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== Introduction == | == Introduction == | ||
'''Inline transformation''', also known as ''function inlining'' or ''inline expansion'', is a compiler optimization that replaces a function call with the actual body of the called function | '''Inline transformation''', also known as ''function inlining'' or ''inline expansion'', is a compiler optimization that replaces a function call with the actual body of the called function. Instead of executing a separate call instruction and incurring the overhead of passing arguments and returning a result, the compiler inserts the function’s code directly at each call site. This is conceptually similar to a preprocessor macro expansion, but it is performed by the compiler on the intermediate code without altering the original source text. The primary goal of inlining is to improve performance by eliminating function-call overhead and enabling further optimizations. Modern compilers can automatically inline functions they deem profitable, and languages like C/C++ provide an <code>inline</code> keyword to ''suggest'' inlining (though the compiler is free to ignore it). Inline expansion can occur at compile time or even later – for instance, '''link-time optimization (LTO)''' allows inlining across object files, and Just-In-Time (JIT) runtimes (like the Java HotSpot VM) perform inlining at runtime using profiling information. | ||
== Mechanism of Inline Transformation == | == Mechanism of Inline Transformation == | ||
When a function call is inlined, the compiler treats the function much like a code snippet to be substituted into the caller. It will evaluate and assign the function’s arguments to local temporaries (as it would for a normal call), then insert the entire function body at the call site, adjusting variable references and control flow as needed | When a function call is inlined, the compiler treats the function much like a code snippet to be substituted into the caller. It will evaluate and assign the function’s arguments to local temporaries (as it would for a normal call), then insert the entire function body at the call site, adjusting variable references and control flow as needed. This means that the normal process of jumping to the function’s code and then returning is bypassed. Ordinarily, a function call requires a branch (transfer of control) to the function, plus setup and teardown instructions (such as saving registers, pushing arguments, and later restoring registers on return) – with inlining, these steps are eliminated so that execution “drops through” directly into the inlined code without a call or return instruction. | ||
<ref>C Programming Techniques: Function Call Inlining - Fabien Le Mentec https://www.embeddedrelated.com/showarticle/172.php</ref> ''Function call overhead.'' A normal function call introduces extra instructions for saving state, passing arguments, the call itself, and restoring state after returning (overhead shown in red) separate from the function’s useful computation (blue). | <ref>C Programming Techniques: Function Call Inlining - Fabien Le Mentec https://www.embeddedrelated.com/showarticle/172.php</ref> ''Function call overhead.'' A normal function call introduces extra instructions for saving state, passing arguments, the call itself, and restoring state after returning (overhead shown in red) separate from the function’s useful computation (blue). | ||
<ref>C Programming Techniques: Function Call Inlining - Fabien Le Mentec https://www.embeddedrelated.com/showarticle/172.php</ref> ''Inline expansion removes overhead.'' After inlining, the function’s code is substituted at the call site, so the call and its prologue/epilogue overhead are removed (the red overhead boxes are gone). The program continues executing the inlined body as if it were part of the caller | <ref>C Programming Techniques: Function Call Inlining - Fabien Le Mentec https://www.embeddedrelated.com/showarticle/172.php</ref> ''Inline expansion removes overhead.'' After inlining, the function’s code is substituted at the call site, so the call and its prologue/epilogue overhead are removed (the red overhead boxes are gone). The program continues executing the inlined body as if it were part of the caller. | ||
Because the function body is now part of the caller, the compiler can optimize across what was once a call boundary. For example, if certain arguments are constants at the call site, those constant values may propagate into the inlined function, allowing the compiler to simplify calculations or remove branches inside the function <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. In effect, inlining can make two separate functions behave as one larger function, which often enables additional compiler optimizations that would not be possible otherwise. | Because the function body is now part of the caller, the compiler can optimize across what was once a call boundary. For example, if certain arguments are constants at the call site, those constant values may propagate into the inlined function, allowing the compiler to simplify calculations or remove branches inside the function <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. In effect, inlining can make two separate functions behave as one larger function, which often enables additional compiler optimizations that would not be possible otherwise. | ||
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'''Advantages:''' | '''Advantages:''' | ||
