XCISE 2.0.6 Dead Code Elimination

by Exploiting ASX

Ian Davis

University of Waterloo

April 22, 2023 Dead Code Elimination 2

Capturing the build process

Wrap gcc & g++ Capture build history

What program What parameters Where invoked

Repeat the build process Adjusting parameters Request compilation to assembler Present each assembler built to ASX

April 22, 2023 Dead Code Elimination 3

Key Idea Compile source to assembler Scan the assembler for:

Function declarations Function invocation

Direct Function Calls Indirect Function Calls via pointer Functions called as part of initialisation

Class information Vtables, Typeinfo structs, etc.

Map back to the source code Source & line no function body starts at

April 22, 2023 Dead Code Elimination 4

Relevant Standards

Intel 386 assembler Function boundaries, calls & pointer manipulation Source file name and line no information

Dwarf 3.0 (Symbolic Information) Recover frame base, and class of this pointer Recover vtable mapping to functions

Itanium Application Binary Interface (ABI) Unmangle g++ function signatures

April 22, 2023 Dead Code Elimination 5

Virtual Function Call Pattern With any registers + interleaved assembler movq -24(%rbp), %rax

// Class pointer -> %rax movq (%rax), %rax

// Vtable -> %rax addq $8, %rax

// Optional -- absent if $0 movq (%rax), %rax

// Function pointer call *%raxApril 22, 2023 Dead Code Elimination 6

Output TA

Output from ASX Consolidated call graph Viewable using LSEdit Readable by C++ and Java code Human readable ascii file

April 22, 2023 Dead Code Elimination 7

April 22, 2023 Dead Code Elimination 8

Phase Two

XCISE Load graph into a C++ program From all mainline functions

main Function calls as part of variable assignment

Flag all reachable functions Report those not reachable Remove them from source code

April 22, 2023 Dead Code Elimination 9

Basic Problems Requires holistic approach

Will break compilation if we remove casually What about nasty invocations through pointer

Problematic issues.. Function declarations in class files Macros Friend classes Polymorphism

Abstract classes Implicit polymorphism

Class .v. Namespace

April 22, 2023 Dead Code Elimination 10

What to remove Constructors / Destructors ? Default constructors/destructors ! Illegal function invocation trapping code Classes ?

Friends Inheritance

Risks re polymorphism Risks re abstract classes If we don’t remove a dead function

Can’t remove anything it calls

April 22, 2023 Dead Code Elimination 11

What about functions in header files Inlined function declarations

Can appear in multiple assembler files Static functions in header files

Likewise Consolidate based on file and line number Inviolate header files

/usr/include Call back mechanisms from libraries

Really problematic

April 22, 2023 Dead Code Elimination 12

What about template functions

Can identify template function by signature No explicit function body that can be removed Can only remove template if no instantiations Templates occur in header files..

Not all templated classes likely to be used Do we even want to remove them

Can’t presume usage over time unchanged

April 22, 2023 Dead Code Elimination 13

Polymorphism

See a polymorphic call to A::foo() But for all X : X subclass of A might be

Call to X::foo() Visit A::foo() polymorphically

=> Visit all such X::foo() polymorphically Discovery using vtable & typeinfo structures

April 22, 2023 Dead Code Elimination 14

Dead Code Removal Given only file and line no of start of body

Want to remove function with this body Hard to parse source

Comments Macros

Run the source through cpp Use original build parameters re source Use file and line number info to map back to

source Eliminates all macros, comments, etc.

April 22, 2023 Dead Code Elimination 15

Removal strategy g++ -Dlinux –Uwin32 –Dversion=v2 main.cpp

#if !defined(_DEAD_) && defined(linux) && !defined(win32) && defined(version) && (version == v2) #define _DEAD_20110312203041 #endif /* _DEAD_ */

Eliminate #ifndef _DEAD_20110312203041 int foo() {} #endif /* _DEAD_20110312203041 */

April 22, 2023 Dead Code Elimination 16

Removal choices Create new output file

Can’t then compile upgraded source Overwrite existing source

Risky Create backup

Really hard to debug introduced errors Massive systemic changes hard to debug Hard to work out why something illegally removed

Possibly provide incremental removal Remove top level dead code first

April 22, 2023 Dead Code Elimination 17

April 22, 2023 Dead Code Elimination 18

Current status Removal of regular functions works well Removal of all dead functions problematic

Not catching all polymorphic calls ? Not catching other types of function addressing Not sure why things don’t compile Don’t want to plough through 1’000s of error messages Can’t break problem down easily Might all be resolved with a single bug fix Might not get resolved with many bug fixes

April 22, 2023 Dead Code Elimination 19

Source Code

ASX 3.0.8 www.swag.uwaterloo.ca/asx

XCISE 2.0.6 www.swag.uwaterloo.ca/xcise

LSEdit 7.3.14 www.swag.uwaterloo.ca/lsedit

ThankYou