University project whose porpouse is to create a compiler which takes C snippets and translate them to assembly
The aim of the project is to create a compiler which translates C language snippets into assembly (RISC-V) language code. The C microbenchmarks are taken from the past exams exercises of the course "Computer Architecture" held at the Universtity of Siena at the computer engineering department.
Understanding Flex/Lex
As stated in the "Compiler Construction using Flex and Bison" by Anthony A. Bay:
Lex and Flex are tools for generating scanners: programs which recognize lexical patterns in text. Flex is a faster version of Lex.
Lets start with and example to understand better the syntax of Flex.
We want to build a "hello Counter", a program wich counts the repetitions of "hello" in a text file as input.
Create a text file with the .l extension
Flex has this type of syntax
%{
//Definitions
}%
%%
Recognized_patter Action
%%
// C code
Definitions are the variables to declare or costants. Recognized_pattern are the characters we want to be recognized by Flex. They will correspond to an action. Action Action which corresponds to a recognized pattern.
Regarding our examples, we want to count the number of "hello" and the number of lines in a given text, so we would type
%{
int num_lines=0,num_hello=0;
}%
After that, it is time to write the patterns and the corresponding actions
%%
"hello" ++num_hello;
"\n" ++num_lines;
. ;
%%
For every hello increment num_hello ++num_hello
For every line feed \n increment num_lines ++num_lines
For every other character . do nothing ;
Now it is time to write the main.
We will call the yylex() function
Lex generates a file containing the function yylex() which returns an integer denoting the token recognized.
main(){
yylex();
printf("The number of hellos is = %d\n The number of lines is= %d\n",num_hello,num_lines);
}
So the actual code would look like this:
%{
int num_lines=0,num_hello=0;
%}
%%
"hello" ++num_hello;
"\n" ++num_lines;
. ;
%%
main(){
yylex();
printf("The number of hellos is = %d\n The number of lines is= %d\n",num_hello,num_lines);
}
As an example we could use the below input-text.
hello, I am Marco, how are you?
hello Marco, I am Giovanni, I am good.
hello, I am Antonio.
I am Andrea.
Why didn't you say hello Andrea?
Compile it:
flex hello_counter.l
gcc lex.yy.c -ll
./a.out < hello-text
Output: The number of hellos is = 4 The number of lines is= 5