fix: use GPU tree builder only for Poseidon hashes (#1515)

The GPU tree builder (Neptune) only supports Poseidon hashes. Switch
to the CPU tree builder if other hash algorithms are used (which isn't
the case in Filecoin at the moment).

Closes #1512.

storage-proofs-porep/src/stacked/vanilla/proof.rs

@@ -1,3 +1,4 @@
+use std::any::TypeId;
 use std::fs;
 use std::marker::PhantomData;
 use std::panic::panic_any;
@@ -7,7 +8,7 @@ use std::sync::Mutex;
 use anyhow::Context;
 use bincode::deserialize;
 use fdlimit::raise_fd_limit;
-use filecoin_hashers::{Domain, HashFunction, Hasher, PoseidonArity};
+use filecoin_hashers::{poseidon::PoseidonHasher, Domain, HashFunction, Hasher, PoseidonArity};
 use generic_array::typenum::{Unsigned, U0, U11, U2, U8};
 use lazy_static::lazy_static;
 use log::{error, info, trace};
@@ -428,7 +429,9 @@ impl<'a, Tree: 'static + MerkleTreeTrait, G: 'static + Hasher> StackedDrg<'a, Tr
         ColumnArity: 'static + PoseidonArity,
         TreeArity: PoseidonArity,
     {
-        if SETTINGS.use_gpu_column_builder {
+        if SETTINGS.use_gpu_column_builder
+            && TypeId::of::<Tree::Hasher>() == TypeId::of::<PoseidonHasher>()
+        {
             Self::generate_tree_c_gpu::<ColumnArity, TreeArity>(
                 layers,
                 nodes_count,
@@ -806,7 +809,10 @@ impl<'a, Tree: 'static + MerkleTreeTrait, G: 'static + Hasher> StackedDrg<'a, Tr
     where
         TreeArity: PoseidonArity,
     {
-        if SETTINGS.use_gpu_tree_builder {
+        // The GPU tree builder only support Poseidon hashes.
+        if SETTINGS.use_gpu_tree_builder
+            && TypeId::of::<Tree::Hasher>() == TypeId::of::<PoseidonHasher>()
+        {
             Self::generate_tree_r_last_gpu::<TreeArity>(
                 data,
                 nodes_count,