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Tiny C Compiler Reference Documentation

















Tiny C Compiler Reference Documentation




Table of Contents




  1 Introduction
  2 Command line invocation
  
    2.1 Quick start
    2.2 Option summary
  
  3 C language support
  
    3.1 ANSI C
    3.2 ISOC99 extensions
    3.3 GNU C extensions
    3.4 TinyCC extensions
  
  4 TinyCC Assembler
  
    4.1 Syntax
    4.2 Expressions
    4.3 Labels
    4.4 Directives
    4.5 X86 Assembler
  
  5 TinyCC Linker
  
    5.1 ELF file generation
    5.2 ELF file loader
    5.3 PE-i386 file generation
    5.4 GNU Linker Scripts
  
  6 TinyCC Memory and Bound checks
  7 The libtcc library
  8 Developer’s guide
  
    8.1 File reading
    8.2 Lexer
    8.3 Parser
    8.4 Types
    8.5 Symbols
    8.6 Sections
    8.7 Code generation
    
      8.7.1 Introduction
      8.7.2 The value stack
      8.7.3 Manipulating the value stack
      8.7.4 CPU dependent code generation
    
    8.8 Optimizations done
  
  Concept Index







Next: Introduction, Previous: (dir), Up: (dir)   [Contents][Index]


Tiny C Compiler Reference Documentation

This manual documents version 0.9.27 of the Tiny C Compiler.


• Introduction:    Introduction to tcc.

• Invoke:    Invocation of tcc (command line, options).

• Clang:    ANSI C and extensions.

• asm:    Assembler syntax.

• linker:    Output file generation and supported targets.

• Bounds:    Automatic bounds-checking of C code.

• Libtcc:    The libtcc library.

• devel:    Guide for Developers.








Next: Invoke, Up: Top   [Contents][Index]


1 Introduction

TinyCC (aka TCC) is a small but hyper fast C compiler. Unlike other C
compilers, it is meant to be self-relying: you do not need an
external assembler or linker because TCC does that for you.

TCC compiles so fast that even for big projects Makefiles may
not be necessary.

TCC not only supports ANSI C, but also most of the new ISO C99
standard and many GNUC extensions including inline assembly.

TCC can also be used to make C scripts, i.e. pieces of C source
that you run as a Perl or Python script. Compilation is so fast that
your script will be as fast as if it was an executable.

TCC can also automatically generate memory and bound checks
(see Bounds) while allowing all C pointers operations. TCC can do
these checks even if non patched libraries are used.

With libtcc, you can use TCC as a backend for dynamic code
generation (see Libtcc).

TCC mainly supports the i386 target on Linux and Windows. There are alpha
ports for the ARM (arm-tcc) and the TMS320C67xx targets
(c67-tcc). More information about the ARM port is available at
http://lists.gnu.org/archive/html/tinycc-devel/2003-10/msg00044.html.

For usage on Windows, see also tcc-win32.txt.





Next: Clang, Previous: Introduction, Up: Top   [Contents][Index]


2 Command line invocation


2.1 Quick start


usage: tcc [options] [infile1 infile2…] [-run infile args…]


TCC options are a very much like gcc options. The main difference is that TCC
can also execute directly the resulting program and give it runtime
arguments.

Here are some examples to understand the logic:


‘tcc -run a.c’
Compile a.c and execute it directly


‘tcc -run a.c arg1’
Compile a.c and execute it directly. arg1 is given as first argument to
the main() of a.c.


‘tcc a.c -run b.c arg1’
Compile a.c and b.c, link them together and execute them. arg1 is given
as first argument to the main() of the resulting program. 


‘tcc -o myprog a.c b.c’
Compile a.c and b.c, link them and generate the executable myprog.


‘tcc -o myprog a.o b.o’
link a.o and b.o together and generate the executable myprog.


‘tcc -c a.c’
Compile a.c and generate object file a.o.


‘tcc -c asmfile.S’
Preprocess with C preprocess and assemble asmfile.S and generate
object file asmfile.o.


‘tcc -c asmfile.s’
Assemble (but not preprocess) asmfile.s and generate object file
asmfile.o.


‘tcc -r -o ab.o a.c b.c’
Compile a.c and b.c, link them together and generate the object file ab.o.




