The as-if rule
Allows any and all code transformations that do not change the observable behavior of the program.
Explanation
The C++ compiler is permitted to perform any changes to the program as long as the following remains true:
|
1) At every sequence point, the values of all volatile objects are stable (previous evaluations are complete, new evaluations not started).
|
(until C++11) |
|
1) Accesses (reads and writes) to volatile objects occur strictly according to the semantics of the expressions in which they occur. In particular, they are not reordered with respect to other volatile accesses on the same thread.
|
(since C++11) |
ON, the changes to the floating-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 (see FLT_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 is
OFF).
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, e.g. due to access to an array out of bounds, modification of a const object, evaluation order violations, etc, are free from the as-if rule: they 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 because signed 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.
|
New-expression has another exception from the as-if rule: the compiler may remove calls to the replaceable allocation functions even if a user-defined replacement is provided and has observable side-effects. |
(since C++14) |
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 ON for (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;
Example
int& preinc(int& n) { return ++n; } int add(int n, int m) { return n + m; } // volatile input to prevent constant folding volatile int input = 7; // volatile output to make the result a visible side-effect volatile int 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 overlapped int m = add(preinc(n), preinc(n)); result = m; }
Output:
# 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;