Re: [Tinycc-devel] manually inlining functions

[Michael Matz]

Hello,

On Tue, 4 May 2021, uso ewin wrote:

> Tinycc handle inline function like if they were macro, so using inline
> in tcc always inline functions.

That's not accurate.  Inline functions in TCC are emitted like normal 
functions but only when there's reason to emit them (which are involved 
and depend on if they are called (or address-taken) or if they are c99 
inline or gnu inline semantics, and static or global).  But they are _not_ 
emitted like macros at every use, rather a normal argument setup and call 
is done, and the body is emitted only once as a normal function.

Either way, the discussion about speed of inline functions in TCC is 
fairly academic, code performance is low for many other, more important, 
reasons.  There is no optimizer, there is no intermediate representation 
that could be optimized, there is nothing that could benefit from inlining 
except the call instruction and the prologue/epilogue itself would be 
removed.

Not even the argument setup would be saved very often: TCCs compilation 
model would require evaluating the arguments to local temporaries which 
are often spilled to stack right soon, in the end needing exactly the same 
work as the argument setup for a call.

I somewhen wrote a simple function inliner for tcc handling most normal 
cases (those when the inline doesn't contain a local static variable, and 
when no variadic args are used), but the result was somewhat so-and-so, 
there simply wasn't much point to having it before having a better 
single-pass optimizer / register allocator.


Ciao,
Michael.