Scripting:

TCC can be invoked from scripts, just as shell scripts. You just
need to add #!/usr/local/bin/tcc -run at the start of your C source:


#!/usr/local/bin/tcc -run
#include <stdio.h>

int main() 
{
    printf("Hello World\n");
    return 0;
}


TCC can read C source code from standard input when - is used in 
place of infile. Example:


echo 'main(){puts("hello");}' | tcc -run -



2.2 Option summary

General Options:


-c
Generate an object file.


-o outfile
Put object file, executable, or dll into output file outfile.


-run source [args...]
Compile file source and run it with the command line arguments
args. In order to be able to give more than one argument to a
script, several TCC options can be given after the
-run option, separated by spaces:

tcc "-run -L/usr/X11R6/lib -lX11" ex4.c

In a script, it gives the following header:

#!/usr/local/bin/tcc -run -L/usr/X11R6/lib -lX11



-v
Display TCC version.


-vv
Show included files.  As sole argument, print search dirs.  -vvv shows tries too.


-bench
Display compilation statistics.




Preprocessor options:


-Idir
Specify an additional include path. Include paths are searched in the
order they are specified.

System include paths are always searched after. The default system
include paths are: /usr/local/include, /usr/include
and PREFIX/lib/tcc/include. (PREFIX is usually
/usr or /usr/local).


-Dsym[=val]
Define preprocessor symbol ‘sym’ to
val. If val is not present, its value is ‘1’. Function-like macros can
also be defined: -DF(a)=a+1


-Usym
Undefine preprocessor symbol ‘sym’.


-E
Preprocess only, to stdout or file (with -o).




Compilation flags:

Note: each of the following options has a negative form beginning with
-fno-.


-funsigned-char
Let the char type be unsigned.


-fsigned-char
Let the char type be signed.


-fno-common
Do not generate common symbols for uninitialized data.


-fleading-underscore
Add a leading underscore at the beginning of each C symbol.


-fms-extensions
Allow a MS C compiler extensions to the language. Currently this
assumes a nested named structure declaration without an identifier
behaves like an unnamed one.


-fdollars-in-identifiers
Allow dollar signs in identifiers




Warning options:


-w
Disable all warnings.




Note: each of the following warning options has a negative form beginning with
-Wno-.


-Wimplicit-function-declaration
Warn about implicit function declaration.


-Wunsupported
Warn about unsupported GCC features that are ignored by TCC.


-Wwrite-strings
Make string constants be of type const char * instead of char
*.


-Werror
Abort compilation if warnings are issued.


-Wall
Activate all warnings, except -Werror, -Wunusupported and
-Wwrite-strings.




Linker options:


-Ldir
Specify an additional static library path for the -l option. The
default library paths are /usr/local/lib, /usr/lib and /lib.


-lxxx
Link your program with dynamic library libxxx.so or static library
libxxx.a. The library is searched in the paths specified by the
-L option and LIBRARY_PATH variable.


-Bdir
Set the path where the tcc internal libraries (and include files) can be
found (default is PREFIX/lib/tcc).


-shared
Generate a shared library instead of an executable.


-soname name
set name for shared library to be used at runtime


-static
Generate a statically linked executable (default is a shared linked
executable).


-rdynamic
Export global symbols to the dynamic linker. It is useful when a library
opened with dlopen() needs to access executable symbols.


-r
Generate an object file combining all input files.


-Wl,-rpath=path
Put custom search path for dynamic libraries into executable.


-Wl,--enable-new-dtags
When putting a custom search path for dynamic libraries into the executable,
create the new ELF dynamic tag DT_RUNPATH instead of the old legacy DT_RPATH.


-Wl,--oformat=fmt
Use fmt as output format. The supported output formats are:

elf32-i386
ELF output format (default)

binary
Binary image (only for executable output)

coff
COFF output format (only for executable output for TMS320C67xx target)




-Wl,-subsystem=console/gui/wince/...
Set type for PE (Windows) executables.


-Wl,-[Ttext=# | section-alignment=# | file-alignment=# | image-base=# | stack=#]
Modify executable layout.


-Wl,-Bsymbolic
Set DT_SYMBOLIC tag.


-Wl,-(no-)whole-archive
Turn on/off linking of all objects in archives.




Debugger options:


-g
Generate run time debug information so that you get clear run time
error messages:  test.c:68: in function 'test5()': dereferencing
invalid pointer instead of the laconic Segmentation
fault.


-b
Generate additional support code to check
memory allocations and array/pointer bounds. -g is implied. Note
that the generated code is slower and bigger in this case.

