Bug in C codegen

[Smit-create]

I was trying to compile and run the C code generated from integration_tests/test_builtin_str.py (#1109 ). I found the following issue with the C codegen:
Python:

def f():
    s: str
    s = "abcde"
    c: str
    for c in s[::-1]:
        print(c)

f()

The generated C code on the main branch is:

void f()
{
    int32_t __explicit_iterator;
    char * __tmp_assign_for_loop = (char*) malloc(40 * sizeof(char));
    char * c = (char*) malloc(40 * sizeof(char));
    char * s = (char*) malloc(40 * sizeof(char));
    strcpy(s, "abcde");
    for (__explicit_iterator=0; __explicit_iterator<=strlen(__tmp_assign_for_loop) - 1; __explicit_iterator++) {
        strcpy(c, (char *)memcpy((char *)calloc(2U, sizeof(char)), __tmp_assign_for_loop + __explicit_iterator + 1 - 1, 1U));
        printf("%s\n", c);
    }
}

The above code is incorrect because we expect __tmp_assign_for_loop to contain the value of s[::-1]. This is because of this line in the ASR frontend:

lpython/src/lpython/semantics/python_ast_to_asr.cpp

Line 3653 in 91cb772

std::string tmp_assign_name = current_scope->get_unique_name("__tmp_assign_for_loop");

The variable value is ignored in C code generation as seen in the above code __tmp_assign_for_loop does not contain the correct value and so essentially will give segmentation fault due to memory errors.

I tried to fix that by using the following diff(changed made to wherever variables are declared):

diff --git a/src/libasr/codegen/asr_to_c_cpp.h b/src/libasr/codegen/asr_to_c_cpp.h
index b696d8d1e..bed985fc9 100644
--- a/src/libasr/codegen/asr_to_c_cpp.h
+++ b/src/libasr/codegen/asr_to_c_cpp.h
@@ -520,6 +520,17 @@ R"(#include <stdio.h>
             }
         }

+        for (auto &item : x.m_symtab->get_scope()) {
+            if (ASR::is_a<ASR::Variable_t>(*item.second)) {
+                ASR::Variable_t *v = ASR::down_cast<ASR::Variable_t>(item.second);
+                if (v->m_value) {
+                    self().visit_expr(*v->m_value);
+                    std::string name(v->m_name);
+                    decl += name +  " = " + src + ";\n";
+                }
+            }
+        }
+

And got the following C code:

void f()
{
    int32_t __explicit_iterator;
    char * __tmp_assign_for_loop = (char*) malloc(40 * sizeof(char));
    char * c = (char*) malloc(40 * sizeof(char));
    char * s = (char*) malloc(40 * sizeof(char));
    __tmp_assign_for_loop = _lfortran_str_slice(s, 0, 0, -1, false, false);
    strcpy(s, "abcde");
    for (__explicit_iterator=0; __explicit_iterator<=strlen(__tmp_assign_for_loop) - 1; __explicit_iterator++) {
        strcpy(c, (char *)memcpy((char *)calloc(2U, sizeof(char)), __tmp_assign_for_loop + __explicit_iterator + 1 - 1, 1U));
        printf("%s\n", c);
    }
}

That still won't work because __tmp_assign_for_loop is assigned before s is assigned. Any suggestions to fix it?

[czgdp1807]

We convert initialisation at declaration to assignment statement in the body for user defined variables (see #662 for background, related issue #651). We should do this for temporary variables as well.

In addition self().visit_expr(*v->m_value); should be self().visit_expr(*v->m_symbolic_value);. Variable::m_value is the compile time constant value and Variable::m_symbolic_value is the initial value provided at the time of variable declaration (see #651 (comment)). However we shouldn't be doing this in any backend anyways as I said above all such initialisation at declaration should be converted into assignment statements in the body itself.

That still won't work because __tmp_assign_for_loop is assigned before s is assigned. Any suggestions to fix it?

This is due to the order in which we are visiting variables. We will look into this once the above bug is fixed. Please feel free to send a PR.