ConvertSecretInsertToStaticInsertPasses

`-convert-secret-insert-to-static-insert`

Convert tensor.insert operations on secret index to static insert operations.

Converts tensor.insert operations that write to secret index to alternative static tensor.insert operations that inserts the inserted value at each index and conditionally selects the newly produced tensor that contains the value at the secret index.

Note: Running this pass alone does not result in a data-oblivious program; we have to run the --convert-if-to-select pass to the resulting program to convert the secret-dependent If-operation to a Select-operation.

Example input:

func.func @main(%secretTensor: !secret.secret<tensor<32xi16>>, %secretIndex: !secret.secret<index>)) -> !secret.secret<i16> {
  ...
  %0 = secret.generic ins(%secretTensor, %secretIndex : !secret.secret<tensor<32xi16>>, !secret.secret<index>) {
  ^bb0(%tensor: tensor<32xi16>, %index: index):
    // Violation: tensor.insert writes value at secret index
    %inserted = tensor.insert %newValue into %tensor[%index] : tensor<16xi32>
  ...
}

Output:

func.func @main(%secretTensor: !secret.secret<tensor<32xi16>>, %secretIndex: !secret.secret<index>)) -> !secret.secret<i16> {
  ...
  %0 = secret.generic ins(%secretTensor, %secretIndex : !secret.secret<tensor<32xi16>>, !secret.secret<index>) {
  ^bb0(%tensor: tensor<32xi16>, %index: index):
    %inserted = affine.for %i=0 to 16 iter_args(%inputArg = %tensor) -> tensor<16xi32> {
      // 1. Check if %i matches the %index
      %cond = arith.cmpi eq, %i, %index : index
      // 2. Insert %newValue and produce %newTensor
      %newTensor = tensor.insert %value into %inputArg[%i] : tensor<16xi32>
      // 3. If %i matches %inputIndex, yield %newTensor, else yield unchanged input tensor
      %finalTensor = scf.if %cond -> (i32) {
        scf.yield %newTensor : tensor<16xi32>
      } else{
        scf.yield %inputArg : tensor<16xi32>
      }
      // 4. Yield final tensor
      affine.yield %finalTensor : tensor<16xi32>
}
  ...
}