Fix CantWriteReturnFp and other issues related to the initial gas loading (#2015)

* fix issues like WriteReturnFp due to the gas builtin

* try fix tests and update changelog

* fix test

Author: FrancoGiachetta

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 #### Upcoming Changes
 
+* fix: Fix `WriteReturnFp` error due to a bad loading of initial gas [#2015](https://github.com/lambdaclass/cairo-vm/pull/2015)
+
 * refactor: Replaces security anyhow errors with enum variants [#1946](https://github.com/lambdaclass/cairo-vm/pull/1946)
 
 * fix: `mod_builtin_fill_memory` could be stuck in an infinite loop [#1975](https://github.com/lambdaclass/cairo-vm/issues/1975)

cairo1-run/src/cairo_run.rs

@@ -143,8 +143,6 @@ pub fn cairo_run_program(
 
     let main_func = find_function(sierra_program, "::main")?;
 
-    let initial_gas = 9999999999999_usize;
-
     // Fetch return type data
     let return_type_id = match main_func.signature.ret_types.last() {
         // We need to check if the last return type is indeed the function's return value and not an implicit return value
@@ -261,7 +259,7 @@ pub fn cairo_run_program(
         false,
     )?;
     let end = runner.initialize(cairo_run_config.proof_mode)?;
-    load_arguments(&mut runner, &cairo_run_config, main_func, initial_gas)?;
+    load_arguments(&mut runner, &cairo_run_config, main_func)?;
 
     // Run it until the end / infinite loop in proof_mode
     runner.run_until_pc(end, &mut hint_processor)?;
@@ -465,7 +463,6 @@ fn load_arguments(
     runner: &mut CairoRunner,
     cairo_run_config: &Cairo1RunConfig,
     main_func: &Function,
-    initial_gas: usize,
 ) -> Result<(), Error> {
     let got_gas_builtin = main_func
         .signature
@@ -497,12 +494,7 @@ fn load_arguments(
     if got_segment_arena {
         ap_offset += 4;
     }
-    // Load initial gas if GasBuiltin is present
     if got_gas_builtin {
-        runner.vm.insert_value(
-            (runner.vm.get_ap() + ap_offset).map_err(VirtualMachineError::Math)?,
-            Felt252::from(initial_gas),
-        )?;
         ap_offset += 1;
     }
     for arg in cairo_run_config.args {
@@ -617,9 +609,10 @@ fn create_entry_code(
                 ap += 1;
             };
         } else if generic_ty == &GasBuiltinType::ID {
-            // We already loaded the inital gas so we just advance AP
+            // Load initial gas
             casm_build_extend! {ctx,
-                ap += 1;
+                const initial_gas = 9999999999999_usize;
+                tempvar gas = initial_gas;
             };
         } else {
             let ty_size = type_sizes[ty];

cairo1-run/src/main.rs

@@ -447,6 +447,13 @@ mod tests {
         None,
         None
     ))]
+    #[case(
+        "gas_builtin_loading.cairo",
+        "939340725154356279478212603733403581890242362232206720294887278547043341575",
+        "[939340725154356279478212603733403581890242362232206720294887278547043341575]",
+        None,
+        None
+    )]
     fn test_run_program(
         #[case] program: &str,
         #[case] expected_output: &str,

cairo_programs/cairo-1-programs/gas_builtin_loading.cairo

@@ -0,0 +1,10 @@
+use core::poseidon::PoseidonTrait;
+use core::hash::{HashStateTrait, HashStateExTrait};
+use core::testing::get_available_gas;
+
+fn main() -> felt252 {
+    let a: u128 = get_available_gas();
+    let b = PoseidonTrait::new().update_with(a).finalize();
+
+    b
+}

cairo_programs/cairo-1-programs/serialized_output/gas_builtin_loading.cairo

@@ -0,0 +1,13 @@
+use core::poseidon::PoseidonTrait;
+use core::hash::{HashStateTrait, HashStateExTrait};
+use core::testing::get_available_gas;
+
+fn main() -> Array<felt252> {
+    let a: u128 = get_available_gas();
+    let b = PoseidonTrait::new().update_with(a).finalize();
+    let mut ser_output: Array<felt252> = ArrayTrait::new();
+
+    b.serialize(ref ser_output);
+
+    ser_output
+}