Alexey Dobriyan <[email protected]> writes: > On Wed, May 27, 2020 at 09:41:53AM -0500, Eric W. Biederman wrote: >> Kaitao Cheng <[email protected]> writes: >> >> > we don't need {len = PTR_ERR(pathname)} when IS_ERR(pathname) is false, >> > it's better to move it into if(IS_ERR(pathname)){}. >> >> Please look at the generated code. >> >> I believe you will find that your change will generate worse assembly. > > I think patch is good. > > Super duper CPUs which speculate thousands instructions forward won't > care but more embedded ones do. Or in other words 1 unnecessary instruction > on common path is more important for slow CPUs than for fast CPUs.
No. This adds an entire extra basic block, with an extra jump. A good compiler should not even generate an extra instruction for this case. A good compiler will just let len and pathname share the same register. So I think this will hurt your slow cpu case two as it winds up just plain being more assembly code, which stress the size of the slow cpus caches. I do admit a good compiler should be able to hoist the assignment above the branch (as we have today) it gets tricky to tell if hoisting the assignment is safe. > This style separates common path from error path more cleanly. Very arguable. [snip a completely different case] Yes larger cases can have different solutions. Eric
