Adjust std::alloc::alloc() to accommodate upcoming VM changes. (FuelL…

…abs#4245)

sway-lib-std/src/alloc.sw

@@ -23,8 +23,19 @@ library alloc;
 /// ```
 /// For more information, see the Fuel Spec for [VM Initialization](https://fuellabs.github.io/fuel-specs/master/vm#vm-initialization)
 /// and the VM Instruction Set for [Memory Allocation](https://fuellabs.github.io/fuel-specs/master/vm/instruction_set.html#aloc-allocate-memory).
+///
+/// NOTE: See https://github.com/FuelLabs/fuel-specs/pull/464 and related PRs.  There is an upcoming
+/// breaking change to the VM which 'corrects' the above behaviour to be more intuitive.  Instead of
+/// `$hp` pointing to the last byte of free memory it will instead point to the bottom of allocated
+/// memory.  So it will be initialized to `VM_MAX_RAM` and after an `ALOC` it will point directly to
+/// the new buffer.
+///
+/// To avoid the need to synchronize the behaviours between this library and the two allocation
+/// modes, i.e., before and after the breaking change, we allocate 1 extra byte here and still
+/// return `$hp + 1`.  So prior to the VM change every allocation will have an unused byte _after_
+/// the buffer and after the change every allocation will have an unused byte _before_ the buffer.
 pub fn alloc<T>(count: u64) -> raw_ptr {
-    asm(size: __size_of::<T>() * count, ptr) {
+    asm(size: __size_of::<T>() * count + 1, ptr) {
         aloc size;
         // `$hp` points to unallocated space and heap grows downward so
         // our newly allocated space will be right after it.
@@ -48,7 +59,7 @@ pub fn realloc<T>(ptr: raw_ptr, count: u64, new_count: u64) -> raw_ptr {
 
 /// Allocates zeroed memory on the heap in individual bytes.
 pub fn alloc_bytes(count: u64) -> raw_ptr {
-    asm(size: count, ptr) {
+    asm(size: count + 1, ptr) {
         aloc size;
         addi ptr hp i1;
         ptr: raw_ptr

test/src/e2e_vm_tests/test_programs/should_pass/require_contract_deployment/call_basic_storage/src/main.sw

@@ -2,7 +2,7 @@ script;
 use basic_storage_abi::{BasicStorage, Quad};
 
 fn main() -> u64 {
-    let addr = abi(BasicStorage, 0x558d7f3f2cc625751596ca69a5c670a02651f1b84852e93adc9675368df26959);
+    let addr = abi(BasicStorage, 0x89d66ade6874264291ca6416a6e746ac568970c385e19297d0f576ff05463be5);
     let key = 0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff;
     let value = 4242;
 

test/src/e2e_vm_tests/test_programs/should_pass/stdlib/alloc/test.toml

@@ -1,3 +1,5 @@
-category = "run"
+# This test is making assumptions about how `$hp` is used with allocations.  It needs to be
+# re-enabled after the VM introduces the new `ALOC` behaviour.
+category = "disabled"
 expected_result = { action = "return", value = 1 }
 validate_abi = true