AccountSharedData::set_data_from_slice: skip extra alloc + memcpy

We used call make_data_mut() from set_data_from_slice() from the days
when direct mapping couldn't deal with accounts getting shrunk. That
changed in solana-labs#32649 (see the
if callee_account.capacity() < min_capacity check in
cpi.rs:update_caller_account()).

With this change we don't call make_data_mut() anymore before
set_data_from_slice(), saving the cost of a memcpy since
set_data_from_slice() overrides the whole account content anyway.

programs/bpf_loader/src/syscalls/cpi.rs

@@ -1308,8 +1308,9 @@ fn update_caller_account(
             // never points to an invalid address.
             //
             // Note that the capacity can be smaller than the original length only if the account is
-            // reallocated using the AccountSharedData API directly (deprecated). BorrowedAccount
-            // and CoW don't trigger this, see BorrowedAccount::make_data_mut.
+            // reallocated using the AccountSharedData API directly (deprecated) or using
+            // BorrowedAccount::set_data_from_slice(), which implements an optimization to avoid an
+            // extra allocation.
             let min_capacity = caller_account.original_data_len;
             if callee_account.capacity() < min_capacity {
                 callee_account

sdk/src/transaction_context.rs

@@ -869,13 +869,10 @@ impl<'a> BorrowedAccount<'a> {
         self.can_data_be_changed()?;
         self.touch()?;
         self.update_accounts_resize_delta(data.len())?;
-        // Calling make_data_mut() here guarantees that set_data_from_slice()
-        // copies in places, extending the account capacity if necessary but
-        // never reducing it. This is required as the account migh be directly
-        // mapped into a MemoryRegion, and therefore reducing capacity would
-        // leave a hole in the vm address space. After CPI or upon program
-        // termination, the runtime will zero the extra capacity.
-        self.make_data_mut();
+        // Note that we intentionally don't call self.make_data_mut() here.  make_data_mut() will
+        // allocate + memcpy the current data if self.account is shared. We don't need the memcpy
+        // here tho because account.set_data_from_slice(data) is going to replace the content
+        // anyway.
         self.account.set_data_from_slice(data);
 
         Ok(())