Note: -b is only available on i386 when using libtcc for the moment.


-bt N
Display N callers in stack traces. This is useful with -g or
-b.




Misc options:


-MD
Generate makefile fragment with dependencies.


-MF depfile
Use depfile as output for -MD.


-print-search-dirs
Print the configured installation directory and a list of library
and include directories tcc will search.


-dumpversion
Print version.




Target specific options:


-mms-bitfields
Use an algorithm for bitfield alignment consistent with MSVC. Default is
gcc’s algorithm.


-mfloat-abi (ARM only)
Select the float ABI. Possible values: softfp and hard


-mno-sse
Do not use sse registers on x86_64


-m32, -m64
Pass command line to the i386/x86_64 cross compiler.




Note: GCC options -Ox, -fx and -mx are
ignored.

Environment variables that affect how tcc operates.


CPATH
C_INCLUDE_PATH
A colon-separated list of directories searched for include files,
directories given with -I are searched first.


LIBRARY_PATH
A colon-separated list of directories searched for libraries for the
-l option, directories given with -L are searched first.










Next: asm, Previous: Invoke, Up: Top   [Contents][Index]


3 C language support


3.1 ANSI C

TCC implements all the ANSI C standard, including structure bit fields
and floating point numbers (long double, double, and
float fully supported).


3.2 ISOC99 extensions

TCC implements many features of the new C standard: ISO C99. Currently
missing items are: complex and imaginary numbers.

Currently implemented ISOC99 features:


 variable length arrays.

 64 bit long long types are fully supported.

 The boolean type _Bool is supported.

 __func__ is a string variable containing the current
function name.

 Variadic macros: __VA_ARGS__ can be used for
   function-like macros:

    #define dprintf(level, __VA_ARGS__) printf(__VA_ARGS__)


dprintf can then be used with a variable number of parameters.

 Declarations can appear anywhere in a block (as in C++).

 Array and struct/union elements can be initialized in any order by
  using designators:

    struct { int x, y; } st[10] = { [0].x = 1, [0].y = 2 };

    int tab[10] = { 1, 2, [5] = 5, [9] = 9};

    
 Compound initializers are supported:

    int *p = (int []){ 1, 2, 3 };

to initialize a pointer pointing to an initialized array. The same
works for structures and strings.

 Hexadecimal floating point constants are supported:

          double d = 0x1234p10;


is the same as writing 

          double d = 4771840.0;


 inline keyword is ignored.

 restrict keyword is ignored.



3.3 GNU C extensions

TCC implements some GNU C extensions:


 array designators can be used without ’=’: 

    int a[10] = { [0] 1, [5] 2, 3, 4 };


 Structure field designators can be a label: 

    struct { int x, y; } st = { x: 1, y: 1};

instead of

    struct { int x, y; } st = { .x = 1, .y = 1};


 \e is ASCII character 27.

 case ranges : ranges can be used in cases:

    switch(a) {
    case 1 … 9:
          printf("range 1 to 9\n");
          break;
    default:
          printf("unexpected\n");
          break;
    }











 The keyword __attribute__ is handled to specify variable or
function attributes. The following attributes are supported:
  
 aligned(n): align a variable or a structure field to n bytes
(must be a power of two).

 packed: force alignment of a variable or a structure field to
  1.

 section(name): generate function or data in assembly section
name (name is a string containing the section name) instead of the default
section.

 unused: specify that the variable or the function is unused.

 cdecl: use standard C calling convention (default).

 stdcall: use Pascal-like calling convention.

 regparm(n): use fast i386 calling convention. n must be
between 1 and 3. The first n function parameters are respectively put in
registers %eax, %edx and %ecx.

 dllexport: export function from dll/executable (win32 only)



Here are some examples:

    int a __attribute__ ((aligned(8), section(".mysection")));


align variable a to 8 bytes and put it in section .mysection.


    int my_add(int a, int b) __attribute__ ((section(".mycodesection"))) 
    {
        return a + b;
    }


generate function my_add in section .mycodesection.

 GNU style variadic macros:

    #define dprintf(fmt, args…) printf(fmt, ## args)

    dprintf("no arg\n");
    dprintf("one arg %d\n", 1);


 __FUNCTION__ is interpreted as C99 __func__ 
(so it has not exactly the same semantics as string literal GNUC
where it is a string literal).

 The __alignof__ keyword can be used as sizeof 
to get the alignment of a type or an expression.

