RTEMS GNU Tools On-Line Library

Optimize Options

Using and Porting the GNU Compiler Collection (GCC)

2.8: Options That Control Optimization

These options control various sorts of optimizations:

-O

-O1

Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.

Without `-O', the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.

Without `-O', the compiler only allocates variables declared register in registers. The resulting compiled code is a little worse than produced by PCC without `-O'.

With `-O', the compiler tries to reduce code size and execution time.

When you specify `-O', the compiler turns on `-fthread-jumps' and `-fdefer-pop' on all machines. The compiler turns on `-fdelayed-branch' on machines that have delay slots, and `-fomit-frame-pointer' on machines that can support debugging even without a frame pointer. On some machines the compiler also turns on other flags.

-O2

Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. The compiler does not perform loop unrolling or function inlining when you specify `-O2'. As compared to `-O', this option increases both compilation time and the performance of the generated code.

`-O2' turns on all optional optimizations except for loop unrolling, function inlining, and strict aliasing optimizations. It also turns on the `-fforce-mem' option on all machines and frame pointer elimination on machines where doing so does not interfere with debugging.

-O3

Optimize yet more. `-O3' turns on all optimizations specified by `-O2' and also turns on the `inline-functions' option.

-O0

Do not optimize.

-Os

Optimize for size. `-Os' enables all `-O2' optimizations that do not typically increase code size. It also performs further optimizations designed to reduce code size.

If you use multiple `-O' options, with or without level numbers, the last such option is the one that is effective.

Options of the form `-fflag' specify machine-independent flags. Most flags have both positive and negative forms; the negative form of `-ffoo' would be `-fno-foo'. In the table below, only one of the forms is listed---the one which is not the default. You can figure out the other form by either removing `no-' or adding it.

-ffloat-store: Do not store floating point variables in registers, and inhibit other options that might change whether a floating point value is taken from a register or memory.
This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use `-ffloat-store' for such programs, after modifying them to store all pertinent intermediate computations into variables.
-fno-default-inline: Do not make member functions inline by default merely because they are defined inside the class scope (C++ only). Otherwise, when you specify `-O', member functions defined inside class scope are compiled inline by default; i.e., you don't need to add `inline' in front of the member function name.
-fno-defer-pop: Always pop the arguments to each function call as soon as that function returns. For machines which must pop arguments after a function call, the compiler normally lets arguments accumulate on the stack for several function calls and pops them all at once.
-fforce-mem: Force memory operands to be copied into registers before doing arithmetic on them. This produces better code by making all memory references potential common subexpressions. When they are not common subexpressions, instruction combination should eliminate the separate register-load. The `-O2' option turns on this option.
-fforce-addr: Force memory address constants to be copied into registers before doing arithmetic on them. This may produce better code just as `-fforce-mem' may.
-fomit-frame-pointer: Don't keep the frame pointer in a register for functions that don't need one. This avoids the instructions to save, set up and restore frame pointers; it also makes an extra register available in many functions. It also makes debugging impossible on some machines.
On some machines, such as the Vax, this flag has no effect, because the standard calling sequence automatically handles the frame pointer and nothing is saved by pretending it doesn't exist. The machine-description macro FRAME_POINTER_REQUIRED controls whether a target machine supports this flag. See Registers.
-fno-inline: Don't pay attention to the inline keyword. Normally this option is used to keep the compiler from expanding any functions inline. Note that if you are not optimizing, no functions can be expanded inline.
-finline-functions: Integrate all simple functions into their callers. The compiler heuristically decides which functions are simple enough to be worth integrating in this way.
If all calls to a given function are integrated, and the function is declared static, then the function is normally not output as assembler code in its own right.
-finline-limit-n: By default, gcc limits the size of functions that can be inlined. This flag allows the control of this limit for functions that are explicitly marked as inline (ie marked with the inline keyword or defined within the class definition in c++). n is the size of functions that can be inlined in number of pseudo instructions (not counting parameter handling). The default value of n is 10000. Increasing this value can result in more inlined code at the cost of compilation time and memory consumption. Decreasing usually makes the compilation faster and less code will be inlined (which presumably means slower programs). This option is particularly useful for programs that use inlining heavily such as those based on recursive templates with c++.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way, it represents a count of assembly instructions and as such its exact meaning might change from one release to an another.
-fkeep-inline-functions: Even if all calls to a given function are integrated, and the function is declared static, nevertheless output a separate run-time callable version of the function. This switch does not affect extern inline functions.
-fkeep-static-consts: Emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if the variable was referenced, regardless of whether or not optimization is turned on, use the `-fno-keep-static-consts' option.
-fno-function-cse: Do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
-ffast-math: This option allows GCC to violate some ANSI or IEEE rules and/or specifications in the interest of optimizing code for speed. For example, it allows the compiler to assume arguments to the sqrt function are non-negative numbers and that no floating-point values are NaNs.
This option should never be turned on by any `-O' option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ANSI rules/specifications for math functions.

