Incomputer programming, athunk is asubroutine used to inject a calculation into another subroutine. Thunks are primarily used to delay a calculation until its result is needed, or to insert operations at the beginning or end of the other subroutine. They have many other applications incompiler code generation andmodular programming.
The term originated as a whimsicalirregular form of the verbthink. It refers to the original use of thunks inALGOL 60 compilers, which required special analysis (thought) to determine what type of routine to generate.[1][2]
The early years ofcompiler research saw broad experimentation with differentevaluation strategies. A key question was how to compile a subroutine call if the arguments can be arbitrary mathematical expressions rather than constants. One approach, known as "call by value", calculates all of the arguments before the call and then passes the resulting values to the subroutine. In the rival "call by name" approach, the subroutine receives the unevaluated argument expression and must evaluate it.
A simple implementation of "call by name" might substitute the code of an argument expression for each appearance of the corresponding parameter in the subroutine, but this can produce multiple versions of the subroutine and multiple copies of the expression code. As an improvement, the compiler can generate a helper subroutine, called athunk, that calculates the value of the argument. The address and environment[a] of this helper subroutine are then passed to the original subroutine in place of the original argument, where it can be called as many times as needed. Peter Ingerman first described thunks in reference to the ALGOL 60 programming language, which supports call-by-name evaluation.[4]
Although the software industry largely standardized on call-by-value andcall-by-reference evaluation,[5] active study of call-by-name continued in thefunctional programming community. This research produced a series oflazy evaluation programming languages in which some variant of call-by-name is the standard evaluation strategy. Compilers for these languages, such as theGlasgow Haskell Compiler, have relied heavily on thunks, with the added feature that the thunks save their initial result so that they can avoid recalculating it;[6] this is known asmemoization orcall-by-need.
Functional programming languages have also allowed programmers to explicitly generate thunks. This is done insource code by wrapping an argument expression in ananonymous function that has no parameters of its own. This prevents the expression from being evaluated until a receiving function calls the anonymous function, thereby achieving the same effect as call-by-name.[7] The adoption of anonymous functions into other programming languages has made this capability widely available.
Thunks are useful inobject-oriented programming platforms that allow aclass toinherit multiple interfaces, leading to situations where the samemethod might be called via any of several interfaces. The following code illustrates such a situation inC++.
classA{public:virtualintAccess()const{returnvalue_;}private:intvalue_;};classB{public:virtualintAccess()const{returnvalue_;}private:intvalue_;};classC:publicA,publicB{public:intAccess()constoverride{returnbetter_value_;}private:intbetter_value_;};intuse(B*b){returnb->Access();}intmain(){// ...Bsome_b;use(&some_b);Csome_c;use(&some_c);}
In this example, the code generated for each of the classes A, B and C will include adispatch table that can be used to callAccess on an object of that type, via a reference that has the same type. Class C will have an additional dispatch table, used to callAccess on an object of type C via a reference of type B. The expressionb->Access() will use B's own dispatch table or the additional C table, depending on the type of object b refers to. If it refers to an object of type C, the compiler must ensure that C'sAccess implementation receives aninstance address for the entire C object, rather than the inherited B part of that object.[8]
As a direct approach to this pointer adjustment problem, the compiler can include an integer offset in each dispatch table entry. This offset is the difference between the reference's address and the address required by the method implementation. The code generated for each call through these dispatch tables must then retrieve the offset and use it to adjust the instance address before calling the method.
The solution just described has problems similar to the naïve implementation of call-by-name described earlier: the compiler generates several copies of code to calculate an argument (the instance address), while also increasing the dispatch table sizes to hold the offsets. As an alternative, the compiler can generate anadjustor thunk along with C's implementation ofAccess that adjusts the instance address by the required amount and then calls the method. The thunk can appear in C's dispatch table for B, thereby eliminating the need for callers to adjust the address themselves.[9]
Thunks have been widely used to provide interoperability between software modules whose routines cannot call each other directly. This may occur because the routines have differentcalling conventions, run in differentCPU modes oraddress spaces, or at least one runs in avirtual machine. A compiler (or other tool) can solve this problem by generating a thunk that automates the additional steps needed to call the target routine, whether that is transforming arguments, copying them to another location, or switching the CPU mode. A successful thunk minimizes the extra work the caller must do compared to a normal call.
Much of the literature on interoperability thunks relates to variousWintel platforms, includingMS-DOS,OS/2,[10]Windows[11][12][13][14] and.NET, and to the transition from16-bit to32-bit memory addressing. As customers have migrated from one platform to another, thunks have been essential to supportlegacy software written for the older platforms.UEFI CSM is another example to do thunk for legacyboot loaders.
The transition from 32-bit to 64-bit code on x86 also uses a form of thunking (WoW64). However, because the x86-64 address space is larger than the one available to 32-bit code, the old "generic thunk" mechanism could not be used to call 64-bit code from 32-bit code.[15] The only case of 32-bit code calling 64-bit code is in the WoW64's thunking of Windows APIs to 32-bit.
On systems that lack automaticvirtual memory hardware, thunks can implement a limited form of virtual memory known asoverlays. With overlays, a developer divides a program's code into segments that can be loaded and unloaded independently, and identifies theentry points into each segment. A segment that calls into another segment must do so indirectly via abranch table. When a segment is in memory, its branch table entries jump into the segment. When a segment is unloaded, its entries are replaced with "reload thunks" that can reload it on demand.[16]
Similarly, systems thatdynamically link modules of a program together at run-time can use thunks to connect the modules. Each module can call the others through a table of thunks that the linker fills in when it loads the module. This way the modules can interact without prior knowledge of where they are located in memory.[17]
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