 The typeof(x) returns the type of x. 
x is an expression or a type.

 Computed gotos: &&label returns a pointer of type 
void * on the goto label label. goto *expr can be
used to jump on the pointer resulting from expr.

 Inline assembly with asm instruction:




static inline void * my_memcpy(void * to, const void * from, size_t n)
{
int d0, d1, d2;
__asm__ __volatile__(
        "rep ; movsl\n\t"
        "testb $2,%b4\n\t"
        "je 1f\n\t"
        "movsw\n"
        "1:\ttestb $1,%b4\n\t"
        "je 2f\n\t"
        "movsb\n"
        "2:"
        : "=&c" (d0), "=&D" (d1), "=&S" (d2)
        :"0" (n/4), "q" (n),"1" ((long) to),"2" ((long) from)
        : "memory");
return (to);
}



TCC includes its own x86 inline assembler with a gas-like (GNU
assembler) syntax. No intermediate files are generated. GCC 3.x named
operands are supported.

 __builtin_types_compatible_p() and __builtin_constant_p() 
are supported.

 #pragma pack is supported for win32 compatibility.




3.4 TinyCC extensions


 __TINYC__ is a predefined macro to indicate that you use TCC.

 #! at the start of a line is ignored to allow scripting.

 Binary digits can be entered (0b101 instead of
5).

 __BOUNDS_CHECKING_ON is defined if bound checking is activated.







Next: linker, Previous: Clang, Up: Top   [Contents][Index]


4 TinyCC Assembler

Since version 0.9.16, TinyCC integrates its own assembler. TinyCC
assembler supports a gas-like syntax (GNU assembler). You can
deactivate assembler support if you want a smaller TinyCC executable
(the C compiler does not rely on the assembler).

TinyCC Assembler is used to handle files with .S (C
preprocessed assembler) and .s extensions. It is also used to
handle the GNU inline assembler with the asm keyword.


4.1 Syntax

TinyCC Assembler supports most of the gas syntax. The tokens are the
same as C.


 C and C++ comments are supported.

 Identifiers are the same as C, so you cannot use ’.’ or ’$’.

 Only 32 bit integer numbers are supported.




4.2 Expressions


 Integers in decimal, octal and hexa are supported.

 Unary operators: +, -, ~.

 Binary operators in decreasing priority order:


 *, /, %
 &, |, ^
 +, -


 A value is either an absolute number or a label plus an offset. 
All operators accept absolute values except ’+’ and ’-’. ’+’ or ’-’ can be
used to add an offset to a label. ’-’ supports two labels only if they
are the same or if they are both defined and in the same section.




4.3 Labels


 All labels are considered as local, except undefined ones.

 Numeric labels can be used as local gas-like labels. 
They can be defined several times in the same source. Use ’b’
(backward) or ’f’ (forward) as suffix to reference them:


 1:
      jmp 1b /* jump to '1' label before */
      jmp 1f /* jump to '1' label after */
 1:





4.4 Directives
























All directives are preceded by a ’.’. The following directives are
supported:


 .align n[,value]
 .skip n[,value]
 .space n[,value]
 .byte value1[,...]
 .word value1[,...]
 .short value1[,...]
 .int value1[,...]
 .long value1[,...]
 .quad immediate_value1[,...]
 .globl symbol
 .global symbol
 .section section
 .text
 .data
 .bss
 .fill repeat[,size[,value]]
 .org n
 .previous
 .string string[,...]
 .asciz string[,...]
 .ascii string[,...]



4.5 X86 Assembler


All X86 opcodes are supported. Only ATT syntax is supported (source
then destination operand order). If no size suffix is given, TinyCC
tries to guess it from the operand sizes.

Currently, MMX opcodes are supported but not SSE ones.





Next: Bounds, Previous: asm, Up: Top   [Contents][Index]


5 TinyCC Linker



5.1 ELF file generation


TCC can directly output relocatable ELF files (object files),
executable ELF files and dynamic ELF libraries without relying on an
external linker.

Dynamic ELF libraries can be output but the C compiler does not generate
position independent code (PIC). It means that the dynamic library
code generated by TCC cannot be factorized among processes yet.

TCC linker eliminates unreferenced object code in libraries. A single pass is
done on the object and library list, so the order in which object files and
libraries are specified is important (same constraint as GNU ld). No grouping
options (--start-group and --end-group) are supported.


