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#include <cstdlib>
#include <limits>
// nonconst with external linkage, to disable constant folding
extern int large_num ; // invariant: large_num > 0
extern int divisor ; // 1007000000 // invariant: divisor > 0
extern int ubound ; // upper bound on value of int_from_calculation
// invariant: ubound > 0
// invariant: ubound < ( std::numeric_limits<int>::max()  divisor )
// invariant: lower bound on value of int_from_calculation > 0
// optimal if expected average value of int_from_calculation is
// comparable to std::numeric_limits<int>::max() / 2
int foo()
{
int sum = 0 ;
for( int i=0 ; i<large_num ; ++i )
{
sum += std::rand() % ubound ;
sum %= divisor;
}
return sum ;
/*
GCC 4.9 i386 with std=c++11 pedanticerrors Wall Wextra fomitframepointer c S
__Z3foov:
subl $28, %esp
movl $0, 12(%esp)
movl $0, 8(%esp)
jmp L4
; *** loop starts here ***
L5:
call _rand
movl _ubound, %ecx
cltd
idivl %ecx ; *** division each time through the loop
movl %edx, %eax
addl %eax, 12(%esp)
movl _divisor, %ecx
movl 12(%esp), %eax
cltd
idivl %ecx
movl %edx, 12(%esp)
addl $1, 8(%esp)
L4:
movl _large_num, %eax
cmpl %eax, 8(%esp)
jl L5
; *** and ends here ***
movl 12(%esp), %eax
addl $28, %esp
ret
*/
}
// optimal if expected average value of int_from_calculation is
// much smaller than std::numeric_limits<int>::max() / 2
int bar() // ( a mod c + b mod c ) == (a+b) mod c, verifirs that a+b does not result in UB
{
int sum = 0 ;
const int maxv = std::numeric_limits<int>::max()  ubound ;
for( int i=0 ; i<large_num ; ++i )
{
sum += std::rand() % ubound ;
if( sum > maxv ) sum %= divisor ;
// or: if( sum > maxv ) sum = divisor ;
// %= is more expensive, = will execute more often
}
return sum ;
/*
GCC 4.9 i386 with std=c++11 pedanticerrors Wall Wextra fomitframepointer c S
__Z3barv:
subl $28, %esp
movl $0, 12(%esp)
call __ZNSt14numeric_limitsIiE3maxEv
movl _ubound, %edx
subl %edx, %eax
movl %eax, 4(%esp)
movl $0, 8(%esp)
jmp L8
; *** loop starts here ***
L10:
call _rand
movl _ubound, %ecx
cltd
idivl %ecx
movl %edx, %eax
addl %eax, 12(%esp)
movl 12(%esp), %eax
cmpl 4(%esp), %eax ; *** comparison each time through the loop
jle L9
; *** division only if unavoidable
movl _divisor, %ecx
movl 12(%esp), %eax
cltd
idivl %ecx
movl %edx, 12(%esp)
L9:
addl $1, 8(%esp)
L8:
movl _large_num, %eax
cmpl %eax, 8(%esp)
jl L10
; *** and ends here ***
movl 12(%esp), %eax
addl $28, %esp
ret
*/
}
// optimal if the invariant:
// std::numeric_limits<int>::max() > (large_num*ubound) holds
int baz() // ( a mod c + b mod c ) == (a+b) mod c, a+b does not result in UB
{
int sum = 0 ;
for( int i=0 ; i<large_num ; ++i )
sum += std::rand() % ubound ;
return sum % divisor ;
/*
GCC 4.9 i386 with std=c++11 pedanticerrors Wall Wextra fomitframepointer c S
__Z3bazv:
subl $28, %esp
movl $0, 12(%esp)
movl $0, 8(%esp)
jmp L13
; *** loop starts here ***
L14:
call _rand
movl _ubound, %ecx
cltd
idivl %ecx
movl %edx, %eax
addl %eax, 12(%esp)
addl $1, 8(%esp)
L13:
movl _large_num, %eax
cmpl %eax, 8(%esp)
jl L14
; *** and ends here ***
movl _divisor, %ecx
movl 12(%esp), %eax
cltd
idivl %ecx ; *** division only once, at the end
movl %edx, %eax
addl $28, %esp
ret
*/
}