* '''Eliminating call overhead:''' Inlining avoids the runtime cost of pushing function arguments, jumping to the function, and returning from it | * '''Eliminating call overhead:''' Inlining avoids the runtime cost of pushing function arguments, jumping to the function, and returning from it. This can make execution faster, especially for very small or frequently called functions where the call overhead is a significant portion of the runtime cost. | ||
* '''Enabling further optimizations:''' By merging the function code into the caller, the compiler gains a wider scope for optimization. It can perform constant propagation, common subexpression elimination, loop optimizations, and other transformations across what used to be a function boundary <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html | * '''Enabling further optimizations:''' By merging the function code into the caller, the compiler gains a wider scope for optimization. It can perform constant propagation, common subexpression elimination, loop optimizations, and other transformations across what used to be a function boundary <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. For instance, inlined code might expose that a condition is always true in a particular context, allowing dead code elimination in the combined code. | ||
* '''Potentially improved performance and smaller code for tiny functions:''' If a function is very simple (e.g., just returns a calculation or a field) and is called often, inlining it might not only speed up execution but could ''reduce'' code size by removing the call/return sequence. (In some cases, the overhead of a call is larger than the function body itself.) Inlining such small functions can both save time and avoid duplicate function call setup code, yielding faster and ''sometimes'' smaller binaries <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. | * '''Potentially improved performance and smaller code for tiny functions:''' If a function is very simple (e.g., just returns a calculation or a field) and is called often, inlining it might not only speed up execution but could ''reduce'' code size by removing the call/return sequence. (In some cases, the overhead of a call is larger than the function body itself.) Inlining such small functions can both save time and avoid duplicate function call setup code, yielding faster and ''sometimes'' smaller binaries <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. | ||
* '''Better use of instruction pipeline:''' Eliminating function calls can help the CPU’s instruction pipeline and branch predictor. With no branch to an external function, there’s no risk of misprediction or pipeline flush for that call, which can improve the instruction flow continuity (though this benefit is context-dependent). | * '''Better use of instruction pipeline:''' Eliminating function calls can help the CPU’s instruction pipeline and branch predictor. With no branch to an external function, there’s no risk of misprediction or pipeline flush for that call, which can improve the instruction flow continuity (though this benefit is context-dependent). | ||
'''Disadvantages:''' | '''Disadvantages:''' | ||
* '''Code size increase (code bloat):''' The biggest drawback of inlining is that it duplicates the function’s code at every call site. If a function is inlined in N places, there will be N copies of its body in the final program | * '''Code size increase (code bloat):''' The biggest drawback of inlining is that it duplicates the function’s code at every call site. If a function is inlined in N places, there will be N copies of its body in the final program. This can dramatically increase the compiled code size, especially for larger functions or many call sites. A larger binary can negatively impact instruction cache usage and paging. | ||
* '''Instruction cache pressure:''' Excessive inlining can hurt performance by filling up the CPU’s instruction cache with repeated code | * '''Instruction cache pressure:''' Excessive inlining can hurt performance by filling up the CPU’s instruction cache with repeated code. When code size grows too large, it may no longer fit in fast instruction caches, leading to more cache misses and slower execution. In other words, beyond a certain point, the lost I-cache efficiency outweighs the saved function call overhead. As a rule of thumb, some inlining improves speed at a minor cost in space, but too much inlining can ''reduce'' speed due to cache effects and increased binary size. | ||
* '''Diminishing returns for large functions:''' Inlining a large function can embed a lot of code into callers, which might not be worth the small constant overhead of a function call. Compilers often refuse to inline functions that are “too large” because the benefit doesn’t justify the cost. In fact, most compilers ignore an <code>inline</code> request if the function’s size exceeds certain heuristics | * '''Diminishing returns for large functions:''' Inlining a large function can embed a lot of code into callers, which might not be worth the small constant overhead of a function call. Compilers often refuse to inline functions that are “too large” because the benefit doesn’t justify the cost. In fact, most compilers ignore an <code>inline</code> request if the function’s size exceeds certain heuristics. Inlining deeply recursive functions is also usually avoided (or limited to a few unrolls) because it would blow up code size or even be impossible to fully inline infinite recursion. | ||
