He means that if you have two functions 'a' and 'b' and a tail calls b, the call instruction could be eliminated because it's just going to the next instruction anyway.
This is not just inlining, it's a function with multiple entry points, which is not popular.
I would call it two functions with multiple tails.
And that was very popular when memory was scarce. The typical BASIC for the 6502 used it. See for example http://hackzapple.org/scripts_php/index.php?menu=14&mod=8517... (search for “The CHARGET Subroutine”). Here, it is used for speed, too (otherwise, at least one of the functions would have to do an indirect load, which is cumbersome and slow on a 6502. The self-modifying code is a lot faster)
It also often was used when one had, say, a function to output a character and another function that printed a specific character. Combined with a creative use of the BIT instruction, one could even have functions printing _any_ character, a space, a question mark, or a CR share their tails (https://retrocomputing.stackexchange.com/a/11132)
But yes, nowadays I guess it is rare, although, with an ABI designed for it, it could be used to make a call with a default argument value fall through into the more generic code)
One issue I can think of is that compilers/linkers assume they can reorder and delete unused symbols in a C program.
You'd turn it off at the object file level if you're writing asm and doing tricks there (x264 asm has some fallthroughs like this), but there isn't a way to say only these two functions need to be in the same order.
You wouldn’t have to say it. Linkers could figure it out on their own (but compilers dropping hints would make it easier)
Some linkers already can merge functions that happen to compile to the same code, even though that corrupts stack frames (foo calls bar, but the linker makes it call baz instead. https://stackoverflow.com/a/61865960)
It’s ‘just’ a matter of learning the build system new tricks.
“The P3 SSE stuff we've done generates multiple entry points,
but it does that within the backend prologue expander. There's nothing to generate multiple regular prologues. Though it shouldn't be that hard to do.
It's not impossible to believe that the bulk of the compiler
would work with them, as long as flow knows how to properly
create the CFG. LABEL_ALTERNATE_NAME was invented for this,
though it appears that the code to properly deal with it is
sitting on a branch waiting for accounting to say it has been
paid for.“
That was in February 2000. I wouldn’t know whether that comment was close to the truth or what gcc currently supports.
Most compilers do that, for most compiled languages.