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its running time it can also enable further optimizations by simplifying program structure dead code includes code that can never be executed unreachable code and code that only affects dead variables written to but never read again that is irrelevant to the program examples edit consider the following example written in c int foo void int a 24 int b 25 assignment to dead variable int c c a 4 return c b 24 unreachable code return 0 trivial analysis of the control flow would mark lines 7 and 8 for removal as they are never reached after return c if the procedure had a more complex control flow such as a label after the return statement and a goto elsewhere in the procedure then a feasible execution path might exist to the assignment to b furthermore simple analysis of the uses of values would then show that the value of b is only written to and never read from b is declared as a local variable inside foo so its value cannot be used outside foo thus the variable b is dead and an optimizer can reclaim its storage space and eliminate its initialization also even though some calculations are performed in the function their values are not stored in locations accessible outside the scope of this function furthermore given the function returns a static value 96 it may be simplified to the value it returns this simplification is called constant folding resulting in int foo void return 96 most advanced compilers have options to activate dead code elimination sometimes at varying levels a lower level might only remove instructions that cannot be executed a higher level might also not reserve space for unused variables a yet higher level might determine instructions or functions that serve no purpose and eliminate them a common use of dead code elimination is as an alternative to optional code inclusion via a preprocessor consider the following code set debug_mode to false constexpr bool debug_mode false int main void int a 5 int b 6 int c c a b 2 if debug_mode printf d n c return c because the constant debug_mode will always evaluate to false due to being defined as so the code inside the if statement can never be executed and dead code elimination would remove it entirely from the optimized program this technique is common in debugging to optionally activate blocks of code using an optimizer with dead code elimination eliminates the need for using a preprocessor to perform the same task e g define debug_mode 0 then if debug_mode in practice much of the dead code that an optimizer finds is created by other transformations in the optimizer for example the classic techniques for operator strength reduction insert new computations into the code and render the older more expensive computations dead 2 subsequent dead code elimination removes those calculations and completes the effect without complicating the strength reduction algorithm historically dead code elimination was performed using information derived from data flow analysis 3 an algorithm based on static single assignment form ssa appears in the original journal article on ssa form by ron cytron et al 4 robert shillingsburg aka shillner improved on the algorithm and developed a companion algorithm for removing useless control flow operations 5 time of elimination edit compile time edit in the above example we eliminate the dead code at compile time doing so only allows removal of code that is unconditionally dead and can be shown by the optimizer to be the case the presence of compilation unit cu boundaries hinder the determination of deadness by the optimizer link time edit consider for example a unix static library a which contains a number of object files o at link time the linker ld examines the symbols referenced by other pieces of code and chooses to only include the necessary object files ones that contain the referenced symbols both by the original code and by the object files pulled in to satisfy the needs of the original object file this consists a very coarse grained version of dead code elimination object files consist of relatively independent sections that may reference each other when an object file is broken down into more sections e g with each function and or variable in their own section ffunction sections fdata sections and if the linker is told to analyze the inter dependencies in a section level granularity gc sections a more complete form of link time dce can be achieved fdata sections may counter productively inflate the size of the binary by creating more relocation entries 6 the gold standard for link time dce is to turn link time into another compile time i e link time optimization in this setup object files contain intermediate representations used by the compiler instead of or in addition to machine code in this way the optimizer is no longer hindered by cu boundaries since it has access to the entire program enabling it to prove many more properties of the code that can be used for optimization this comes with the downside of extended compile times for processing such a large representation of the program dynamic edit in practice it is also common for code sections to represent dead or unreachable code only under certain conditions which may not be known at the time of compilation or assembly such conditions may be imposed by different runtime environments for example different versions of an operating system or different sets and combinations of drivers or services loaded in a particular target environment which may require different sets of special cases in the code but at the same time become conditionally dead code for the other cases 7 8 also the software for example a driver or resident service may be configurable to include or exclude certain features depending on user preferences rendering unused code portions useless in a particular scenario 7 8 while modular software may be developed to dynamically load libraries on demand only in most cases it is not possible to load only the relevant routines from a particular library and even if this would be supported a routine may still include code sections which can be considered dead code in a given scenario but could not be ruled out at compile time already the techniques used to dynamically detect demand identify and resolve dependencies remove such conditionally dead code and to recombine the remaining code at load or runtime are called dynamic dead code elimination 9 10 11 or dynamic dead instruction elimination 12 most programming languages compilers and operating systems offer no or little more support than dynamic loading of libraries and late linking therefore software utilizing dynamic dead code elimination is very rare in conjunction with languages compiled ahead of time or written in assembly language 13 14 15 however language implementations doing just in time compilation may dynamically optimize for dead code elimination 11 16 17 although with a rather different focus similar approaches are sometimes also utilized for dynamic software updating and hot patching see also edit redundant code simplification symbolic computation partial redundancy elimination conjunction elimination dynamic software updating dynamic coupling computing self relocation software cruft tree shaking bundle time dce for dynamic languages analogous to link time dce post pass optimization profile guided optimization superoptimizer function multi versioning references edit malavolta ivano et al javascript dead code identification