© 2006 Andrew R. BernatMarch 2006Generalized Code Relocation Generalized Code Relocation for Instrumentation and Efficiency Andrew R. Bernat University of Wisconsin

– 2 –© 2006 Andrew R. Bernat Generalized Code Relocation Design Objectives  Whole-program instrumentation Instrument every instruction in the program … and all control flow edges as well  Efficient instrumentation No traps! Minimize extraneous jumps Restrict register save/restores  Flexible, extensible instrumentation system Laying the groundwork for binary rewriting

– 3 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramps  Whole-program instrumentation All instructions, including neighbors All control flow edges  One trampoline per basic block Reduces number of extra branches  Hierarchical code generation Extensible Allows for a variety of optimizations

– 4 –© 2006 Andrew R. Bernat Generalized Code Relocation Function Relocation  Efficient instrumentation Blocks too small for branch to instrumentation Instrumentation too far away No traps!  Shared functions Copy to remove sharing  Function rewriting Undo optimizations

– 5 –© 2006 Andrew R. Bernat Generalized Code Relocation Old Instrumentation Overview Application Program Function foo Base Trampoline Save Regs instr2 Mini Trampolines Instrumentation Code Instrumentation Code instr1 instr2 instr3 Restore Regs Save Regs Restore Regs

– 6 –© 2006 Andrew R. Bernat Generalized Code Relocation Old Instrumentation - Consecutive Application Program Function foo Multiple Base Trampolines Multiple Base Trampolines Mini Trampolines instr2 instr1 instr2 instr3 instr1

– 7 –© 2006 Andrew R. Bernat Generalized Code Relocation Old Instrumentation – Uninstrumentable Neighbors Application Program Function foo Base Trampoline Save Regs instr2 Mini Trampolines Instrumentation Code Instrumentation Code instr1 instr2 instr3 instr1 instr3 Restore Regs Save Regs Restore Regs

– 8 –© 2006 Andrew R. Bernat Generalized Code Relocation Edge instrumentation Application Program Function foo Base Trampolines branch ‘Edge’ Trampoline save/restore branch Instrument edges via another level of indirection (plus extra branches) pre-branch fallthrough jump taken

– 9 –© 2006 Andrew R. Bernat Generalized Code Relocation Limitations of Old Instrumentation  Incomplete instrumentation coverage Often could not instrument “near-by” instructions  Inefficient instrumentation Edges, consecutive instructions require extra branches  Platform specific implementation Inextensible and bug-prone

– 10 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramp Principles  Basic-block instrumentation One jump to/from per block Efficient instrumentation of neighbor instructions  Logical view: a control flow graph Relocated instructions + instrumentation Apply compiler techniques to dynamic instrumentation

– 11 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramps Application Program Function foo Multitramp Basic Block Base Tramp Instruction Base Tramp Branch Fallthrough Target

– 12 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramp Implementation  A multitramp is a tree of code objects  Code objects provide the following: Maximum space required (worst case) Generate, install, and link callbacks Map relocated to original address  Single mechanism for both instruction and edge instrumentation

– 13 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramp Example Base Tramp 1 Instruction Base Tramp 2 Branch Mini Tramp 4 Base Tramp 3 Mini Tramp 3 Mini Tramp 1 Mini Tramp 2 save ; BT 1 branch restore ; BT 1 branch save ; BT 2 branch restore ; BT 2 return save ; BT 3 branch restore ; BT 3 return

– 14 –© 2006 Andrew R. Bernat Generalized Code Relocation In-Line Instrumentation  Current out-of-line model is based on the requirements of Paradyn Frequent insertion/removal of instrumentation  Limited opportunity for optimization Particularly register saves and restores  What about long-lived instrumentation?

– 15 –© 2006 Andrew R. Bernat Generalized Code Relocation In-Line Instrumentation  In-line instrumentation into a single code sequence: Relocated instructions Save/restore code Instrumentation  Replace entire sequence when something changes! BPatch::setMergeTramp(true)

– 16 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramp Status  Extensible implementation Can add new code objects to multitramp CFG: –Raw binary sections. –Control flow-altering code  In-line instrumentation POWER, x86-64  Platform-independent design Encapsulated platform-dependent sections Included with all platforms in Dyninst 5.0

– 17 –© 2006 Andrew R. Bernat Generalized Code Relocation Multitramp Results Whole-program instrumentation Instrument every instruction in the program … and all control flow edges as well Efficient instrumentation No traps! Minimize extraneous jumps Restrict register save/restores Flexible, extensible instrumentation system Laying the groundwork for binary rewriting

– 18 –© 2006 Andrew R. Bernat Generalized Code Relocation Function Relocation  The basic block may be too small to contain a branch to instrumentation IA-32, x86-64  We may not have the available registers to construct a long branch POWER, SPARC  Solution: relocate on a function level Sufficient space to fit large branches Dead registers that can be used to branch

– 19 –© 2006 Andrew R. Bernat Generalized Code Relocation Old Approach  One-time relocation Preemptively expand possible instrumentation sites: –Function entry, exit, call sites; loop entry, exits –But what about everything else?  Linear scan of the function, ignoring control flow. Dangerous with in-lined data

– 20 –© 2006 Andrew R. Bernat Generalized Code Relocation Incremental Function Relocation  A function is a list of basic blocks  Accumulate modifications to each block Ex: block must be 5 bytes long  Generate relocated versions on-the-fly Only modify what is necessary  Add instrumentation to the new function

– 21 –© 2006 Andrew R. Bernat Generalized Code Relocation Function Relocation - Example Block 2 is too small to patch in a jump block 1 block 5 block 4 block 3 block 2 block 1 block 5 block 4 block 3 block 2 1.Copy the function 2.Enlarge block 2 3.Replace Add modification

– 22 –© 2006 Andrew R. Bernat Generalized Code Relocation Other Uses for Relocation  Overlapping functions Relocation disambiguates code Instrument unique per-function copy  Undo optimizations Rewrite the function during relocation Example: unwinding a tail call

– 23 –© 2006 Andrew R. Bernat Generalized Code Relocation Function Relocation Status  Platform-independent function relocation engine IA-32, x86-64, POWER, SPARC  Support for multiple relocated versions On-the-fly code relocation  Extensible modification interface Block must be 5 bytes long Modify the instructions in the block

– 24 –© 2006 Andrew R. Bernat Generalized Code Relocation Design Objectives Whole-program instrumentation Instrument every instruction in the program … and all control flow edges as well Efficient instrumentation No traps! Minimize extraneous jumps Restrict register save/restores Flexible, extensible instrumentation system Laying the groundwork for binary rewriting

– 25 –© 2006 Andrew R. Bernat Generalized Code Relocation Conclusion  Multitramps Whole-program instrumentation approach  Function relocation Instrument everywhere (without traps)  People Drew Bernat – Multitramps Nate Rosenblum – Function relocation Nick Rutar – Register optimizations