chore: uses if_then_zero only in HPU ERC20 whitepaper (to be updated when encrypt_trivial becomes available on HPU), adds test of if_then_zero for both CPU & HPU

tfhe-benchmark/benches/high_level_api/erc20.rs

@@ -34,12 +34,12 @@ where
 {
     let has_enough_funds = (from_amount).ge(amount);
     let amount_to_transfer = {
-        #[cfg(feature = "gpu")]
+        #[cfg(not(feature = "hpu"))]
         {
             let zero_amount = FheType::encrypt_trivial(0u64);
             has_enough_funds.select(amount, &zero_amount)
         }
-        #[cfg(not(feature = "gpu"))]
+        #[cfg(feature = "hpu")]
         {
             has_enough_funds.if_then_zero(amount)
         }

tfhe/src/high_level_api/integers/unsigned/tests/cpu.rs

@@ -386,6 +386,12 @@ fn test_if_then_else() {
     super::test_case_if_then_else(&client_key);
 }
 
+#[test]
+fn test_if_then_zero() {
+    let client_key = setup_default_cpu();
+    super::test_case_if_then_zero(&client_key);
+}
+
 #[test]
 fn test_flip() {
     let client_key = setup_default_cpu();

tfhe/src/high_level_api/integers/unsigned/tests/hpu.rs

@@ -89,6 +89,12 @@ fn test_case_if_then_else_hpu() {
     super::test_case_if_then_else(&client_key);
 }
 
+#[test]
+fn test_case_if_then_zero_hpu() {
+    let client_key = setup_default_hpu();
+    super::test_case_if_then_zero(&client_key);
+}
+
 #[test]
 fn test_case_flip_hpu() {
     let client_key = setup_default_hpu();

tfhe/src/high_level_api/integers/unsigned/tests/mod.rs

@@ -568,6 +568,28 @@ fn test_case_if_then_else(client_key: &ClientKey) {
     );
 }
 
+fn test_case_if_then_zero(client_key: &ClientKey) {
+    let clear_a = 42u8;
+    let clear_b = 128u8;
+
+    let a = FheUint8::encrypt(clear_a, client_key);
+    let b = FheUint8::encrypt(clear_b, client_key);
+
+    let result = a.le(&b).if_then_zero(&a);
+    let decrypted_result: u8 = result.decrypt(client_key);
+    assert_eq!(
+        decrypted_result,
+        if clear_a <= clear_b { clear_a } else { 0 }
+    );
+
+    let result = a.ge(&b).if_then_zero(&a);
+    let decrypted_result: u8 = result.decrypt(client_key);
+    assert_eq!(
+        decrypted_result,
+        if clear_a >= clear_b { clear_a } else { 0 }
+    );
+}
+
 fn test_case_flip(client_key: &ClientKey) {
     let clear_a = rand::random::<u32>();
     let clear_b = rand::random::<u32>();