* '''Compilation time and memory:''' While not usually a major concern, inlining can increase compile time and memory usage in the compiler, since the optimizer now has to work with larger functions after inlining. Extremely aggressive inlining (especially via compiler flags) might slow down compilation and produce larger intermediate code for the compiler to process. | * '''Compilation time and memory:''' While not usually a major concern, inlining can increase compile time and memory usage in the compiler, since the optimizer now has to work with larger functions after inlining. Extremely aggressive inlining (especially via compiler flags) might slow down compilation and produce larger intermediate code for the compiler to process. | ||
* '''Debugging and profiling complexity:''' When a function is inlined, it no longer exists as a separate entity in the compiled output, which can complicate debugging. For example, setting breakpoints or getting stack traces for inlined functions is harder because they don’t have their own stack frame. Similarly, performance profilers might attribute time spent in an inlined function to the caller, which can be confusing. (Modern debuggers and profilers do have support for inlined code, but it can still be less straightforward than with regular function calls.) | * '''Debugging and profiling complexity:''' When a function is inlined, it no longer exists as a separate entity in the compiled output, which can complicate debugging. For example, setting breakpoints or getting stack traces for inlined functions is harder because they don’t have their own stack frame. Similarly, performance profilers might attribute time spent in an inlined function to the caller, which can be confusing. (Modern debuggers and profilers do have support for inlined code, but it can still be less straightforward than with regular function calls.) | ||
In general, inlining is most beneficial for '''small, frequently-called functions''' (such as simple getters or arithmetic functions) and in performance-critical code paths | In general, inlining is most beneficial for '''small, frequently-called functions''' (such as simple getters or arithmetic functions) and in performance-critical code paths. It is usually counterproductive for large or infrequently-called functions. Compilers use sophisticated heuristics to decide an optimal balance (discussed below), and they may ignore a programmer’s inline suggestion if it would lead to worse results. | ||
== Implementation in Popular Compilers == | == Implementation in Popular Compilers == | ||
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It’s worth noting that in C++ programs, GCC automatically treats any function defined ''inside a class definition'' as inline (this is mandated by the C++ standard). GCC will attempt to inline such functions even without the <code>inline</code> keyword <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. Also, if a function is declared <code>inline</code>, GCC still emits a standalone function definition for it ''unless'' it can prove that every call was inlined and no external reference is needed. This means an inline function might not actually be inlined everywhere, but the one-definition rule is respected by outputting one copy if needed (you can prevent outputting unused inline functions with the <code>-fkeep-inline-functions</code> flag, for instance <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>). | It’s worth noting that in C++ programs, GCC automatically treats any function defined ''inside a class definition'' as inline (this is mandated by the C++ standard). GCC will attempt to inline such functions even without the <code>inline</code> keyword <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>. Also, if a function is declared <code>inline</code>, GCC still emits a standalone function definition for it ''unless'' it can prove that every call was inlined and no external reference is needed. This means an inline function might not actually be inlined everywhere, but the one-definition rule is respected by outputting one copy if needed (you can prevent outputting unused inline functions with the <code>-fkeep-inline-functions</code> flag, for instance <ref>Inline - Using the GNU Compiler Collection (GCC) https://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Inline.html</ref>). | ||
'''Summary (GCC):''' Use the <code>inline</code> keyword to hint inlining, and the <code>__attribute__((always_inline))</code> to strongly force it (typically combined with <code>inline</code> in the definition). At <code>-O3</code> or with <code>-finline-functions</code>, GCC becomes more aggressive about inlining automatically. The compiler will ignore inline hints for overly large functions or if other constraints prevent inlining | '''Summary (GCC):''' Use the <code>inline</code> keyword to hint inlining, and the <code>__attribute__((always_inline))</code> to strongly force it (typically combined with <code>inline</code> in the definition). At <code>-O3</code> or with <code>-finline-functions</code>, GCC becomes more aggressive about inlining automatically. The compiler will ignore inline hints for overly large functions or if other constraints prevent inlining. | ||
=== Clang/LLVM === | === Clang/LLVM === | ||
