gas: Optimize variable reference on assembly

We were moving the stack data for variable reference to another stack
position ending up with two pointer to the same value.

    // a: i32 = 1;
    mov $1, -8(%rbp)
    // b: i32 = a;
    mov -8(%rbp), %rax
    mov %rax, -24(%rbp)
    mov -24(%rbp), %rax
    mov %rax, -16(%rbp)

After this changes, we wont create a new temp space on stack if we don't
need it. See bellow the example after the optimization:

    // a: i32 = 1;
    mov $1, -8(%rbp)
    // b: i32 = a;
    mov -8(%rbp), %rax
    mov %rax, -16(%rbp)

Signed-off-by: Johnny Richard <johnny@johnnyrichard.com>
Co-authored-by: Carlos Maniero <carlosmaniero@gmail.com>

3 files changed, 32 insertions, 7 deletions

diff --git a/examples/variables.pipa b/examples/variables.pipa
index 9c395ce..25cbd59 100644
--- a/examples/variables.pipa
+++ b/examples/variables.pipa
@@ -3,5 +3,6 @@ main(): i32 {
   b: i32 = 32;
   c: i32 = 2 * (b + a);
   d: i32 = (c - 33) + 1;
-  return d / 2;
+  e: i32 = d;
+  return e / 2;
 }
diff --git a/src/gas_assembly_generator.c b/src/gas_assembly_generator.c
index b9b8537..dd5d240 100644
--- a/src/gas_assembly_generator.c
+++ b/src/gas_assembly_generator.c
@@ -54,6 +54,13 @@ gas_assembly_generator_set_latest_evaluation_to_rax(gas_assembly_generator_t *ge
   gen->latest_evaluation = (evaluation_result_t){ .kind = EVALUATION_RESULT_ON_RAX };
 }
 
+static void
+gas_assembly_generator_set_latest_evaluation_on_stack(gas_assembly_generator_t *gen, int stack_offset)
+{
+  gen->latest_evaluation =
+    (evaluation_result_t){ .kind = EVALUATION_RESULT_ON_STACK, .data = { .stack_offset = stack_offset } };
+}
+
 typedef struct ref_entry_t
 {
   ast_identifier_t *id;
@@ -167,11 +174,15 @@ gas_assembly_generator_compile_variable_declaration(gas_assembly_generator_t *ge
 
   switch (gen->latest_evaluation.kind) {
     case EVALUATION_RESULT_AS_LITERAL_INTEGER:
-      fprintf(gen->stream, "  mov $%ld, %d(%%rbp)\n", gen->latest_evaluation.data.literal_int, entry->stack_offset);
+      fprintf(gen->stream, "  movq $%ld, %d(%%rbp)\n", gen->latest_evaluation.data.literal_int, entry->stack_offset);
       break;
     case EVALUATION_RESULT_ON_RAX:
       fprintf(gen->stream, "  mov %%rax, %d(%%rbp)\n", entry->stack_offset);
       break;
+    case EVALUATION_RESULT_ON_STACK:
+      fprintf(gen->stream, "  movq %d(%%rbp), %%rax\n", gen->latest_evaluation.data.stack_offset);
+      fprintf(gen->stream, "  movq %%rax, %d(%%rbp)\n", entry->stack_offset);
+      break;
     case EVALUATION_RESULT_VOID:
       assert(false && "Unexpected void result for variable declaration");
       // FIXME: store the latest node at latest evaluation and print_ast
@@ -190,8 +201,7 @@ gas_assembly_generator_compile_variable(gas_assembly_generator_t *gen, ast_varia
     }
   }
   assert(entry && "reference not found");
-  fprintf(gen->stream, "  mov %d(%%rbp), %%rax\n", entry->stack_offset);
-  gas_assembly_generator_set_latest_evaluation_to_rax(gen);
+  gas_assembly_generator_set_latest_evaluation_on_stack(gen, entry->stack_offset);
 }
 
 static void
@@ -228,15 +238,27 @@ gas_assembly_generator_binary_operation(gas_assembly_generator_t *gen, ast_binar
       fprintf(gen->stream, "  mov %d(%%rbp), %%rcx\n", gen->stack_offset);
       gen->stack_offset += 8;
       break;
+    case EVALUATION_RESULT_ON_STACK:
+      fprintf(gen->stream, "  movq %d(%%rbp), %%rcx\n", right_evaluation.data.stack_offset);
+      break;
     case EVALUATION_RESULT_VOID:
-      assert(false && "Unexpected void result for variable declaration");
+      assert(false && "Unexpected void result for binary operation");
       // FIXME: store the latest node at latest evaluation and print_ast
   }
 
   gas_assembly_generator_set_latest_evaluation_to_rax(gen);
 
-  if (left_evaluation.kind == EVALUATION_RESULT_AS_LITERAL_INTEGER) {
-    fprintf(gen->stream, "  mov $%ld, %%rax\n", left_evaluation.data.literal_int);
+  switch (left_evaluation.kind) {
+    case EVALUATION_RESULT_AS_LITERAL_INTEGER:
+      fprintf(gen->stream, "  mov $%ld, %%rax\n", left_evaluation.data.literal_int);
+      break;
+    case EVALUATION_RESULT_ON_STACK:
+      fprintf(gen->stream, "  movq %d(%%rbp), %%rax\n", left_evaluation.data.stack_offset);
+      break;
+    case EVALUATION_RESULT_ON_RAX:
+      break;
+    case EVALUATION_RESULT_VOID:
+      assert(false && "Unexpected void result for binary operation");
   }
 
   if (binary_operation->kind == AST_BINOP_ADITION) {
diff --git a/src/gas_assembly_generator.h b/src/gas_assembly_generator.h
index 8501a67..92972f9 100644
--- a/src/gas_assembly_generator.h
+++ b/src/gas_assembly_generator.h
@@ -25,12 +25,14 @@ typedef enum evaluation_result_kind_t
 {
   EVALUATION_RESULT_VOID,
   EVALUATION_RESULT_ON_RAX,
+  EVALUATION_RESULT_ON_STACK,
   EVALUATION_RESULT_AS_LITERAL_INTEGER
 } evaluation_result_kind_t;
 
 typedef union evaluation_result_data_t
 {
   int64_t literal_int;
+  int stack_offset;
 } evaluation_result_data_t;
 
 typedef struct evaluation_result_t