elimination and empirical assessment ieee transactions on software engineering 49 7 2023 3692 3714 web allen frances cocke john kennedy ken june 1981 reduction of operator strength in jones neil d muchnick steven stanley eds program flow analysis theory application prentice hall isbn 0 13729681 9 kennedy ken june 1981 a survey of data flow analysis techniques in jones neil d muchnick steven stanley eds program flow analysis theory application prentice hall isbn 0 13729681 9 cytron ron k ferrante jeanne rosen barry k zadeck f kenneth 1991 efficiently computing static single assignment form and the program dependence graph acm toplas 13 4 cooper keith d torczon linda 2003 2002 01 01 engineering a compiler morgan kaufmann pp 498ff isbn 978 1 55860698 2 query on ffunction section fdata sections options of gcc stack overflow 1 2 paul matthias r 2002 04 03 2001 06 18 fd dev ctrl alt del freedos dev retrieved 2017 09 09 any of the options can be permanently excluded at installation time will also save the memory for the corresponding code excerpts due to our dynamic dead code elimination or it can be disabled or enabled at any later time via api functions in case someone wants to keep a user from being able to reboot the machine we are considering to add more synchronous cache flush calls due to our dynamic dead code elimination method this would not cause any kind of bloating when not needed in a particular target configuration as a particular cache flush call would be included in freekeyb s runtime image only if the corresponding disk cache is loaded as well or freekeyb was told by command line switches to load the corresponding support cite web cs1 maint deprecated archival service link 1 2 paul matthias r 2002 04 06 fd dev ctrl alt del freedos dev retrieved 2019 04 27 freekeyb builds the driver s runtime image at initialization time depending on the type of machine it is being loaded on the type of keyboard layout country and code page used the type of mouse and video adapter s installed the other drivers loaded on that system the operating system and the load and relocation method s used the individual features included and the configuration options specified in the command line due to the large number of command line switches and options supported around fifty switches with multiple possible settings there is a high number of feature combinations with uncountable dependencies resulting in endless number of different target images freekeyb s dynamic dead code elimination technique manages to resolve these dependencies and remove dead code and data is not restricted to include or exclude a somewhat limited number of modules or whole sub routines and fix up some dispatch tables as in classical tsr programming but works at byte level able to remove individual instructions in the middle of larger routines distributed all over the code to handle a particular case or support a specific feature special tools are used to analyze the code and create fixup tables automated using conditional defines to declare the various cases not only optional at assembly time but at initialization time without the overhead of having at least some amount of dead code left in the runtime image to keep track of all the dependencies between these conditionals dynamically build and relocate the runtime image fix up all the references between these small changing and moving binary parts still allowing to use the tiny com sys style model is done at initialization time api to import and export object structures between freekeyb and the calling application to transparently resize and move them internally at runtime cite web cs1 maint deprecated archival service link thammanur sathyanarayan 2001 01 31 a just in time register allocation and code optimization framework for embedded systems ms thesis university of cincinnati engineering computer engineering ucin982089462 archived 2019 07 28 at the wayback machine archived 2019 07 28 at the wayback machine kubice jan 2024 10 17 dynamic dead code elimination optimizing for flexibility 1 2 conway andrew 1995 12 04 cyclic data structures newsgroup comp lang functional retrieved 2017 07 03 lazy evaluation is basically dynamic dead code elimination cite newsgroup cs1 maint deprecated archival service link nb possibly the first public use of the term dynamic dead code elimination though only conceptually and with a focus on lazy evaluation in functional languages butts j adam sohi guri october 2002 dynamic dead instruction detection and elimination pdf san jose ca usa computer science department university of wisconsin madison asplos x acm 1 58113 574 2 02 0010 retrieved 2017 06 23 cite web cs1 maint deprecated archival service link paul matthias r frinke axel c 1997 10 13 first published 1991 freekeyb enhanced dos keyboard and console driver user manual v6 5 ed nb freekeyb is a unicode based dynamically configurable successor of k3plus supporting most keyboard layouts code pages and country codes utilizing an off the shelf macro assembler as well as a framework of automatic pre and post processing analysis tools to generate dependency and code morphing meta data to be embedded into the executable file alongside the binary code and a self discarding relaxing and relocating loader the driver implements byte level granular dynamic dead code elimination and relocation techniques at load time as well as self modifying code and reconfigurability at run time to minimize its memory footprint downto close the canonical form depending on the underlying hardware operating system and driver configuration as well as the selected feature set and locale about sixty configuration switches with hundreds of options for an almost unlimited number of possible combinations this complexity and the dynamics are hidden from users who deal with a single executable file just like they would do with a conventional driver k3plus was an extended keyboard driver for dos widely distributed in germany at its time with adaptations to a handful of other european languages available it supported a sub set of features already but did not implement dynamic dead code elimination paul matthias r frinke axel c 2006 01 16 freekeyb advanced international dos keyboard and console driver user manual v7 preliminary ed paul matthias r 2001 04 10 ann freedos beta 6 released in german newsgroup de comp os msdos retrieved 2017 07 02 brandneue s feature der dynamischen dead code elimination die die jeweils notwendigen bestandteile des treibers erst zum installationszeitpunkt zusammenbastelt und reloziert so daß keine ungenutzten code oder datenbereiche mehr resident bleiben z b wenn jemand ein bestimmtes freekeyb feature nicht benötigt cite newsgroup cs1 maint deprecated archival service link nb this represents the first known implementation of byte level granular dynamic dead code elimination for software assembled or compiled ahead of time johng yessong danielsson per ehnsiö per hermansson mats jolanki mika moore scott strander lars wettergren lars 2002 11 08 chapter 5 java overview and iseries implementation 5 1 1 miscellaneous components intentia movex java on the ibm iseries server an implementation guide overview of movex java on the iseries server movex java on iseries installation and configuration operational tips and techniques pdf red books ibm corp p 41 isbn 0 73842461 7 sg24 6545 00 archived pdf from the original on 2013 10 08 retrieved 2019 04 20 polito guillermo 2015 virtualization support for application r...