Dear GCC developers:
TL;DR Can I talk to a maintainer of gcc's tail-call optimizer to learn more
about the very impressive technology behind it? Or is there a paper, or
documentation, that explains it?
I am sure you know that gcc does a much better job of tail-call optimizations
than its competitors. Recently I noticed this little gem:
void f( void *p);
int h( int n, void *p)
{
{
void *a[ 10 ];
a[ 0 ]= p;
f(( void *)a);
}
return h(n, p);
}
With the inner curly braces, gcc turns the call to h into a jump. Without the
inner curly braces, gcc does not do tail-call optimization. This is exactly
correct, because we cannot know whether f() stores its argument in a place
where later calls to h() might access it. The inner braces ensure that a[] is
officially deallocated before the call to h.
Clang/llvm does not tail-call-optimize this program.
So, congratulations.
I would enjoy speaking to one of you who is knowledgeable about how gcc does
this.
A few years ago I wrote some compiler textbooks,
Modern Compiler Implementation in C, in ML, in Java (Cambridge University
Press)
and I am still interested in these things. More to the point, I use C as the
target for a functional-language compiler, for which tail-call optimization is
very important.
So please let me know,
* is there a paper somewhere that describes the approach that gcc takes for
TCO?
* I'm particularly interested in how gcc's intermediate language represents the
fact that a[] is deallocated at the first close-curly-brace
* Would someone like to Zoom sometime to tell me about this?
Sincerely,
Andrew Appel
Professor of Computer Science
Princeton University