5.2 ELF file loader

TCC can load ELF object files, archives (.a files) and dynamic
libraries (.so).


5.3 PE-i386 file generation


TCC for Windows supports the native Win32 executable file format (PE-i386).  It
generates EXE files (console and gui) and DLL files.

For usage on Windows, see also tcc-win32.txt.


5.4 GNU Linker Scripts







Because on many Linux systems some dynamic libraries (such as
/usr/lib/libc.so) are in fact GNU ld link scripts (horrible!),
the TCC linker also supports a subset of GNU ld scripts.

The GROUP and FILE commands are supported. OUTPUT_FORMAT
and TARGET are ignored.

Example from /usr/lib/libc.so:

/* GNU ld script
   Use the shared library, but some functions are only in
   the static library, so try that secondarily.  */
GROUP ( /lib/libc.so.6 /usr/lib/libc_nonshared.a )






Next: Libtcc, Previous: linker, Up: Top   [Contents][Index]


6 TinyCC Memory and Bound checks



This feature is activated with the -b (see Invoke).

Note that pointer size is unchanged and that code generated
with bound checks is fully compatible with unchecked
code. When a pointer comes from unchecked code, it is assumed to be
valid. Even very obscure C code with casts should work correctly.

For more information about the ideas behind this method, see
http://www.doc.ic.ac.uk/~phjk/BoundsChecking.html.

Here are some examples of caught errors:


Invalid range with standard string function:

{
    char tab[10];
    memset(tab, 0, 11);
}



Out of bounds-error in global or local arrays:

{
    int tab[10];
    for(i=0;i<11;i++) {
        sum += tab[i];
    }
}



Out of bounds-error in malloc’ed data:

{
    int *tab;
    tab = malloc(20 * sizeof(int));
    for(i=0;i<21;i++) {
        sum += tab4[i];
    }
    free(tab);
}



Access of freed memory:

{
    int *tab;
    tab = malloc(20 * sizeof(int));
    free(tab);
    for(i=0;i<20;i++) {
        sum += tab4[i];
    }
}



Double free:

{
    int *tab;
    tab = malloc(20 * sizeof(int));
    free(tab);
    free(tab);
}









Next: devel, Previous: Bounds, Up: Top   [Contents][Index]


7 The libtcc library

The libtcc library enables you to use TCC as a backend for
dynamic code generation. 

Read the libtcc.h to have an overview of the API. Read
libtcc_test.c to have a very simple example.

The idea consists in giving a C string containing the program you want
to compile directly to libtcc. Then you can access to any global
symbol (function or variable) defined.





Previous: Libtcc, Up: Top   [Contents][Index]


8 Developer’s guide

This chapter gives some hints to understand how TCC works. You can skip
it if you do not intend to modify the TCC code.


8.1 File reading

The BufferedFile structure contains the context needed to read a
file, including the current line number. tcc_open() opens a new
file and tcc_close() closes it. inp() returns the next
character.


8.2 Lexer

next() reads the next token in the current
file. next_nomacro() reads the next token without macro
expansion.

tok contains the current token (see TOK_xxx)
constants. Identifiers and keywords are also keywords. tokc
contains additional infos about the token (for example a constant value
if number or string token).


8.3 Parser

The parser is hardcoded (yacc is not necessary). It does only one pass,
except:


 For initialized arrays with unknown size, a first pass 
is done to count the number of elements.

 For architectures where arguments are evaluated in 
reverse order, a first pass is done to reverse the argument order.




8.4 Types

The types are stored in a single ’int’ variable. It was chosen in the
first stages of development when tcc was much simpler. Now, it may not
be the best solution.


#define VT_INT        0  /* integer type */
#define VT_BYTE       1  /* signed byte type */
#define VT_SHORT      2  /* short type */
#define VT_VOID       3  /* void type */
#define VT_PTR        4  /* pointer */
#define VT_ENUM       5  /* enum definition */
#define VT_FUNC       6  /* function type */
#define VT_STRUCT     7  /* struct/union definition */
#define VT_FLOAT      8  /* IEEE float */
#define VT_DOUBLE     9  /* IEEE double */
#define VT_LDOUBLE   10  /* IEEE long double */
#define VT_BOOL      11  /* ISOC99 boolean type */
#define VT_LLONG     12  /* 64 bit integer */
#define VT_LONG      13  /* long integer (NEVER USED as type, only
                            during parsing) */
#define VT_BTYPE      0x000f /* mask for basic type */
#define VT_UNSIGNED   0x0010  /* unsigned type */
#define VT_ARRAY      0x0020  /* array type (also has VT_PTR) */
#define VT_VLA        0x20000 /* VLA type (also has VT_PTR and VT_ARRAY) */
#define VT_BITFIELD   0x0040  /* bitfield modifier */
#define VT_CONSTANT   0x0800  /* const modifier */
#define VT_VOLATILE   0x1000  /* volatile modifier */
#define VT_DEFSIGN    0x2000  /* signed type */