The following options control specific optimizations. The `-O2' option turns on all of these optimizations except `-funroll-loops' `-funroll-all-loops', and `-fstrict-aliasing'. On most machines, the `-O' option turns on the `-fthread-jumps' and `-fdelayed-branch' options, but specific machines may handle it differently.

You can use the following flags in the rare cases when ``fine-tuning'' of optimizations to be performed is desired.

-fstrength-reduce

Perform the optimizations of loop strength reduction and elimination of iteration variables.

-fthread-jumps

Perform optimizations where we check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false.

-fcse-follow-jumps

In common subexpression elimination, scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE will follow the jump when the condition tested is false.

-fcse-skip-blocks

This is similar to `-fcse-follow-jumps', but causes CSE to follow jumps which conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, `-fcse-skip-blocks' causes CSE to follow the jump around the body of the if.

-frerun-cse-after-loop

Re-run common subexpression elimination after loop optimizations has been performed.

-frerun-loop-opt

Run the loop optimizer twice.

-fgcse

Perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

-fexpensive-optimizations

Perform a number of minor optimizations that are relatively expensive.

-foptimize-register-moves

-fregmove

Attempt to reassign register numbers in move instructions and as operands of other simple instructions in order to maximize the amount of register tying. This is especially helpful on machines with two-operand instructions. GCC enables this optimization by default with `-O2' or higher.

Note -fregmove and -foptimize-register-moves are the same optimization.

-fdelayed-branch

If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after delayed branch instructions.

-fschedule-insns

If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to required data being unavailable. This helps machines that have slow floating point or memory load instructions by allowing other instructions to be issued until the result of the load or floating point instruction is required.

-fschedule-insns2

Similar to `-fschedule-insns', but requests an additional pass of instruction scheduling after register allocation has been done. This is especially useful on machines with a relatively small number of registers and where memory load instructions take more than one cycle.

-ffunction-sections

-fdata-sections

Place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file.

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. HPPA processors running HP-UX and Sparc processors running Solaris 2 have linkers with such optimizations. Other systems using the ELF object format as well as AIX may have these optimizations in the future.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker will create larger object and executable files and will also be slower. You will not be able to use gprof on all systems if you specify this option and you may have problems with debugging if you specify both this option and `-g'.

-fcaller-saves

Enable values to be allocated in registers that will be clobbered by function calls, by emitting extra instructions to save and restore the registers around such calls. Such allocation is done only when it seems to result in better code than would otherwise be produced.

This option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.

For all machines, optimization level 2 and higher enables this flag by default.

-funroll-loops

Perform the optimization of loop unrolling. This is only done for loops whose number of iterations can be determined at compile time or run time. `-funroll-loops' implies both `-fstrength-reduce' and `-frerun-cse-after-loop'.

-funroll-all-loops

Perform the optimization of loop unrolling. This is done for all loops and usually makes programs run more slowly. `-funroll-all-loops' implies `-fstrength-reduce' as well as `-frerun-cse-after-loop'.

-fmove-all-movables

Forces all invariant computations in loops to be moved outside the loop.

-freduce-all-givs

Forces all general-induction variables in loops to be strength-reduced.

Note: When compiling programs written in Fortran, `-fmove-all-movables' and `-freduce-all-givs' are enabled by default when you use the optimizer.

These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.

These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations.

Please let us (gcc@gcc.gnu.org and fortran@gnu.org) know how use of these options affects the performance of your production code. We're very interested in code that runs slower when these options are enabled.

-fno-peephole

Disable any machine-specific peephole optimizations.

-fbranch-probabilities

After running a program compiled with `-fprofile-arcs' (see Debugging Options), you can compile it a second time using `-fbranch-probabilities', to improve optimizations based on guessing the path a branch might take.

With `-fbranch-probabilities', GCC puts a `REG_EXEC_COUNT' note on the first instruction of each basic block, and a `REG_BR_PROB' note on each `JUMP_INSN' and `CALL_INSN'. These can be used to improve optimization. Currently, they are only used in one place: in `reorg.c', instead of guessing which path a branch is mostly to take, the `REG_BR_PROB' values are used to exactly determine which path is taken more often.

-fstrict-aliasing

Allows the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type.

Pay special attention to code like this:

union a_union { 
  int i;
  double d;
};

int f() {
  a_union t;
  t.d = 3.0;
  return t.i;
}

The practice of reading from a different union member than the one most recently written to (called ``type-punning'') is common. Even with `-fstrict-aliasing', type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:

int f() { 
  a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Every language that wishes to perform language-specific alias analysis should define a function that computes, given an tree node, an alias set for the node. Nodes in different alias sets are not allowed to alias. For an example, see the C front-end function c_get_alias_set.

Using and Porting the GNU Compiler Collection (GCC)