Looks like you haven't got my point or I haven't made my point clear. So I will try again. Under UP, #define __IRQ_STAT(cpu, member) ((void)(cpu), irq_stat[0].member) I agree with your point about compiler optimization/performace improvement here as irq_stat[0].member is used instead of irq_stat[cpu].member(this would save the indirection overhead) But, I am worried about the first argument '(void)(cpu)' in the definition. Instead of doing #define x (y), why should we do #define x (z, y) where z is not used. As you can see, with this, the assignment won't work. Eg, If I wan't to do local_irq_count = 0, it translates (under UP) to (cpu, irq_stat[0].__local_irq_count) = 0, which is syntactically wrong. I see no change in performace/optimization if this definition is changed to #define __IRQ_STAT(cpu, member) (irq_stat[0].member), which will allow me to do local_irq_count = 0, which translates to (irq_stat[0].__local_irq_count = 0). Hope I have made my point clear and have understood your point too :-) Regards, Bharata. On Sun, Jan 06, 2002 at 02:17:27AM +0000, John Levon wrote: > On Thu, Dec 20, 2001 at 05:22:12PM +0530, Bharata B Rao wrote: > > > On Thu, Dec 20, 2001 at 11:41:38AM +0000, John Levon wrote: > > > On Wed, Dec 19, 2001 at 11:07:11AM +0530, Bharata B Rao wrote: > > > > > > > Please consider the following definitions from include/linux/irq_cpustat.h > > > > -------------------------- > > > > #ifdef CONFIG_SMP > > > > #define __IRQ_STAT(cpu, member) (irq_stat[cpu].member) > > > > #else > > > > #define __IRQ_STAT(cpu, member) ((void)(cpu), irq_stat[0].member) > > > > #endif > > > > > > I can only assume this is for performance. cpu might not be constant to the compiler, > > > so the first would still have an indirection overhead on UP. The second one, however, > > > lets the compiler optimise as the value of cpu is discarded (assume its evaluation has > > > not side effects that is ;) > > > > If that is so, why not just > > #define __IRQ_STAT(cpu, member) (irq_stat[0].member) > > for UP, so that the macros like local_irq_count(cpu) can be used as Lvalue. > > (I am concerned about not being able to use this as Lvalue in UP) > > haven't I just answered this above ? > > whether it makes a difference or not performance-wise is a different matter. IF you want > it to change, you should prove that it makes no real difference. > > john > > -- > "Unless everyone else on earth is attending meetings I haven't been told > about." > - /. paranoia at its finest > -- > Kernelnewbies: Help each other learn about the Linux kernel. > Archive: http://mail.nl.linux.org/kernelnewbies/ > IRC Channel: irc.openprojects.net / #kernelnewbies > Web Page: http://www.kernelnewbies.org/ -- Bharata B Rao, IBM Linux Technology Center, IBM Software Lab, Bangalore. Ph: 91-80-5044962 Mail: bharata@in.ibm.com -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
