1. Correct Alignment of a RAS after Call and Return Mispredictions Ghent University Veerle Desmet Yiannakis Sazeides Constantinos Kourouyiannis Koen De Bosschere University of Cyprus

2. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20052 Motivation Return-Address-Stack (RAS) 1. Correct alignment 2. Effect deeper pipelines by Veerle Desmet Ghent University

3. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20053 Return-Address-Stack (RAS) return from printf main(): my_printf() my_function() my_printf() return main: my_printf() my_function() my_printf() return function calls push return address on RAS 1 my_printf() RAS return from printf my_printf: for(condition){ printf() } return printf() TOS printf: /* print */ return returns predicted by popping from RAS 2 return from my_printf return from my_printf return from my_function return from main my_function: /* fun */ return my_function()

4. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20054 Mispredictions fetch checkpoint RAS return from printf TOS return from my_printf return from main  recovery wrong path Speculative RAS updates due to wrong path calls

5. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20055 Bottlenecks for RAS performance De-alignment `unbalanced # of call/return’ Corruption `RAS content overwritten by wrong path calls’ Overflow `call depth exceeds stack size’ return from main return from printf RAS return from my_printf return from my_printf TOS

6. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20056 Correct Alignment… RAS return from printf return from main TOS Checkpoint TOS 1 Conditional branch misprediction 2 Wrong path 3 Recovery to checkpointed TOS 4 return from wrong path e.g. 1 misspeculated call  return from wrong path return from my_function

7. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20057 … after Call Mispredictions RAS return from printf return from main TOS Checkpoint TOS 1 Call target misprediction + RAS update 2 Wrong path 3 Recovery to checkpointed TOS 4 e.g. 1 misspeculated call  return from my_function return from wrong path return from wrong path return from mispr. call

8. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20058 … after Return Mispredictions RAS return from printf TOS Checkpoint TOS 1 Return target misprediction + RAS update 2 Wrong path 3 Recovery to checkpointed TOS 4 e.g. no misspeculated calls or returns  return from mispr return return from mispr return return from main

9. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 20059 return from printf TOS return from mispred call Correct Alignment Incorrect Alignment Correct Alignment TOS mispred. return addr Conditional branch misprediction Call misprediction Return misprediction RAS

10. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200510 Correct Alignment 0% 5% 10% 15% 20% 25% 30% compress95 gcc95 go95 ijpeg95 li95 m88ksim95 vortex95 mcf00 parser00 vortex00 mesa00 average RAS misprediction rate 0,94 0,96 0,98 1,00 1,02 1,04 1,06 1,08 1,10 1,12 compress95 gcc95 go95 ijpeg95 li95 m88ksim95 vortex95 mcf00 parser00 vortex00 mesa00 average Speedup Incorrect Alignment Correct Alignment Speedup can be affected by up to 10%

11. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200511 A lot of published work… SimpleScalar HydraScalar Yeh, Intel patent SimWattch Simca Jourdan Eickemeyer, Hoyt, Hummel, McDonald, McMahan Steely INCORRECT = LOWER PERFORMING CORRECTUNCLEAR

12. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200512 Bottlenecks for RAS performance De-alignment `unbalanced # of call/return’ Corruption `RAS content overwritten by wrong path calls’ Overflow `call depth exceeds stack size’ return from main return from printf RAS return from my_printf return from my_printf TOS

13. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200513 RAS content recovery 0,90 0,95 1,00 1,05 1,10 1,15 1,20 compress95 gcc95 go95 ijpeg95 li95 m88ksim95 vortex95 mcf00 parser00 vortex00 mesa00 average Speedup Incorrect Alignment Correct Alignment Also checkpoints/recovers top of stack data 2% speedup on average [Skadron et al. @ MICRO 1998] top of stack data

14. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200514 Motivation Return-Address-Stack (RAS) 1. Correct alignment 2. Effect deeper pipelines by Veerle Desmet Ghent University

15. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200515 Deeper Pipelines… 0,88 0,90 0,92 0,94 0,96 0,98 1,00 51015202530 pipeline stages relative IPC to full RAS recovery 32-entry RAS 16-entry RAS 8-entry RAS 64-entry RAS On average, reasonable scaling… Independence on RAS size

16. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200516 But... per benchmark -7% -5% 32-entry RAS 15 stages 10 stages 5 stages 20 stages 25 stages 30 stages

17. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200517 RAS Corruptions per kilo instructions 32-entry RAS … Backward (tos,1,…) more destructive than forward corruption (31,30,…) 31 tos 1 2 3 4 30 corruption distance 1

18. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200518 Individual benchmarks 32-entry RAS 20 stage pipeline

19. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200519 TOS behavior time li95gcc95 time 32-entry RAS 20 stage pipeline Mainly forward corruption More backward corruption wrong path RAS updates

20. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200520 Bottlenecks for RAS performance De-alignment `unbalanced # of call/return’ Corruption `RAS content overwritten by wrong path calls’ Overflow `call depth exceeds stack size’ return from main return from printf RAS return from my_printf return from my_printf TOS

21. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200521 TOS Behavior no overflow 32-entry RAS 20 stage pipeline time li95gcc95 overflow

22. Workshop on Duplicating Deconstructing and Debunking (WDDD 2005) --- June 4, 200522 Summary Correct Aligned RAS Return misprediction decrease with 40% Speedup of up to 10% Deeper Pipelines One of the best performing RAS recovery techniques Satisfactory on average Performance decrease up to 7% for some programs May need to checkpoint more content Paper: Possible implementations Call uncorruption optimization for free How to fix correct alignment in SimpleScalar

23. Correct Alignment of a RAS after Call and Return Mispredictions Ghent University Veerle Desmet Yiannakis Sazeides Constantinos Kourouyiannis Koen De Bosschere University of Cyprus