First off, I am not looking for a way to force the compiler to inline the implementation of every function.
To reduce the level of misguided answers make sure you understand what the inline keyword actually means. Here is good description, inline vs static vs extern.
So my question, why not mark every function definition inline? ie Ideally, the only compilation unit would be main.cpp. Or possibly a few more for the functions that cannot be defined in a header file (pimpl idiom, etc).
The theory behind this odd request is it would give the optimizer maximum information to work with. It could inline function implementations of course, but it could also do "cross-module" optimization as there is only one module. Are there other advantages?
Has any one tried this in with a real application? Did the performance increase? decrease?!?
What are the disadvantages of marking all function definitions inline?
- Compilation might be slower and will consume much more memory.
- Iterative builds are broken, the entire application will need to be rebuilt after every change.
- Link times might be astronomical
All of these disadvantage only effect the developer. What are the runtime disadvantages?
Did you really mean #include everything? That would give you only a single module and let the optimizer see the entire program at once.
Actually, Microsoft's Visual C++ does exactly this when you use the /GL (Whole Program Optimization) switch, it doesn't actually compile anything until the linker runs and has access to all code. Other compilers have similar options.
sqlite uses this idea. During development it uses a traditional source structure. But for actual use there is one huge c file (112k lines). They do this for maximum optimization. Claim about 5-10% performance improvement
We (and some other game companies) did try it via making one uber-.CPP that #includeed all others; it's a known technique. In our case, it didn't seem to affect runtime much, but the compile-time disadvantages you mention turned out to be utterly crippling. With a half an hour compile after every single change, it becomes impossible to iterate effectively. (And this is with the app divvied up into over a dozen different libraries.)
We tried making a different configuration such that we would have multiple .objs while debugging and then have the uber-CPP only in release-opt builds, but then ran into the problem of the compiler simply running out of memory. For a sufficiently large app, the tools simply are not up to compiling a multimillion line cpp file.
We tried LTCG as well, and that provided a small but nice runtime boost, in the rare cases where it didn't simply crash during the link phase.
Interesting question! You are certainly right that all of the listed disadvantages are specific to the developer. I would suggest, however, that a disadvantaged developer is far less likely to produce a quality product. There may be no runtime disadvantages, but imagine how reluctant a developer will be to make small changes if each compile takes hours (or even days) to complete.
I would look at this from a "premature optimization" angle: modular code in multiple files makes life easier for the programmer, so there is an obvious benefit to doing things this way. Only if a specific application turns out to run too slow, and it can be shown that inlining everything makes a measured improvement, would I even consider inconveniencing the developers. Even then, it would be after a majority of the development has been done (so that it can be measured) and would probably only be done for production builds.
This is semi-related, but note that Visual C++ does have the ability to do cross-module optimization, including inline across modules. See http://msdn.microsoft.com/en-us/library/0zza0de8%28VS.80%29.aspx for info.
To add an answer to your original question, I don't think there would be a downside at run time, assuming the optimizer was smart enough (hence why it was added as an optimization option in Visual Studio). Just use a compiler smart enough to do it automatically, without creating all the problems you mention. :)
Little benefit
On a good compiler for a modern platform, inline will affect only a very few functions. It is just a hint to the compiler, modern compilers are fairly good at making this decision themselves, and the the overhead of a function call has become rather small (often, the main benefit of inlining is not to reduce call overhead, but opening up further optimizations).
Compile time
However, since inline also changes semantics, you will have to #include everything into one huge compile unit. This usually increases compile time significantly, which is a killer on large projects.
Code Size
if you move away from current desktop platforms and its high performance compilers, things change a lot. In this case, the increased code size generated by a less clever compiler will be a problem - so much that it makes the code significantly slower. On embedded platforms, code size is usually the first restriction.
Still, some projects can and do profit from "inline everything". It gives you the same effect as link time optimization, at least if your compiler doesn't blindly follow the inline.
