GCC

Extensions to C

Extensions to the C language are documented at the online gcc docs, and in the Info pages distributed with gcc.

__builtin_ functions

• __builtin_expect(expr,expectedp) - Instruct gcc that expr is or is not likely to be true (0 for unlikely), affecting generation of conditional code (normally, gcc assumes that if conditionals are taken in most/all cases).

Attributes

Attributes are preceded by the keyword __attribute__ and in some cases followed by a parenthesized argument list; the attribute name and any argument list are both then enclosed within double parentheses. All are non-standard extensions.

Function Attributes

• See the gcc documentation at http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
• aligned - takes a single parameter, the minimum number of bytes at which to align the function. -falign-functions will override this, if larger.
• malloc - indicates that any value returned does not alias any other currently-valid pointers
• alloc_size - takes one or two parameters and indicates that the function will return a pointer to an allocated chunk of memory having either the size provided by a single argument, or the product of two arguments. This is necessary for __builtin_object_size's correct functioning.
• pure (2.96) - indicates that the function has no side-effects save its return value, which is based only on calls to other pure functions, the function's own parameters and/or non-volatile global memory.
• const (2.5) - stronger than pure, a pure function which does not dereference any pointer parameters, use global memory or call non-const functions.
• warn_unused_result - warn if the return value is not used, for instance in a wrapper to open(2) or malloc(3)
• cold (4.3) - indicates the code does not lie on any hotpaths, resulting in optimization for size, location within the .text section, and automatic application of __builtin_expect((x),0) to conditionals on a calling path. Disabled by -fprofile-use.
• hot (4.3) - opposite of cold.
• nothrow (3.3) - marks the function as never throwing an exception, for optimization purposes.
• noreturn (2.5) - marks the function as never normally returning (longjmp(3) and exceptions may still be used).
• unused - a function is (possibly) unused. Calls may still be made to it, but -Wunused-functions warnings will not be generated.

Inline Assembly

• There's no need to use inline assembly for SIMD; use Target-Specific Builtins and Vector Extensions, or autovectorization if applicable.
• Functions only referenced by inline assembly might not have code generated for them; use of the used function attribute will force generation.
• The GNU assembler (gas) is used for assembly and syntax.
• Statements can be arbitrarily reordered by default, or anchored with the volatile qualifier
• outputs, inputs and clobbers are expressed in a colon-delimited list of comma-delimited lists of the form [asmsymbol] "constraints" (c symbol)
  • operand constraints are properties of the assembly code, not the values
  • without "=" or "+" constraint modifier, operands are assumed to be read-only
  • compiler verifies that all outputs are lvalues. types of operands are not checked.
  • "cc" logical register ought be listed as a clobber if the conditional code register is changed
  • "memory" must be listed as clobbered if memory is touched in an unpredictable fashion
• constraint modifiers:
  • "=": operand is write-only (previous value needn't be preserved until write)
  • "+": operand is read-write (can't be arbitrarily used)
  • "&": operand is clobbered early (prior to use of all inputs), and thus can't be placed atop an input operand's

Intermediate Representations

• RTL: The Register Transfer Language, GCC's older IR (still used for late optimization passes)
• GENERIC: A loose IR to which frontends must now compile
• GIMPLE: A restricted subset of GENERIC, on which most optimizations are performed
• Graphite: GIMPLE as Polyhedra, an optimization framework making use of polyhedral methods (especially for autovectorization)

See also

• Compiler Design
• Building Libraries
• Nick Clifton keeps a good monthly-or-so livejournal regarding the GNU toolchain
• "An Implementation of Global Value Numbering in the GNU Compiler Collection, with Performance Measurements" (Kongstad 2004)
• GNU Compiler Collection Internals