1. Wish Branches Combining Conditional Branching and Predication for Adaptive Predicated Execution
Hyesoon Kim
Onur Mutlu
Jared Stark*
Yale N. Patt
The University of Texas at Austin *Oregon Microarchitecture Lab
Electrical and Computer Engineering Intel Corporation

2. Talk Outline Problem Wish Branches Experimental Methodology Results
Conclusion

3. Predicated Execution C B D A B C D A if (cond) { b = 0; } else {
(normal branch code) C B D A T N
p1 = (cond)
branch p1, TARGET
mov b, 1
jmp JOIN
TARGET:
mov b, 0
(predicated code) B C D A
if (cond) {
b = 0; }
else {
b = 1; A
p1 = (cond)
(!p1) mov b, 1
(p1) mov b, 0 B C D
add x, b, 1
Convert control flow dependency to data dependency
Pro: Eliminate hard-to-predict branches
Cons: (1) Fetch blocks B and C all the time
(2) Wait until p1 is resolved

4. The Overhead of Predicated Execution
-2%
16%
13%
non-predicated A
p1 = (cond)
(0) mov b,1
(1) mov b,0
p1 = (cond)
(!p1) mov b, 1
(p1) mov b, 0 B C D
add x, b, 1
(Predicated code)
If all overhead is ideally eliminated, predicated execution would provide 16% improvement in average execution time

5. The Problem
Due to the predication overhead, predicated execution sometimes reduces performance
Branch misprediction characteristics are dependent on run-time behavior: input set, control-flow path and phase behavior The compiler cannot accurately estimate the run-time behavior of branches

6. Talk Outline Problem Wish Branches Experimental Methodology Results
Conclusion

7. Wish Branches
A new type of control flow instruction types: wish jump/join and wish loop
The compiler generates code (with wish branches) that can be executed either as predicated code or non-predicated code (normal branch code)
The hardware decides to execute predicated code or normal branch code at run-time based on the confidence of branch prediction
Easy to predict: normal branch code
Hard to predict: predicated code

8. Wish Jump/Join A wish jump C B D A nop B C D A B wish join Taken
High Confidence
Low Confidence A
wish jump C B D A T N
mov b, 1
jmp JOIN
TARGET:
mov b,0
normal branch code
nop B C D A
p1 = (cond)
(!p1) mov b,1
(p1) mov b,0
predicated code B
wish join
Taken
Not-Taken C D A
p1=(cond)
wish.jump p1 TARGET
p1 = (cond)
branch p1, TARGET B
nop
(!p1) mov b,1
wish.join !p1 JOIN
(1) mov b,1
wish.join (1) JOIN C
TARGET:
(1) mov b,0
TARGET:
(p1) mov b,0 D
JOIN:
wish jump/join code

9. Wish Loop H X X High Confidence Low Confidence Y Y T T N N H X X (1) Y
do {
a++;
i++;
} while (i<N); X T N N
High Confidence
Low Confidence Y Y H
mov p1, 1
LOOP:
(p1) add a, a, 1
(p1) add i, i, 1
(p1) p1 = (cond)
wish. loop p1, LOOP
EXIT: X X
LOOP:
add a, a, 1
add i, i, 1
p1 = (i<N)
branch p1, LOOP
EXIT:
(1) Y Y
normal backward branch code
wish loop code

10. Mispredicted Case 1: Early-ExitH
Correct
execution: H X1 X2 X3 Y T T N X T
Early-exit:
(Low confidence)
Flush pipeline N H X1 X2 Y … T N Y X3 Y N
Compared to normal branch code:
predicate data dependency and one extra instruction (-)

11. Mispredicted Case 2: Late-ExitH
Correct
execution: H X1 X2 X3 Y T T N X T
nop
nop
Late-exit:
(Low confidence) N H X1 X2 X3 X4 X5 Y … T T T T N Y
Compared to normal branch code:
pro: reduce flush penalty (+++)
cons: predicate data dependency and one extra instruction (-)

12. Mispredicted Case 3: No-ExitH
Correct
execution: H X1 X2 X3 Y T T N
Flush pipeline X T
No-exit:
(Low confidence) N H X1 X2 X3 X4 X5 X6 … T T T T T T Y Y
Compared to normal branch code:
predicate data dependency and one extra instruction (-)

13. Advantages/Disadvantages of Wish Branches
Advantages compared to predicated execution
Reduce the overhead of predication
Increase the benefits of predicated code by allowing the compiler to generate more aggressively-predicated code
Provide a mechanism to exploit predication to reduce the branch misprediction penalty for backward branches (Wish loops)
Make predicated code less dependent on machine configuration (eg. branch predictor)

14. Advantages/Disadvantages of Wish Branches
Disadvantages compared to predicated execution
Extra branch instructions use machine resources
Extra branch instructions increase the contention for branch predictor table entries
May constrain the compiler’s scope for code optimizations

15. Wish Branch Support ISA Support Compiler Support Hardware Support
predicated execution, wish branch instruction
Compiler Support
Wish branch generation algorithms
The compiler needs to decide which branches are predicated, which are converted to wish branches, and which stay as normal branches
Hardware Support
Confidence estimator
Front-end and branch misprediction detection/recovery module

16. Talk Outline Problem Wish Branches Experimental Methodology Results
Conclusion

17. Experimental Infrastructure
Source
Code
IA-64
Binary
IA-64
Trace
µops
IA-64
Compiler
(ORC)
Trace
generation
module
Micro-op
Translator
Micro-op
Simulator
IA-64 provides full support for predication
Convert IA-64 traces to micro-ops to simulate an out-of-order superscalar processor model

18. Simulation Methodology
Nine SPEC 2000 integer benchmarks
Baseline Processor Configuration
Front End
Large and accurate branch predictor (64KB hybrid branch predictor: gshare + local)
Minimum 30-cycle branch misprediction penalty
64KB, 2-cycle latency I-cache
Execution Core
8-wide out-of-order processor
512-entry instruction window
Confidence Estimator
1KB tagged 16-bit history JRS confidence estimator (Jacobsen et al. MICRO-29)

19. Talk Outline Problem Wish Branches Experimental Methodology Results
Conclusion

20. Performance Improvement
-4%
14%
2.02 8%
24%
non-predicated
16% over conditional branch prediction (w/o mcf)
11% over selective-predication (w/o mcf)
7 % over aggressive predication (w/o mcf)
14% over conditional branch prediction and
13% over selective-predication and
16% over aggressive-predication
12% over conditional branch prediction
11% over selective-predication
13 % over aggressive predication
SELECTIVE-PREDICATION: branches are selectively predicated using compile-time cost-benefit analysis
AGGRESSIVE-PREDICATION: all branches that are suitable for if-conversion are predicated

21. Talk Outline Problem Wish Branches Experimental Methodology Results
Conclusion

22. Conclusion
New control flow instructions: wish branches (jump/join/loop)
Wish branches improve performance by dividing the work of predication between the compiler and the microarchitecture
Compiler: analyzes the control-flow graph and generates code
Microarchitecture: makes run-time decision to use predication
Wish branches provide significant performance benefits
16% compared to conditional branch prediction
13% compared to selectively predicated code
Wish branches can make predicated execution more viable and effective in high performance processors
By enabling adaptive and aggressive predicated execution