#define VT_STRUCT_SHIFT 18   /* structure/enum name shift (14 bits left) */


When a reference to another type is needed (for pointers, functions and
structures), the 32 - VT_STRUCT_SHIFT high order bits are used to
store an identifier reference.

The VT_UNSIGNED flag can be set for chars, shorts, ints and long
longs.

Arrays are considered as pointers VT_PTR with the flag
VT_ARRAY set. Variable length arrays are considered as special
arrays and have flag VT_VLA set instead of VT_ARRAY.

The VT_BITFIELD flag can be set for chars, shorts, ints and long
longs. If it is set, then the bitfield position is stored from bits
VT_STRUCT_SHIFT to VT_STRUCT_SHIFT + 5 and the bit field size is stored
from bits VT_STRUCT_SHIFT + 6 to VT_STRUCT_SHIFT + 11.

VT_LONG is never used except during parsing.

During parsing, the storage of an object is also stored in the type
integer:


#define VT_EXTERN  0x00000080  /* extern definition */
#define VT_STATIC  0x00000100  /* static variable */
#define VT_TYPEDEF 0x00000200  /* typedef definition */
#define VT_INLINE  0x00000400  /* inline definition */
#define VT_IMPORT  0x00004000  /* win32: extern data imported from dll */
#define VT_EXPORT  0x00008000  /* win32: data exported from dll */
#define VT_WEAK    0x00010000  /* win32: data exported from dll */



8.5 Symbols

All symbols are stored in hashed symbol stacks. Each symbol stack
contains Sym structures.

Sym.v contains the symbol name (remember
an identifier is also a token, so a string is never necessary to store
it). Sym.t gives the type of the symbol. Sym.r is usually
the register in which the corresponding variable is stored. Sym.c is
usually a constant associated to the symbol like its address for normal
symbols, and the number of entries for symbols representing arrays.
Variable length array types use Sym.c as a location on the stack
which holds the runtime sizeof for the type.

Four main symbol stacks are defined:


define_stack
for the macros (#defines).


global_stack
for the global variables, functions and types.


local_stack
for the local variables, functions and types.


global_label_stack
for the local labels (for goto).


label_stack
for GCC block local labels (see the __label__ keyword).




sym_push() is used to add a new symbol in the local symbol
stack. If no local symbol stack is active, it is added in the global
symbol stack.

sym_pop(st,b) pops symbols from the symbol stack st until
the symbol b is on the top of stack. If b is NULL, the stack
is emptied.

sym_find(v) return the symbol associated to the identifier
v. The local stack is searched first from top to bottom, then the
global stack.


8.6 Sections

The generated code and data are written in sections. The structure
Section contains all the necessary information for a given
section. new_section() creates a new section. ELF file semantics
is assumed for each section.

The following sections are predefined:


text_section
is the section containing the generated code. ind contains the
current position in the code section.


data_section
contains initialized data


bss_section
contains uninitialized data


bounds_section
lbounds_section
are used when bound checking is activated


stab_section
stabstr_section
are used when debugging is active to store debug information


symtab_section
strtab_section
contain the exported symbols (currently only used for debugging).





8.7 Code generation



8.7.1 Introduction

The TCC code generator directly generates linked binary code in one
pass. It is rather unusual these days (see gcc for example which
generates text assembly), but it can be very fast and surprisingly
little complicated.

The TCC code generator is register based. Optimization is only done at
the expression level. No intermediate representation of expression is
kept except the current values stored in the value stack.

On x86, three temporary registers are used. When more registers are
needed, one register is spilled into a new temporary variable on the stack.


8.7.2 The value stack


When an expression is parsed, its value is pushed on the value stack
(vstack). The top of the value stack is vtop. Each value
stack entry is the structure SValue.

