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Debugging Optimized Code

Debugging Optimized Code. Steven Osman. Debugging Optimized Code is Hard …. Un-optimized code has a correlation between source and object code Optimizing compilers rearrange, add and remove code. This results in: Code location problems Data value problems. Talk Overview.

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Debugging Optimized Code

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  1. Debugging Optimized Code Steven Osman

  2. Debugging Optimized Code is Hard… • Un-optimized code has a correlation between source and object code • Optimizing compilers rearrange, add and remove code. This results in: • Code location problems • Data value problems

  3. Talk Overview • Understanding the problems • Introducing Source-Level Debugging of Scalar Optimized Code • Classifying data-value problems • Identifying data-value problems • What’s next?

  4. The Code Location Problem We want a breakpoint at S2

  5. The Data-Value Problem What is the value of y at S2?

  6. Source-Level Debugging of Scalar Optimized Code • By Ali-Reza Adl-Tabatabai and Thomas Gross • Identifies data-value problems • No changes to the optimized code • Uses knowledge of optimizations

  7. Some Simple Terminology • Expected value = value a variable should have • Actual value = value stored in register • Current = Actual value IS expected value (true for all in unoptimzed code) • Endangered = Not current • Noncurrent = Actual IS NOT expected • Suspect = Actual may be expected

  8. Code Hoisting E0: x=u-v E1: x=y+z Bkpt1 Bkpt2 E2: x=y+z Bkpt3

  9. Code Hoisting E0: x=u-v E1: x=y+z E3: x=y+z Bkpt1 Bkpt2 E2: x=y+z Bkpt3 E2=RedCopy(E3)

  10. Hoist Reaches E0: x=u-v E1: x=y+z E3: x=y+z Bkpt1 Bkpt2 E2: x=y+z Bkpt3 E2=RedCopy(E3)

  11. Dead Code Elimination E0: x=y+z Bkpt1 Bkpt3 E1: …=x Bkpt4 Bkpt2 Bkpt5 E2: x=u-v Bkpt6

  12. Dead Code Elimination E0: x=y+z Bkpt1 Bkpt3 E3: x=y+z E1: …=x Bkpt4 Bkpt2 Bkpt5 E2: x=u-v Bkpt6

  13. Dead Reaching E0: x=y+z Bkpt1 Bkpt3 E3: x=y+z E1: …=x Bkpt4 Bkpt2 Bkpt5 E2: x=u-v Bkpt6

  14. What’s Next? • Good paper, but it only warns programmer • Suggestions about how to get the expected value • Alok Ladsariya’s talk…

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