputs: %result is the selected vector.
  • constraints and limitations: Source vectors and result MUST have matching vector shapes and element types.

pto.vselr

  • syntax: %result = pto.vselr %src0, %src1 : !pto.vreg<NxT>, !pto.vreg<NxT> -> !pto.vreg<NxT>
  • semantics: Select with reversed mask semantics.
for (int i = 0; i < N; i++)
    dst[i] = mask[i] ? src1[i] : src0[i];  // reversed from vsel
  • inputs: %src0 and %src1 are the source vectors.
  • outputs: %result is the selected vector.
  • constraints and limitations: This instruction set preserves reversed-select semantics. If the concrete lowering uses an implicit predicate source, that predicate source MUST be documented by the surrounding IR pattern.

pto.vselrv2

  • syntax: %result = pto.vselrv2 %src0, %src1 : !pto.vreg<NxT>, !pto.vreg<NxT> -> !pto.vreg<NxT>
  • semantics: Variant select form with the same current two-vector operand shape.
  • inputs: %src0 and %src1 are the source vectors.
  • outputs: %result is the selected vector.
  • constraints and limitations: Only the instruction-set shape is recorded here. Lowering MUST preserve the exact A5 variant semantics selected for this form.

Typical Usage

// Clamp negative values to zero (manual ReLU)
%all = pto.pset_b32 "PAT_ALL" : !pto.mask<G>
%zero = pto.vbr %c0_f32 : f32 -> !pto.vreg<64xf32>
%neg_mask = pto.vcmps %input, %c0_f32, %all, "lt" : !pto.vreg<64xf32>, f32, !pto.mask<b32> -> !pto.mask<b32>
%clamped = pto.vsel %zero, %input, %neg_mask : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.vreg<64xf32>

// Element-wise max via compare+select
%gt_mask = pto.vcmp %a, %b, %all, "gt" : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.mask<b32>
%max_ab = pto.vsel %a, %b, %gt_mask : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.vreg<64xf32>

// Threshold filter
%above_thresh = pto.vcmps %scores, %threshold, %all, "ge" : !pto.vreg<64xf32>, f32, !pto.mask<b32> -> !pto.mask<b32>
%filtered = pto.vsel %scores, %zero, %above_thresh : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.vreg<64xf32>

Compare + Select Pattern

// Softmax safe exp: exp(x - max) where x < max returns exp of negative
// but we want to clamp to avoid underflow

%all = pto.pset_b32 "PAT_ALL" : !pto.mask<G>

// 1. Compare against threshold
%too_small = pto.vcmps %x_minus_max, %min_exp_arg, %all, "lt"
    : !pto.vreg<64xf32>, f32, !pto.mask<b32> -> !pto.mask<b32>

// 2. Clamp values below threshold
%clamped = pto.vsel %min_exp_arg_vec, %x_minus_max, %too_small
    : !pto.vreg<64xf32>, !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.vreg<64xf32>

// 3. Safe exp
%exp_result = pto.vexp %clamped, %all : !pto.vreg<64xf32>, !pto.mask<b32> -> !pto.vreg<64xf32>