http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51680
--- Comment #6 from miles at gnu dot org 2011-12-28 01:04:05 UTC ---
Well, it's just an impression ... :]
I think one reason is that unlike normal functions, template functions are
implicitly sort of "local" (by necessity), in that they can have a definition
in many compilation units without causing a link conflict. To get this effect
for normal functions, one must use the "static" or "inline" keywords -- so the
impression (rightly or wrongly) is that template functions definitions are like
one of those.
... and of course if a [normal] function definition has "static" or "inline"
attached, gcc does do inlining differently. As I mentioned in my original
report, gcc 4.6, seems to treat template functions this way w/r/t inlining, as
if they were static.[*]
On another note, I'd kinda expect gcc these days to be making pretty reasonable
decisions about inlining even without hints; that's why I'm surprised at
behavior of my example, where the functions involved are so trivial that
inlining seems almost always a good bet...
[*] For instance, here's a "normal function" version of my example: extern
void process (float);
void process_fun_at (float (&fun)(float), float x)
{
process (fun (x));
}
static float add1 (float x)
{
return x + 1;
}
void test (float i)
{
process_fun_at (add1, i);
}
gcc 4.6 will compile this differently if "process_fun_at" is declared "static";
if it's static, everything gets inlined, otherwise, nothing is inlined. gcc
4.6 completely inlines the original template example with no special
declaration for the "process_fun_at" template function, meaning it's being
treated kind of like it's static for inlining purposes.
gcc 4.7 compiles the normal-function example the same as gcc 4.6 for both
static and non-static cases, but _doesn't_ inline the template version by
default. So if nothing else, this is a change in behavior from gcc 4.6;
whether it's good or bad I dunno.
[and yeah, I know inlining heuristics are a big ball of hair that nobody wants
to mess with if they can help it.... sorry.... :]
Thanks,
-miles
