[Advcomparch] Ερώτηση σχετικά με λάθος στο branch prediction

[Spyridon Mastorakis]

ÊáôÜëáâá.
Óáò åõ÷áñéóôþ ðïëý ãéá ôçí áðÜíôçóç.

Óôéò 25 Ìáú 2013, 12:30 ì.ì., ï/ç Konstantinos Nikas <knikas at cslab.ece.ntua.gr> Ýãñáøå:

ÊáëçìÝñá,

ãéáôß íá êÜíåéò flush ôéò caches; ¼ôáí Ý÷åéò Ýíá branch misprediction ðñÝðåé íá êÜíåéò ôá åîÞò:

1. Íá ãõñßóåéò óôï óùóôü architectural state. Áí ÷ñçóéìïðïéåßò ROB üðùò åßäáìå êáé óôï ìÜèçìá äå 
÷ñåéÜæåôáé íá êÜíåéò áðïëýôùò ôßðïôá, ìéáò êáé ôï in order commit óïõ åããõÜôáé ôï state óïõ åßíáé 
ïê.

2. Íá óôáìáôÞóåéò íá åêôåëåßò á÷ñåßáóôåò åíôïëÝò. Ãéá áõôü ôï ëüãï êáé êÜíåéò flush ôï pipeline, 
ðñïêåéìÝíïõ íá áðåëåõèåñùèïýí ôá äéÜöïñá components êáé íá ÷ñçóéìïðïéçèïýí áðü ôéò óùóôÝò 
åíôïëÝò ÷ùñßò êáèõóôÝñçóç. Ôéò caches ãéáôß üìùò íá ôéò êÜíåéò flush? ÊáôÜ ôçí åêôÝëåóç ôïõ  
mispredicted path, Ý÷åéò öÝñåé óôçí ÷åéñüôåñç ðåñßðôùóç êÜðïéá "á÷ñåßáóôá" loads, ôá ïðïßá 
óßãïõñá åßíáé ðïëý ëéãüôåñá áðü ôïí óõíïëéêü áñéèìü ôùí blocks ðïõ èá ÷Üóåéò êÜíïíôáò flush. 

Ê.

2013/5/23 Spyridon Mastorakis <el09195 at mail.ntua.gr>
ÊáëçóðÝñá óáò.

¸÷ù ìéá åñþôçóç ðïõ áöïñÜ ôçí ðåñßðôùóç ðïõ êÜíïõìå ëÜèïò óôï branch prediction, ïðüôå êáé
÷ñåéÜæåôáé íá êÜíïõìå flush ôï pipeline.

Óôçí ðåñßðôùóç áõôÞ , åêôüò áðü ôï pipeline , èá êÜíïõìå flush êáé ôéò caches. ÓùóôÜ?
ÕðÜñ÷åé, êÜðïéï Üëëï unit ðïõ èá ðñÝðåé íá ãßíåé flush óôçí ðåñßðôùóç áõôÞ?

Óáò åõ÷áñéóôþ ðïëý åê' ôùí ðñïôÝñùí ãéá ôçí áðÜíôçóç.
Óðýñïò ÌáóôïñÜêçò.

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-- 
Dr. Konstantinos Nikas
Computing Systems Laboratory
School of Electrical and Computer Engineering
National Technical University of Athens

Tel: +30-210-7724159
e-mail: knikas at cslab.ece.ntua.gr
http://www.cslab.ece.ntua.gr/~knikas