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constexpr-8cc: Compile-time C Compiler
constexpr-8cc is a compile-time C compiler implemented as C++14 constant expressions. This enables you to compile while you compile! This project is a port of 8cc built on ELVM Infrastructure.
Constant expressions in C++ are expressions that can be evaluated at compile-time. In C++14, by relaxing constrains, constant expressions became so powerful that a C compiler can be implemented in!
In constexpr-8cc, the main routine for compilations of C programs is implemented in a C++14 constexpr
function.
Therefore, if you compile 8cc.cpp
to a binary file by g++, compilation of a C program will be performed as a compile-time computation and the result of this C compilation will be embedded into the generated binary.
In this sense, constexpr-8cc is a compile-time C compiler.
The following is the main
function in 8cc.cpp.
int main() {
// Compile-time
constexpr buffer buf = eight_cc(); // Compile C code into ELVM IR
constexpr unsigned int output_size = buf.size;
static_assert(0 <= output_size && output_size < EIGHT_CC_OUTPUT_LIMIT, "8cc: Error");
// Run-time
for(int i = 0; i < output_size; ++i) {
putchar(buf.b[i]);
}
}
In this program, the return value of eight_cc
is stored into the variable buf
with a constexpr
specifier.
Thus, you will find that the compilation of a C program is done in compile-time.
Requirements
constexpr-8cc
requires Linux with >g++-6.2. I confirmed ./test/hello.c
can be compiled with g++-6.2
, g++-8.3
and g++-9.3
at least.
- g++-6.2 worked without any extra flag related to constexpr as there was no limitation of constant's loop counts at this version.
- With g++-8.3, I needed to enlarge constexpr's loop count with
-fconstexpr-loop-limit
. The maximum number we can specify is2**31 - 1
. - With g++-9.3, in addition to
-fconstexpr-loop-limit
, enlarging-fconstexpr-ops-limit
was needed. - There is no guarantee that it works with other versions of g++.
- I couldn't make it work with clang++ as clang++ has more strict limitation of constexpr loop counts.
How to run
Compilation by run_8cc.py
In order to try constexpr-8cc easily, use run_8cc.py
.
$ ./run_8cc.py x86 ./test/hello.c -o ./hello.exe # It takes about 3 minutes on my laptop
$ chmod +x ./hello.exe # 'hello.exe' is i386-linux binary
$ ./hello.exe
Hello, world!
You can change the target language of compilations like the following:
$ ./run_8cc.py py ./test/hello.c -o ./hello.py # target language is Python
$ python ./hello.py
Hello, world!
For more information about this script, type $ ./run_8cc.py -h
.
Compilation by hand
If you want to compile 8cc.cpp
manually, please look at config.hpp
.
In this file, the variable EIGHT_CC_INPUT_FILE
is defined.
EIGHT_CC_INPUT_FILE
should be a name of a file that contains a source C program as a C++ string literal.
This string will be embedded in 8cc.cpp at pre-processing-time and used as an input of the compile-time computation.
So, before compiling 8cc.cpp
manually, you have to convert a raw program to a string literal like the following:
$ sed '1s/^/R"(/' ./test/hello.c | sed '$s/$/\n)"/' > ./test/hello.c.txt # Convert C to string literal
$ g++-6 ./8cc.cpp -o eir_gen.out
$ ./eir_gen.out > ./test/hello.eir # eir_gen.out outputs ELVM IR
$ sed -i '1s/^/R"(x86/' ./test/hello.eir # Convert IR to string literal
$ sed -i '$s/$/\n)"/' ./test/hello.eir
$ g++-6 ./elc.cpp -o exe_gen.out
$ ./exe_gen.out > ./hello.exe # exe_gen.out outputs i386-linux binary
$ chmod +x ./hello.exe
$ ./hello.exe
Hello, world!
How was constexpr-8cc generated?
When you see 8cc.hpp
, you will know this program was not written by hand.
Actually, I used ELVM Compiler Infrastructure to generate it.
I just implemented a translator from ELVM IR to C++14 constexpr here.
Author
Keiichi Watanabe (udon.watanabe [at] gmail.com)
Links
-
- A very cool C compiler. constexpr-8cc is a C++14's constexpr port of 8cc.
-
- ELVM(EsoLang Virtual Machine) is a parody project of LLVM, but dedicated to Esoteric Languages. constexpr-8cc is built on ELVM infrastructure.
-
8cc.vim (@rhysd), 8cc.tex (@hak7a3)
- constexpr-8cc is influenced by these projects.
-
Compile-time Brainf*ck compiler (@bolero-MURAKAMI)
- I got several ideas from this program.
-
TMP-8cc: Another Compile-time C Compiler
- Another compile-time C compiler I created, which is implemented in C++ Template Metaprogramming
- This is generated by ELVM's C++ Template Metaprogramming backend.
- Since
TMP
neads huge amount of memories,TMP-8cc
unfortunately does not work on real machines.