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Deciding ''when'' to inline a function is a complex problem, and compilers use sophisticated heuristics to make this decision. Inlining provides a trade-off between speed and size, and the “right” amount of inlining can depend on the target CPU, the overall program structure, and runtime behavior of the code. Some of the challenges and considerations include: | Deciding ''when'' to inline a function is a complex problem, and compilers use sophisticated heuristics to make this decision. Inlining provides a trade-off between speed and size, and the “right” amount of inlining can depend on the target CPU, the overall program structure, and runtime behavior of the code. Some of the challenges and considerations include: | ||
* '''Predicting performance impact is non-trivial:''' While removing a function call generally improves execution speed, the net effect on a large program is not always positive. Inlining can ''increase'' or ''decrease'' performance depending on many factors | * '''Predicting performance impact is non-trivial:''' While removing a function call generally improves execution speed, the net effect on a large program is not always positive. Inlining can ''increase'' or ''decrease'' performance depending on many factors. For example, inlining might speed up one part of the code but cause another part to slow down due to cache misses. The compiler has to predict whether inlining a particular function at a particular call site will be beneficial overall, which is undecidable with perfect accuracy. As studies and experience have shown, ''no compiler can always make the optimal inlining decision'' because it lacks full knowledge of runtime execution patterns and hardware microarchitectural effects <ref>c++ - Link-time optimization and inline - Stack Overflow https://stackoverflow.com/questions/7046547/link-time-optimization-and-inline</ref>. The instruction cache behavior is especially critical: a program that fit in cache before might overflow it after inlining one too many functions, causing performance to drop. These complex interactions mean that inlining decisions are essentially heuristic guesses aimed at a balance. | ||
* '''Compiler heuristics:''' Modern compilers treat the inlining decision as an optimization problem. They often set a '''“budget” for code growth''' and try to inline the most beneficial calls without exceeding that budget | * '''Compiler heuristics:''' Modern compilers treat the inlining decision as an optimization problem. They often set a '''“budget” for code growth''' and try to inline the most beneficial calls without exceeding that budget. This is sometimes modeled like a knapsack problem – choosing which function calls to inline to maximize estimated performance gain for a given allowable increase in code size. The heuristics involve metrics such as the size of the function (in internal intermediate representation instructions), the number of call sites, and the estimated frequency of each call. For instance, a call inside a loop that runs thousands of times is more profitable to inline than a call in a one-off initialization function. Compilers also consider whether inlining a function will enable ''subsequent optimizations'': if inlining a function exposes a constant or a branch that can simplify the code, the compiler gives that more weight. These factors are combined into a cost/benefit analysis for each call site. If the estimated benefit (e.g., saved cycles) outweighs the cost (e.g., added instructions and bytes of code), the call is inlined – otherwise it’s left as a regular call <ref>c++ - Link-time optimization and inline - Stack Overflow https://stackoverflow.com/questions/7046547/link-time-optimization-and-inline</ref>. Different compilers (and even different versions of the same compiler) use different formulas and thresholds for this. For example, GCC and Clang assign a certain “cost” to the function based on IR instruction count and adjust it if the function is marked <code>inline</code> (which gives a hint benefit) or if the call is in a hot path, etc. MSVC similarly has internal thresholds and will inline more aggressively at <code>/Ob2</code> than at <code>/Ob1</code>. These heuristics are continually refined to produce good results across typical programs. | ||
* '''Profile-guided inlining:''' One way to improve inlining decisions is to use ''profile-guided optimization (PGO)''. PGO involves compiling the program, running it on sample workloads to gather actual execution frequencies of functions and branches, and then feeding that profile data back into a second compilation. With PGO, the compiler knows which functions are actually hot (called frequently in practice) and which call sites are executed often. This information can greatly inform the inlining heuristics – for example, the compiler might inline a function it knows is called millions of times a second, but not inline another function that is rarely used, even if they are similar in size. Using PGO, compilers can be more bold about inlining hot paths and avoid code bloat on cold paths. That said, the gains from PGO-based inlining, while real, are often in the single-digit percentages of performance <ref>c++ - Link-time optimization and inline - Stack Overflow https://stackoverflow.com/questions/7046547/link-time-optimization-and-inline</ref>. It helps the compiler make more informed decisions, but it doesn’t fundamentally eliminate the trade-offs. In some cases PGO might even cause slight regressions if the profile data misleads the heuristics (e.g., if the runtime usage differs from the training