1. Overview of Exception Handling Implementation in Open64
Hucheng Zhou
Wenguang Chen
Tsinghua University
10/15/2019

2. Outline Introduction Three-Level C++ EH ABI
Overview of component of EH Mechanism
EH Implementation in Open64
LSDA structure
Call Frame Dwarf Information
How to debug EH implementation
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3. Introduction
Exceptions Handling provides a systematic and robust approach to coping with errors that cannot be recovered from locally at the point where they are detected
There are two main strategies to EH implementation:
“Code” based approach
“Table” based approach
C++ EH ABI defines the convention of “Table” based approach
Most of the existed compiler adopt the “Table” based approach, including gcc, open64, LLVM
Since it is “zero-cost” way that it will not add the burden on the exception-free execution
We will describe the detailed C++ EH ABI and the specific implementation in Open64
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4. Three Level C++ EH ABI According to the bottom-up order Level 1:
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Three Level C++ EH ABI
According to the bottom-up order
Level 1:
Unwind Library
Level 2:
Runtime Library, i.e., libstdc++.so
Level 3:
Unwind descriptor and EH table generated by compiler
This EH ABI is part of C++ ABI for Itanium, but as the de fact ABI for EH.
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5. Three Level EH ABI
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6. Component of EH FLOW
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7. EH Implementation in Open64
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8. EH Implementation in Open64
The implantation in FE, IPA, OPT and CGEMIT is straightforward
Here we mainly focus on CG part:
LSDA (Language Specific Data Area) generation, including call site table, activation record table, type table and type specification table
Unwind Descriptor generation
Call Frame Dwarf Information
.eh_frame section
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9. 2019/10/15
This graph shows the internal data structure for EH table, which is not documented well.
These data structures were dumped through reversing engineering and debug the personality_routine in libstdc++.
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10. Unwind Table
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11. LSDA Structure
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12. LSDA Illustration
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13. Call Frame Dwarf Information
The unwinding operation needs CIE and FDE information to know how to recover the saved registers and how to compute the predecessor’s CFA (Canonical Frame Address) and code location
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14. Example 10/15/2019 2019/10/15 There are 8 4-byte registers:
R0 always 0; R1 hold the return address; R2~R3 caller saved registers; R4~R6 callee saved registers; R7 stack register.
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Example Cont’
The first column indicates an address for every location. The remaining columns contain virtual unwinding rules that are associated with the indicated location given in the first column.
CFA(Canonical Frame Address) is used to identify the current call frame. Typically, CFA is defined to be the values of the stack pointer at the call site in the previous frame.
S = same_value; u = undefined value; r = register(N) rule; c = offset(N) rule; a = architecture rule
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17. Call Frame Dwarf Information
Prologue and epilogue code is not always in distinct blocks at the beginning and end of a program
There are different ways to manage the call frame, sometimes using frame pointer
CFA’s computation will be different at different PC
Some PU hasn’t call frame
Callee saved register can be saved in another register which is a scratch register
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18. Impact on Optimization
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Impact on Optimization
Optimization to exception free application
Generally, there is no runtime overhead. However, EH has a few impacts on compiler optimization
More complex CFG (e.g., There should be an edge from throw point to its landing pad)
Conservative optimization across EH region
Optimization to exception intensive application
Unnecessary EH ranges removable
IPA can help to statically analyze the thrown point and its related landing pad
Profile the exception caught path, and inline the calls during thrown point and its handler function
Generally, table based EH implementation does not add any runtime overhead to exception-free application
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19. How to Debug EH Implementation
1. Add EH related trace mechanism
2. Instrument the personality routine in libstdc++.so
3. Even instrument the functions in unwind library
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20. Thanks
Thank you very much
And any questions?
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