On Wed, Dec 08, 2021 at 07:40:10PM +0100, Morten Brørup wrote: > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > > Sent: Wednesday, 8 December 2021 18.35 > > > > On Fri, Dec 03, 2021 at 11:37:10AM +0100, Morten Brørup wrote: > > > > From: Morten Brørup [mailto:m...@smartsharesystems.com] > > > > Sent: Thursday, 2 December 2021 14.56 > > > > > > > > > > > > I disagree: Negative value does not mean failure. Only -1 means > > > > failure. > > > > > > > > There is no -2 return value. There is no -EINVAL return value. > > > > > > > > Testing for (ret < 0) might confuse someone to think that other > > values > > > > than -1 could be returned as indication of failure, which is not > > the > > > > case when following the convention where the functions set errno > > and > > > > return -1 in case of failure. > > > > > > > > It would be different if following a convention where the functions > > > > return -errno in case of failure. In this case, testing (ret < 0) > > would > > > > be appropriate. > > > > > > > > So explicitly testing (ret == -1) clarifies which of the two > > > > conventions are relevant. > > > > > > > > > > I tested it on Godbolt, and (ret < 0) produces slightly smaller code > > than (ret == -1) on x86-64: > > > > > > https://godbolt.org/z/3xME3jxq8 > > > > > > A binary test (Error or Data) uses 1 byte less, and a tristate test > > (Error, Zero or Data) uses 3 byte less. > > > > > > Although there is no measurable performance difference for a single > > instance of this kind of test, we should consider that this kind of > > test appears many times in the code, so the saved bytes might add up to > > something slightly significant in the instruction cache. > > > > > > My opinion is not so strong anymore... perhaps we should prefer > > performance over code readability, also in this case? > > > > > > > i would not expect many calls that return rte_errno to be made on the > > hot path. most of the use of errno / rte_errno is control but it's good > > to have considered it. if i start seeing a lot of error handling in hot > > paths i ordinarily find a way to get rid of it through various > > techniques. > > Tyler, I think you and I agree perfectly on this topic. > > -1 should be returned as error, and rte_errno should provide details. > > I'm only saying that comparing the return value with < 0 provides marginally > less instruction bytes than comparing it with == -1, so even though -1 is the > canonical indication of error, the comparison could be < 0 instead of == -1 > (if weighing performance higher than code clarity).
sounds about right to me, i appreciate the extra analysis. certainly with explicit -1 it doesn't stop an application gaining the slightly better code generation with < 0. thanks
