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Diffstat (limited to '05/README.md')
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diff --git a/05/README.md b/05/README.md index 0244a4b..a68cbbb 100644 --- a/05/README.md +++ b/05/README.md @@ -1,42 +1,42 @@ # [bootstrap](../README.md) stage 05 -This stage consists of a C compiler capable of compiling TCC (after some modifications -to TCC's source code). +This stage consists of a C compiler capable of compiling tcc (after some modifications +to tcc's source code). Run ``` $ make ``` -to build our C compiler and TCC. This will take some time (approx. 25 seconds on my computer). +to build our C compiler and tcc. This will take some time (approx. 25 seconds on my computer). This also compiles a "Hello, world!" executable, `a.out`, with our compiler. -We can now compile TCC with itself. But first, you'll need to install the header files and library files -which are needed to compile (almost) any program with TCC: +We can now compile tcc with itself. But first, you'll need to install the header files and library files +which are needed to compile (almost) any program with tcc: ``` $ sudo make install-tcc0 ``` The files will be installed to `/usr/local/lib/tcc-bootstrap`. If you want to change this, make sure to change -both the `TCCINST` variable in the makefile, and the `CONFIG_TCCDIR` macro in `config.h`. +both the `TCCINST` variable in the makefile, and the `CONFIG_TCCDIR` macro in `tcc-0.9.27/config.h`. Anyways, once this installation is done, you should be able to compile any C program with `tcc-0.9.27/tcc0`, -including TCC itself: +including tcc itself: ``` $ cd tcc-0.9.27 $ ./tcc0 tcc.c -o tcc1 ``` -Now, let's try doing the same thing, but starting with GCC instead of our C compiler: +Now, let's try doing the same thing, but starting with gcc instead of our C compiler: ``` $ gcc tcc.c -o tcc0a $ ./tcc0a tcc.c -o tcc1a ``` -In theory, these should produce the same files, since the output of TCC shouldn't depend on which compiler it was compiled with. -If they are different, then perhaps a bug *was* introduced in some early version of GCC, and replicated in all C compilers since then! +In theory, these should produce the same files, since the output of tcc shouldn't depend on which compiler it was compiled with. +If they are different, then perhaps a bug *was* introduced in some early version of gcc, and replicated in all C compilers since then! Well, only one way to find out: ``` @@ -53,9 +53,9 @@ $ diff tcc2 tcc1a $ ``` -Yes, after compiling TCC with itself one more time, we get the same executable as the GCC-TCC one. +Yes, after compiling tcc with itself one more time, we get the same executable as the gcc-tcc one. I'm not sure why `tcc1` differs from `tcc2`, but there you go. Turns out there isn't some malicious -self-replicating code hiding in GCC after all.\* +self-replicating code hiding in gcc after all.\* ## the C compiler @@ -69,7 +69,7 @@ idents.b - functions for creating mappings from identifiers to arbitrary 6 preprocess.b - preprocesses C files tokenize.b - turns preprocessing tokens into tokens (see explanation below) parse.b - turns tokens into a nice representation of the program -codegen.b - turns parse.b's representation into actual code +codegen.b - turns parse.b's representation into CPU instructions main.b - puts everything together ``` @@ -290,7 +290,7 @@ Here is a (probably incomplete) list of things we do wrong: - You can't have a variable/function/etc. called `defined`. - Various little things about when macros are evaluated in some contexts. - The horrible, horrible function `setjmp`, which surely no one uses, is not properly supported. -Oh wait, TCC uses it. Fortunately it's not critically important to TCC. +Oh wait, tcc uses it. Fortunately it's not critically important to tcc. - Wide characters and wide character strings are not supported. - The `localtime()` function assumes you are in the UTC+0 timezone. - `mktime()` always fails. @@ -317,17 +317,16 @@ rounds down, but 0.09999999999999999861222121921855432447046041488647460937501 rounds up. ``` -Good luck writing a function which handles that! +Good luck writing code which handles that! - Originally, there was a bug where negative powers of 2 were being interpreted as half of their actual value, e.g. `x = 0.25;` would set `x` to `0.125`, but `x = 4;`, `x = 0.3;`, etc. would all work just fine. -- Writing the functions in `math.h`, although probably not necessary for compiling TCC, +- Writing the functions in `math.h`, although probably not necessary for compiling tcc, was fun! There are quite a few interesting optimizations you can make, and little tricks for avoiding losses in floating-point accuracy. -- The <s>first</s> second non-trivial program I successfully compiled worked perfectly the first time I ran it! -- A very difficult to track down bug happened the first time I ran `tcc`: there was a declaration along +- A very difficult to track down bug happened the first time I ran tcc: there was a declaration along the lines of `char x[] = "a\0b\0c";` but it got compiled as `char x[] = "a";`! -- Originally, I was just treating labels the same as any other statements, but `tcc` actually has code like: +- Originally, I was just treating labels the same as any other statements, but tcc actually has code like: ``` ... goto lbl; @@ -337,7 +336,7 @@ if (some_condition) ``` so the `do_something();` was not being considered as part of the `if` statement. - The first time I compiled tcc with itself (and then with itself again), I actually got a different -executable from the GCC one. After spending a long time looking at disassemblies, I found the culprit: +executable from the gcc one. After spending a long time looking at disassemblies, I found the culprit: ``` # if defined(__linux__) tcc_define_symbol(s, "__linux__", NULL); @@ -345,8 +344,9 @@ executable from the GCC one. After spending a long time looking at disassemblies # endif ``` If the `__linux__` macro is defined (to indicate that the target OS is linux), -TCC will also define the `__linux__` macro. Unlike GCC, our compiler doesn't define the `__linux__` macro, -so when it's used to compile TCC, TCC