Allows any and all code transformations that do not change the observable behavior of the program.

[edit]Explanation

Observable behavior of a program includes the following:

• Prompting text which is sent to interactive devices will be shown before the program waits for input.
• If the ISO C pragma#pragma STDC FENV_ACCESS is supported and is set toON, the changes to thefloating-point environment (floating-point exceptions and rounding modes) are guaranteed to be observed by the floating-point arithmetic operators and function calls as if executed as written, except that
  • the result of any floating-point expression other than cast and assignment may have range and precision of a floating-point type different from the type of the expression (seeFLT_EVAL_METHOD),
  • notwithstanding the above, intermediate results of any floating-point expression may be calculated as if to infinite range and precision (unless#pragma STDC FP_CONTRACT isOFF).

[edit]Notes

Because the compiler is (usually) unable to analyze the code of an external library to determine whether it does or does not perform I/O or volatile access, third-party library calls also aren't affected by optimization. However, standard library calls may be replaced by other calls, eliminated, or added to the program during optimization. Statically-linked third-party library code may be subject to link-time optimization.

Programs with undefined behavior often change observable behavior when recompiled with different optimization settings. For example, if a test for signed integer overflow relies on the result of that overflow, e.g.if(n+1< n) abort();,it is removed entirely by some compilers becausesigned overflow is undefined behavior and the optimizer is free to assume it never happens and the test is redundant.

Copy elision is an exception from the as-if rule: the compiler may remove calls to move- and copy-constructors and the matching calls to the destructors of temporary objects even if those calls have observable side effects.

The count and order of floating-point exceptions can be changed by optimization as long as the state as observed by the next floating-point operation is as if no optimization took place:

#pragma STDC FENV_ACCESS ONfor(i=0; i< n;++i)    x+1;// x + 1 is dead code, but may raise FP exceptions// (unless the optimizer can prove otherwise). However, executing it n times// will raise the same exception over and over. So this can be optimized to:if(0< n)    x+1;

[edit]Example

int& preinc(int& n){return++n;}int add(int n,int m){return n+ m;} // volatile input to prevent constant foldingvolatileint input=7; // volatile output to make the result a visible side-effectvolatileint result; int main(){int n= input;// using built-in operators would invoke undefined behavior//  int m = ++n + ++n;// but using functions makes sure the code executes as-if// the functions were not overlappedint m= add(preinc(n), preinc(n));    result= m;}

# full code of the main() function as produced by the GCC compiler# x86 (Intel) platform:        movl    input(%rip), %eax   # eax = input        leal    3(%rax,%rax), %eax  # eax = 3 + eax + eax        movl    %eax, result(%rip)  # result = eax        xorl    %eax, %eax          # eax = 0 (the return value of main())        ret # PowerPC (IBM) platform:        lwz 9,LC..1(2)        li 3,0          # r3 = 0 (the return value of main())        lwz 11,0(9)     # r11 = input;        slwi 11,11,1    # r11 = r11 << 1;        addi 0,11,3     # r0 = r11 + 3;        stw 0,4(9)      # result = r0;        blr # Sparc (Sun) platform:        sethi   %hi(result), %g2        sethi   %hi(input), %g1        mov     0, %o0                 # o0 = 0 (the return value of main)        ld      [%g1+%lo(input)], %g1  # g1 = input        add     %g1, %g1, %g1          # g1 = g1 + g1        add     %g1, 3, %g1            # g1 = 3 + g1        st      %g1, [%g2+%lo(result)] # result = g1        jmp     %o7+8        nop # in all cases, the side effects of preinc() were eliminated, and the# entire main() function was reduced to the equivalent of result = 2 * input + 3;

[edit]See also

• copy elision

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