Use u16 for program account index

Author: LucasSte

transaction-context/src/instruction.rs

@@ -18,7 +18,9 @@ use {
 #[repr(C)]
 #[derive(Debug)]
 pub struct InstructionFrame {
-    pub program_account_index_in_tx: u32,
+    /// Reserved filed for alignment and potential future usage.
+    pub reserved: u16,
+    pub program_account_index_in_tx: u16,
     pub nesting_level: u16,
     /// This is the index of the parent instruction if this is a CPI and u16::MAX if this is a
     /// top-level instruction
@@ -36,6 +38,7 @@ impl Default for InstructionFrame {
             // Using u64::MAX as the default pointer value, since it shall never be accessible.
             instruction_accounts: VmSlice::new(0, 0),
             instruction_data: VmSlice::new(0, 0),
+            reserved: 0,
         }
     }
 }

transaction-context/src/lib.rs

@@ -299,7 +299,7 @@ impl<'ix_data> TransactionContext<'ix_data> {
             .last_mut()
             .ok_or(InstructionError::CallDepth)?;
 
-        instruction.program_account_index_in_tx = program_index as u32;
+        instruction.program_account_index_in_tx = program_index;
         instruction.configure_vm_slices(
             instruction_index as u64,
             instruction_accounts.len(),