won't define it either, no matter how many times you compile it +tcc will also define the `__linux__` macro in any programs it compiles. +Unlike gcc, our compiler doesn't define the `__linux__` macro, +so when it's used to compile tcc, tcc won't define it either, no matter how many times you compile it with itself! ## modifications of tcc's source code @@ -359,7 +359,7 @@ here. - First, we (and C89) don't allow a comma after the last member in an initializer. In several places, the last comma in an initializer/enum definition was removed, or an irrelevant entry was added to the end. - Global variables were sometimes declared twice, which we don't support. -So, a bunch of duplicate declarations were removed. +So a bunch of duplicate declarations were removed. - The `# if defined(__linux__)` and `# endif` mentioned above were removed. - In a bunch of places, `ELFW(something)` had to be replaced with `ELF64_something` due to subtleties of how we evaluate macros. @@ -368,12 +368,12 @@ some initializers were replaced by functions called at the top of `main`. - In several places, `default:` had to be moved to after every `case` label. - In two places, `-some_long_double_expression` had to be replaced with a function call to `negate_ld` (a function I wrote for negating long doubles). -This is because TCC only supports negating long doubles if -the compiler used to compile it has an 80-bit long double type, which our compiler doesn't. -- `\0` was replaced with `\n` as a separator for keyword names. -- Forced TCC to use `R_X86_64_PC32` relocations, because its `plt` code doesn't seem to work for static +This is because tcc only supports negating long doubles if +the compiler which compiled it has an 80-bit long double type, and our compiler doesn't. +- `\0` was replaced with `\n` as a separator for keyword names in the `tcc_keywords` global variable. +- Forced tcc to use `R_X86_64_PC32` relocations, because its `plt` code doesn't seem to work for static executables. -- Lastly, there's the `config.h` file, which is normally produced by TCC's `configure` script, +- Lastly, there's the `config.h` file, which is normally produced by tcc's `configure` script, but it's easy to write one manually: ``` #define TCC_VERSION "0.9.27" @@ -386,30 +386,62 @@ but it's easy to write one manually: ``` The last line causes the `inline` keyword (added in C99) to be ignored. -Fewer changes would've been needed for an older version of TCC, but older versions didn't support -x86-64 assembly, which might end up being relevant... - ## \*libc -If you look in TCC's source code, you will not find implementations of any of the C standard library functions. -So how can programs compiled with TCC use those functions? +If you look in tcc's source code, you will not find implementations of any of the C standard library functions. +So how can programs compiled with tcc use those functions? -When a program compiled with TCC (under default settings) calls `printf`, say, it actually gets the instructions +When a program compiled with tcc (under default settings) calls `printf`, say, it actually gets the instructions for `printf` from a separate library file (called something like `/usr/lib/x86_64-linux-gnu/libc-2.31.so`). There are very good reasons for this: for example, if there a security bug were found in `printf`, it would be much easier to replace the library file than re-compile every program which uses `printf`. Now this library file is itself compiled from C source files (typically glibc). -So, we can't really say that the self-compiled TCC was built from scratch, and there could be malicious +So, we can't really say that the self-compiled tcc was built from scratch, and there could be malicious self-replicating code in glibc. -You can't compile glibc with TCC, but -it's possible to build an old version of `musl`, an alternate libc +### compiling glibc + +You can't compile glibc with tcc, but +it's possible to build an old version of musl, an alternate libc (you can run `CC=../tcc-0.9.27/tcc0 make` in the `musl-0.6.0` directory here). -You should be able to use musl alongside TCC to build an old version of GCC (git revision -`79a6d9b7ff3822675ee44d8d6cad86027dadd664` seems workable). This also requires -building several tools needed to compile GCC. You should then be able to build (possibly an old version of) -glibc, and with that, a modern version of GCC. -This is all extremely tedious, though, so I'm not planning on doing it anytime soon. +You should be able to use musl alongside tcc to build an old version of gcc. This also requires +building several tools needed to compile gcc. You should then be able to build an old version of +glibc, and with that, a modern version of gcc. + +Well, I tried this. And it is an absolute nightmare. +GNU has created a horrible web of programs that all depend on each other. +According to the recommended build process, you need awk to build awk, sed to build sed, +sed to build grep, etc. Here was a "guide" I was starting to write for how to +get to glibc: + +- install tcc, musl +- build mrsh, make, basic utilities +- chroot +- build & install coreutils +- build & install dash +- build & install sed-4.2 +- build & install ld, as (from binutils) +- build gcc +- build & install grep-3.7 +- build & install awk +- build & install bash +- build & install glibc (didn't work) + +Each of these programs uses a `./configure` script to set up the code and Makefiles. +These scripts are basically impossible to use without already having +most of these programs. So, I resorted to configuring the build with +the ordinary binary versions of `sed`, etc. I had on my machine. +This made broken Makefiles which I spent hours editing by hand +-- and is it really compiled from scratch if it's built from +computer-generated source files and Makefiles? +And although the developers at GNU +refrain from declaring variables after statements, and keep old-style function declarations +to support compilers from the 80s; they *still* manage to use gcc-specific extensions, and +not even extensions that all versions of gcc support! +After hours and hours of fixing compiler errors, I decided to give up. + +THIS WAY LIES MADNESS. + |