SValue.t is the type. SValue.r indicates how the value is
currently stored in the generated code. It is usually a CPU register
index (REG_xxx constants), but additional values and flags are
defined:


#define VT_CONST     0x00f0
#define VT_LLOCAL    0x00f1
#define VT_LOCAL     0x00f2
#define VT_CMP       0x00f3
#define VT_JMP       0x00f4
#define VT_JMPI      0x00f5
#define VT_LVAL      0x0100
#define VT_SYM       0x0200
#define VT_MUSTCAST  0x0400
#define VT_MUSTBOUND 0x0800
#define VT_BOUNDED   0x8000
#define VT_LVAL_BYTE     0x1000
#define VT_LVAL_SHORT    0x2000
#define VT_LVAL_UNSIGNED 0x4000
#define VT_LVAL_TYPE     (VT_LVAL_BYTE | VT_LVAL_SHORT | VT_LVAL_UNSIGNED)



VT_CONST
indicates that the value is a constant. It is stored in the union
SValue.c, depending on its type.


VT_LOCAL
indicates a local variable pointer at offset SValue.c.i in the
stack.


VT_CMP
indicates that the value is actually stored in the CPU flags (i.e. the
value is the consequence of a test). The value is either 0 or 1. The
actual CPU flags used is indicated in SValue.c.i. 

If any code is generated which destroys the CPU flags, this value MUST be
put in a normal register.


VT_JMP
VT_JMPI
indicates that the value is the consequence of a conditional jump. For VT_JMP,
it is 1 if the jump is taken, 0 otherwise. For VT_JMPI it is inverted.

These values are used to compile the || and && logical
operators.

If any code is generated, this value MUST be put in a normal
register. Otherwise, the generated code won’t be executed if the jump is
taken.


VT_LVAL
is a flag indicating that the value is actually an lvalue (left value of
an assignment). It means that the value stored is actually a pointer to
the wanted value. 

Understanding the use VT_LVAL is very important if you want to
understand how TCC works.


VT_LVAL_BYTE
VT_LVAL_SHORT
VT_LVAL_UNSIGNED
if the lvalue has an integer type, then these flags give its real
type. The type alone is not enough in case of cast optimisations.


VT_LLOCAL
is a saved lvalue on the stack. VT_LVAL must also be set with
VT_LLOCAL. VT_LLOCAL can arise when a VT_LVAL in
a register has to be saved to the stack, or it can come from an
architecture-specific calling convention.


VT_MUSTCAST
indicates that a cast to the value type must be performed if the value
is used (lazy casting).


VT_SYM
indicates that the symbol SValue.sym must be added to the constant.


VT_MUSTBOUND
VT_BOUNDED
are only used for optional bound checking.





8.7.3 Manipulating the value stack


vsetc() and vset() pushes a new value on the value
stack. If the previous vtop was stored in a very unsafe place(for
example in the CPU flags), then some code is generated to put the
previous vtop in a safe storage.

vpop() pops vtop. In some cases, it also generates cleanup
code (for example if stacked floating point registers are used as on
x86).

The gv(rc) function generates code to evaluate vtop (the
top value of the stack) into registers. rc selects in which
register class the value should be put. gv() is the most
important function of the code generator.

gv2() is the same as gv() but for the top two stack
entries.


8.7.4 CPU dependent code generation

See the i386-gen.c file to have an example.


load()
must generate the code needed to load a stack value into a register.


store()
must generate the code needed to store a register into a stack value
lvalue.


gfunc_start()
gfunc_param()
gfunc_call()
should generate a function call


gfunc_prolog()
gfunc_epilog()
should generate a function prolog/epilog.


gen_opi(op)
must generate the binary integer operation op on the two top
entries of the stack which are guaranteed to contain integer types.

The result value should be put on the stack.


gen_opf(op)
same as gen_opi() for floating point operations. The two top
entries of the stack are guaranteed to contain floating point values of
same types.


gen_cvt_itof()
integer to floating point conversion.


gen_cvt_ftoi()
floating point to integer conversion.


gen_cvt_ftof()
floating point to floating point of different size conversion.


gen_bounded_ptr_add()
gen_bounded_ptr_deref()
are only used for bounds checking.





8.8 Optimizations done







Constant propagation is done for all operations. Multiplications and
divisions are optimized to shifts when appropriate. Comparison
operators are optimized by maintaining a special cache for the
processor flags. &&, || and ! are optimized by maintaining a special
’jump target’ value. No other jump optimization is currently performed
because it would require to store the code in a more abstract fashion.