run). Still, PGO is a valuable tool for squeezing out extra performance by fine-tuning inlining and other optimizations based on actual usage. | * '''Profile-guided inlining:''' One way to improve inlining decisions is to use ''profile-guided optimization (PGO)''. PGO involves compiling the program, running it on sample workloads to gather actual execution frequencies of functions and branches, and then feeding that profile data back into a second compilation. With PGO, the compiler knows which functions are actually hot (called frequently in practice) and which call sites are executed often. This information can greatly inform the inlining heuristics – for example, the compiler might inline a function it knows is called millions of times a second, but not inline another function that is rarely used, even if they are similar in size. Using PGO, compilers can be more bold about inlining hot paths and avoid code bloat on cold paths. That said, the gains from PGO-based inlining, while real, are often in the single-digit percentages of performance <ref>c++ - Link-time optimization and inline - Stack Overflow https://stackoverflow.com/questions/7046547/link-time-optimization-and-inline</ref>. It helps the compiler make more informed decisions, but it doesn’t fundamentally eliminate the trade-offs. In some cases PGO might even cause slight regressions if the profile data misleads the heuristics (e.g., if the runtime usage differs from the training run). Still, PGO is a valuable tool for squeezing out extra performance by fine-tuning inlining and other optimizations based on actual usage. | ||
* '''Limitations and overrides:''' There are practical limits to inlining. Compilers will not inline a function in certain scenarios, no matter what: for example, a recursive function usually can’t be fully inlined (it would lead to infinite code expansion), although some compilers will unroll a recursion a fixed number of times if marked inline. If a function’s address is taken (meaning a pointer to the function is used), most compilers have to generate an actual function body for it, and they might not inline all calls either because the function now needs to exist independently <ref>Inline Functions (C++) | Microsoft Learn https://learn.microsoft.com/en-us/cpp/cpp/inline-functions-cpp?view=msvc-170</ref>. Virtual function calls in C++ cannot be inlined unless the compiler can deduce the exact target (e.g., the object’s dynamic type is known or the function is devirtualized); thus, inlining across a polymorphic call often requires whole-program analysis or final devirtualization. Additionally, as mentioned earlier, compilers impose certain limits to avoid ''pathological code expansion'': GCC, for instance, has parameters like '''<code>inline-unit-growth</code>''' and '''<code>max-inline-insns-single</code>''' that prevent inlining from blowing up the code more than a certain factor <ref>Optimize Options (Using the GNU Compiler Collection (GCC)) https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html</ref> <ref>Optimize Options (Using the GNU Compiler Collection (GCC)) https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html</ref>. These ensure that even under <code>-O3</code>, the compiler won’t inline everything blindly and will stop if the function grows too large due to inlining. | * '''Limitations and overrides:''' There are practical limits to inlining. Compilers will not inline a function in certain scenarios, no matter what: for example, a recursive function usually can’t be fully inlined (it would lead to infinite code expansion), although some compilers will unroll a recursion a fixed number of times if marked inline. If a function’s address is taken (meaning a pointer to the function is used), most compilers have to generate an actual function body for it, and they might not inline all calls either because the function now needs to exist independently <ref>Inline Functions (C++) | Microsoft Learn https://learn.microsoft.com/en-us/cpp/cpp/inline-functions-cpp?view=msvc-170</ref>. Virtual function calls in C++ cannot be inlined unless the compiler can deduce the exact target (e.g., the object’s dynamic type is known or the function is devirtualized); thus, inlining across a polymorphic call often requires whole-program analysis or final devirtualization. Additionally, as mentioned earlier, compilers impose certain limits to avoid ''pathological code expansion'': GCC, for instance, has parameters like '''<code>inline-unit-growth</code>''' and '''<code>max-inline-insns-single</code>''' that prevent inlining from blowing up the code more than a certain factor <ref>Optimize Options (Using the GNU Compiler Collection (GCC)) https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html</ref> <ref>Optimize Options (Using the GNU Compiler Collection (GCC)) https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html</ref>. These ensure that even under <code>-O3</code>, the compiler won’t inline everything blindly and will stop if the function grows too large due to inlining. | ||
