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- Use -Q -Ox --help=optimizers to determine which optimization flags are enabled for a given -Ox setting (or read the info pages).
- Dataflow analysis is only performed at -O2 or above, and thus at least this level of optimization is necessary for use-of-uninitialized-variable warnings and such.
Extensions to C
- __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 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.
- 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.
- 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
- 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)