Concept Index
Jump to:   _
   


A
   
B
   
C
   
D
   
E
   
F
   
G
   
I
   
J
   
L
   
M
   
O
   
P
   
Q
   
R
   
S
   
T
   
U
   
V
   
W
   


Index Entry    Section
 
_ 
__asm__:   Clang
 
A 
align directive:   asm
aligned attribute:   Clang
ascii directive:   asm
asciz directive:   asm
assembler:   asm
assembler directives:   asm
assembly, inline:   Clang
 
B 
bound checks:   Bounds
bss directive:   asm
byte directive:   asm
 
C 
caching processor flags:   devel
cdecl attribute:   Clang
code generation:   devel
comparison operators:   devel
constant propagation:   devel
CPU dependent:   devel
 
D 
data directive:   asm
directives, assembler:   asm
dllexport attribute:   Clang
 
E 
ELF:   linker
 
F 
FILE, linker command:   linker
fill directive:   asm
flags, caching:   devel
 
G 
gas:   Clang
global directive:   asm
globl directive:   asm
GROUP, linker command:   linker
 
I 
inline assembly:   Clang
int directive:   asm
 
J 
jump optimization:   devel
 
L 
linker:   linker
linker scripts:   linker
long directive:   asm
 
M 
memory checks:   Bounds
 
O 
optimizations:   devel
org directive:   asm
OUTPUT_FORMAT, linker command:   linker
 
P 
packed attribute:   Clang
PE-i386:   linker
previous directive:   asm
 
Q 
quad directive:   asm
 
R 
regparm attribute:   Clang
 
S 
scripts, linker:   linker
section attribute:   Clang
section directive:   asm
short directive:   asm
skip directive:   asm
space directive:   asm
stdcall attribute:   Clang
strength reduction:   devel
string directive:   asm
 
T 
TARGET, linker command:   linker
text directive:   asm
 
U 
unused attribute:   Clang
 
V 
value stack:   devel
value stack, introduction:   devel
 
W 
word directive:   asm
 

Jump to:   _
   


A
   
B
   
C
   
D
   
E
   
F
   
G
   
I
   
J
   
L
   
M
   
O
   
P
   
Q
   
R
   
S
   
T
   
U
   
V
   
W
• Introduction:		Introduction to tcc.
• Invoke:		Invocation of tcc (command line, options).
• Clang:		ANSI C and extensions.
• asm:		Assembler syntax.
• linker:		Output file generation and supported targets.
• Bounds:		Automatic bounds-checking of C code.
• Libtcc:		The libtcc library.
• devel:		Guide for Developers.
	Index Entry	Section

_
	__asm__:	Clang

A
	align directive:	asm
	aligned attribute:	Clang
	ascii directive:	asm
	asciz directive:	asm
	assembler:	asm
	assembler directives:	asm
	assembly, inline:	Clang

B
	bound checks:	Bounds
	bss directive:	asm
	byte directive:	asm

C
	caching processor flags:	devel
	cdecl attribute:	Clang
	code generation:	devel
	comparison operators:	devel
	constant propagation:	devel
	CPU dependent:	devel

D
	data directive:	asm
	directives, assembler:	asm
	dllexport attribute:	Clang

E
	ELF:	linker

F
	FILE, linker command:	linker
	fill directive:	asm
	flags, caching:	devel

G
	gas:	Clang
	global directive:	asm
	globl directive:	asm
	GROUP, linker command:	linker

I
	inline assembly:	Clang
	int directive:	asm

J
	jump optimization:	devel

L
	linker:	linker
	linker scripts:	linker
	long directive:	asm

M
	memory checks:	Bounds

O
	optimizations:	devel
	org directive:	asm
	OUTPUT_FORMAT, linker command:	linker

P
	packed attribute:	Clang
	PE-i386:	linker
	previous directive:	asm

Q
	quad directive:	asm

R
	regparm attribute:	Clang

S
	scripts, linker:	linker
	section attribute:	Clang
	section directive:	asm
	short directive:	asm
	skip directive:	asm
	space directive:	asm
	stdcall attribute:	Clang
	strength reduction:	devel
	string directive:	asm

T
	TARGET, linker command:	linker
	text directive:	asm

U
	unused attribute:	Clang

V
	value stack:	devel
	value stack, introduction:	devel

W
	word directive:	asm