Hi Manish, On Mon, Aug 1, 2011 at 4:45 PM, Manish Katiyar <mkatiyar@gmail.com> wrote:
C absolutely guarantees that the second will NEVER be evaluated before the first. Otherwise, things like this:
if ( ptr && ptr->field )
could fail if ptr was NULL.
In your example yes. But is there a problem if the two conditions are independent ? for eg..
The compiler doesn't care if the two conditions are independent or dependant.
foo() { .... a = 0; b = my_complex_function_call(); if (b && a) { /* Do some non-trivial stuff */ } else { printf("I'm an intelligent compiler\n"); } ..... }
Does C guarantee that even in this case it won't be reordered and not optimized ?
C guarantees that within the if ( b && a ) statement, a won't be evaluated if b evaluates to zero. If you coded if ( 0 && func() ) then func will NEVER be called. if you coded if ( func() && 0 ) then I think that func will always be called since it won't evaluate the 0 until after the left hand side of the && has been evaluated. I tried the following very simple example (compiled with -O3) #include <stdio.h> static inline int func() { printf( "func called\n" ); return 1; } int main( int argc, char **argv ) { if ( func() && 0 ) printf( "path1\n" ); else printf( "path2\n" ); return 0; } The compiler was smart enough to optimize out the printf( "path1\n" ) (i.e. running strings on the executable only showed path2), but it still calls func(). -- Dave Hylands Shuswap, BC, Canada http://www.davehylands.com