* '''Link-time optimization (LTO):''' Traditional compilation limits inlining to within a single source file (translation unit) because the compiler can only see one .c/.cpp file at a time. '''Link-time optimization''' lifts this restriction by allowing inlining (and other optimizations) to occur across translation unit boundaries at link time. With LTO enabled (for example, <code>gcc -flto</code> or MSVC’s <code>/LTCG</code>), the compiler effectively sees the entire program or library, so it can inline functions from one module into another | * '''Link-time optimization (LTO):''' Traditional compilation limits inlining to within a single source file (translation unit) because the compiler can only see one .c/.cpp file at a time. '''Link-time optimization''' lifts this restriction by allowing inlining (and other optimizations) to occur across translation unit boundaries at link time. With LTO enabled (for example, <code>gcc -flto</code> or MSVC’s <code>/LTCG</code>), the compiler effectively sees the entire program or library, so it can inline functions from one module into another. This means even if you didn’t mark a function <code>inline</code> or put it in a header, LTO might inline it if it’s beneficial. For instance, a small utility function defined in one source file and called in another could be inlined during LTO, whereas without LTO that call would remain a regular function call (because the compiler wouldn’t have seen the function’s body while compiling the caller). LTO thus increases the scope of inlining and can yield significant performance improvements for codebases split across many files. One common use of LTO-driven inlining is for library functions: the compiler might inline standard library functions or other library calls if LTO is enabled and it has the library’s code. The downside is that LTO can make compile times (or link times) longer and increase memory usage during compilation, due to the larger optimization scope. Also, the same caution applies: even with whole-program visibility, the compiler still uses heuristics to decide what to inline <ref>c++ - Link-time optimization and inline - Stack Overflow https://stackoverflow.com/questions/7046547/link-time-optimization-and-inline</ref>. Having more opportunities to inline (thanks to LTO) doesn’t mean it will inline everything; it still must choose carefully to avoid overwhelming code bloat or slower performance from cache misses. | ||
* '''Alternate strategies:''' In cases where inlining is not beneficial or possible, other optimizations may be preferable. For example, compilers might use ''outline'' strategies (the opposite of inline) to reduce code size – i.e., they might decide ''not'' to inline to keep code small (especially at <code>-Os</code> or in constrained environments). Another strategy is '''partial inlining''', where the compiler might extract and inline only a portion of a function. GCC introduced something along these lines (sometimes called “IPA-split” or partial inlining) where it tries to inline the hot parts of a function into callers and keep the cold parts out-of-line, as a compromise. This is advanced and not directly under user control, but it shows that inlining doesn’t have to be all-or-nothing. | * '''Alternate strategies:''' In cases where inlining is not beneficial or possible, other optimizations may be preferable. For example, compilers might use ''outline'' strategies (the opposite of inline) to reduce code size – i.e., they might decide ''not'' to inline to keep code small (especially at <code>-Os</code> or in constrained environments). Another strategy is '''partial inlining''', where the compiler might extract and inline only a portion of a function. GCC introduced something along these lines (sometimes called “IPA-split” or partial inlining) where it tries to inline the hot parts of a function into callers and keep the cold parts out-of-line, as a compromise. This is advanced and not directly under user control, but it shows that inlining doesn’t have to be all-or-nothing. | ||
In summary, inline transformation is a powerful optimization, but it must be applied with care. Compilers provide keywords and options to guide inlining, but they also rightfully employ their own models to decide when inlining makes sense. As a developer, a good practice is to trust the compiler for general decisions, and only force inlining in cases where you have clear evidence (via profiling or knowledge of the code) that the compiler’s heuristic might be missing an opportunity. Tools like optimization reports or profile-guided optimization can assist in making those decisions. Ultimately, inline transformation is one of many tools in the compiler’s toolbox, and its effectiveness will vary – some code speeds up dramatically with inlining, while in other cases excessive inlining can degrade performance | In summary, inline transformation is a powerful optimization, but it must be applied with care. Compilers provide keywords and options to guide inlining, but they also rightfully employ their own models to decide when inlining makes sense. As a developer, a good practice is to trust the compiler for general decisions, and only force inlining in cases where you have clear evidence (via profiling or knowledge of the code) that the compiler’s heuristic might be missing an opportunity. Tools like optimization reports or profile-guided optimization can assist in making those decisions. Ultimately, inline transformation is one of many tools in the compiler’s toolbox, and its effectiveness will vary – some code speeds up dramatically with inlining, while in other cases excessive inlining can degrade performance. The key is balancing those effects, a task that modern compilers handle through continual refinement of their inlining algorithms. | ||
==Procedure Inline Transformation in emmtrix Studio== | ==Procedure Inline Transformation in emmtrix Studio== | ||