compilation - C++ compiler optimizations and short-circuit evaluation -

-

this question has answer here:

here code :

b = f() || b;

the function f() has side effect , must executed. normally, right operand can short-circuited , code should work. afraid compilators reverse 2 operands, since it's more efficient short-circuit function evaluation rather simple variable evaluation. know g++ -o3 can break specifications, don't know if code can affected.

so, code risk-free?

i knew is short-circuiting logical operators mandated? , evaluation order? question compilers optimizations, didn't know can't break standards (even if strange).

but afraid compilators reverse 2 operands

these expressions must evaluated left-to-right. covered in standard operators &&, ||, ?, , ,. mention order, enforced sequence points.

§5.14.1 (logical and)

the && operator groups left-to-right. operands both contextually converted bool (clause 4). result true if both operands true , false otherwise. unlike &, && guarantees left-to-right evaluation: second operand not evaluated if first operand false.

§5.15.1 (logical or)

the || operator groups left-to-right. operands both contextually converted bool (clause 4). returns true if either of operands true, , false otherwise. unlike |, || guarantees left-to-right evaluation; moreover, second operand not evaluated if first operand evaluates true.

§5.16.1 (conditional operator)

conditional expressions group right-to-left. first expression contextually converted bool (clause 4). evaluated , if true, result of conditional expression value of second expression, otherwise of third expression. 1 of second , third expressions evaluated. every value computation , side effect associated first expression sequenced before every value computation , side effect associated the second or third expression.

§5.19.1 (comma operator)

the comma operator groups left-to-right. pair of expressions separated comma evaluated left-to-right; left expression discarded value expression (clause 5). every value computation , side effect associated left expression sequenced before every value computation , side effect associated right expression. type , value of result type , value of right operand; result of same value category right operand, , bit-field if right operand glvalue , bit-field. if value of right operand temporary (12.2), result temporary.

regarding concern optimizations violating order, no compilers not allowed change order. compilers must first , foremost (try to) follow standard. then can try make code faster. may not violate standard sake of performance. undermines entire premise of having standard.


Comments

Post a Comment

Popular posts from this blog

c# - Where does the .ToList() go in LINQ query result -

-
when getting data populate data grid, have pulled data, made list , mapped model, this: public list<person> getpeople() { var data = _db.person.tolist(); list<person> people = new list<person>(); foreach (var item in data) { person person = new person() { personid = item.personid, firstname = item.firstname, lastname = item.lastname }; people.add(person); } return people; } this works fine, in interest of writing less code discovered .select method , (what think is) anonymous type. have: public list<person> getpeople() { return _db.person.select(d => new person { personid = item.personid, firstname = item.firstname, lastname = item.lastname }).tolist(); } though not easy read, works fine too. there preference either way? ...
Read more

android - CollapsingToolbarLayout: position the ExpandedText programmatically -

-
i using new android support design view collapsingtoolbarlayout. have need position expandedtitle programmatically. actually way set position expandedtitle in .xml file (example here: https://goo.gl/l5h5fc ) there no public api set text position programmatically , fields private . do know if it's possible without reflection or 'hacky' tricks? i have solved problem exposing properties via reflection ( https://gist.github.com/fpezzato/8e7d45112883dbbc5ffb ), works don't use trick in production code. @google: please no more private or final stuff! :)
Read more

Listeners to visualise results of load test in JMeter -

-
i want run load test in non-gui mode , save results in .csv file. (i have not modified .properties files, standard information saved) i plan load file , visualise results jmeter’s listeners. which listeners suited purpose? should avoid @ cost, might cause me misinterpret information? (e.g. in view results tree response data empty have - - not saved this. however, there has in fact been response. if saw listener might think there hasn’t been one. since want avoid this, listeners have gain information needed .csv file.) i’d know both jmeter listeners plugins. if running test in non-ui mode ideally should disable listeners in script. use '-l' option store test results .jtl extension. can later open jtl file jmeter. aggregate report or summary report should summarize test results , response time , throughput. if want plot graphs see trends on test duration can use http://jmeter-plugins.org/#/ standard , lib set. i suggest following listeners: response ...
Read more