It is done already in some cases. It is very similar to the idea of unity builds, and the advantages and disadvantages are not fa from what you descibe:
- more potential for the compiler to optimize
- link time basically goes away (if everything is in a single translation unit, there is nothing to link, really)
- compile time goes, well, one way or the other. Incremental builds become impossible, as you mentioned. On the other hand, a complete build is going to be faster than it would be otherwise (as every line of code is compiled exactly once. In a regular build, code in headers ends up being compiled in every translation unit where the header is included)
But in cases where you already have a lot of header-only code (for example if you use a lot of Boost), it might be a very worthwhile optimization, both in terms of build time and executable performance.
As always though, when performance is involved, it depends. It's not a bad idea, but it's not universally applicable either.
As far as buld time goes, you have basically two ways to optimize it:
- minimize the number of translation units (so your headers are included in fewer places), or
- minimize the amount of code in headers (so that the cost of including a header in multiple translation units decreases)
C code typically takes the second option, pretty much to its extreme: almost nothing apart from forward declarations and macros are kept in headers. C++ often lies around the middle, which is where you get the worst possible total build time (but PCH's and/or incremental builds may shave some time off it again), but going further in the other direction, minimizing the number of translation units can really do wonders for the total build time.
That's pretty much the philosophy behind Whole Program Optimization and Link Time Code Generation (LTCG) : optimization opportunities are best with global knowledge.
From a practical point of view it's sort of a pain because now every single change you make will require a recompilation of your entire source tree. Generally speaking you need an optimized build less frequently than you need to make arbitrary changes.
I tried this in the Metrowerks era (it's pretty easy to setup with a "Unity" style build) and the compilation never finished. I mention it only to point out that it's a workflow setup that's likely to tax the toolchain in ways they weren't anticipating.
The assumption here is that the compiler cannot optimize across functions. That is a limitation of specific compilers and not a general problem. Using this as a general solution for a specific problem might be bad. The compiler may very well just bloat your program with what could have been reusable functions at the same memory address (getting to use the cache) being compiled elsewhere (and losing performance because of the cache).
Big functions in general cost on optimization, there is a balance between the overhead of local variables and the amount of code in the function. Keeping the number of variables in the function (both passed in, local, and global) to within the number of disposable variables for the platform results in most everything being able to stay in registers and not have to be evicted to ram, also a stack frame is not required (depends on the target) so function calling overhead is noticeably reduced. Hard to do in real world applications all the time, but the alternative a small number of big functions with lots of local variables the code is going to spend a significant amount of time evicting and loading registers with variables to/from ram (depends on the target).
Try llvm it can optimize across the entire program not just function by function. Release 27 had caught up to gcc's optimizer, at least for a test or two, I didnt do exhaustive performance testing. And 28 is out so I assume it is better. Even with a few files the number of tuning knob combinations are too many to mess with. I find it best to not optimize at all until you have the whole program into one file, then perform your optimization, giving the optimizer the whole program to work with, basically what you are trying to do with inlining, but without the baggage.
Suppose foo() and bar() both call some helper(). If everything is in one compilation unit, the compiler might choose not to inline helper(), in order to reduce total instruction size. This causes foo() to make a non-inlined function call to helper().
The compiler doesn't know that a nanosecond improvement to the running time of foo() adds $100/day to your bottom line in expectation. It doesn't know that a performance improvement or degradation of anything outside of foo() has no impact on your bottom line.
Only you as the programmer know these things (after careful profiling and analysis of course). The decision not to inline bar() is a way of telling the compiler what you know.
The problem with inlining is that you want high performance functions to fit in cache. You might think function call overhead is the big performance hit, but in many architectures a cache miss will blow the couple pushes and pops out of the water. For example, if you have a large (maybe deep) function that needs to be called very rarely from your main high performance path, it could cause your main high performance loop to grow to the point where it doesn't fit in L1 icache. That will slow your code down way, way more than the occasional function call.
来源：https://stackoverflow.com/questions/3999806/why-not-mark-everything-inline