参考： dave.cheney.net/2020/04/25/…
What is inlining?
Inlining is the act of combining smaller functions into their respective callers. In the early days of computing this optimisation was typically performed by hand. Nowadays inlining is one of a class of fundamental optimisations performed automatically during the compilation process.
Why is inlining important?
Inlining is important for two reasons. The first is it removes the overhead of the function call itself. The second is it permits the compiler to more effectively apply other optimisation strategies.
Function call overhead
Calling a function in any language carries a cost. There are the overheads of marshalling parameters into registers or onto the stack (depending on the ABI) and reversing the process on return. Invoking a function call involves jumping the program counter from one point in the instruction stream to another which can cause a pipeline stall. Once inside the function there is usually some preamble required to prepare a new stack frame for the function to execute and a similar epilogue needed to retire the frame before returning to the caller.
In Go a function call carries additional costs to support dynamic stack growth. On entry the amount of stack space available to the goroutine is compared to the amount required for the function. If insufficient stack space is available, the preamble jumps into the runtime logic that grows the stack by copying it to a new, larger, location. Once this is done the runtime jumps back to the start of the original function, the stack check is performed again, which now passes, and the call continues. In this way goroutines can start with a small stack allocation which grows only when needed.
This check is cheap–only a few instructions–and because goroutine stacks grows geometrically the check rarely fails. Thus, the branch prediction unit inside a modern processor can hide the cost of the stack check by assuming it will always be successful. In the case where the processor mis-predicts the stack check and has to discard the work done while it was executing speculatively, the cost of the pipeline stall is relatively small compared to the cost of the work needed for the runtime to grow a goroutines stack.
While the overhead of the generic and Go specific components of each function call are well optimised by modern processors using speculative execution techniques, those overheads cannot be entirely eliminated, thus each function call carries with it a performance cost over and above the time it takes to perform useful work. As a function call’s overhead is fixed, smaller functions pay a larger cost relative to larger ones because they tend to do less useful work per invocation.
The solution to eliminating these overheads must therefore be to eliminate the function call itself, which the Go compiler does, under certain conditions, by replacing the call to a function with the contents of the function. This is known as inlining because it brings the body of the function in line with its caller.
Improved optimisation opportunities
Dr. Cliff Click describes inlining as the optimisation performed by modern compilers as it forms the basis for optimisations like constant propagation and dead code elimination. In effect, inlining allows the compiler to see further, allowing it to observe, in the context that a particular function is being called, logic that can be further simplified or eliminated entirely. As inlining can be applied recursively optimisation decisions can be made not only in the context of each individual function, but also applied to the chain of functions in a call path.
