On Thu, Mar 11, 2021 at 11:29:57AM -0300, Alexandre Oliva wrote:
> On Mar 11, 2021, Richard Biener <richard.guent...@gmail.com> wrote:
> > I wonder what the effect of this switch is - it's documented as
> > affecting the call site (different call insn sequence by default), so
> > the best representation would be an attribute on actual call stmts
>
> I don't think that would fulfill its purpose.
That is how it already works though. The option sets a flag that gives
the default for a function attribute. The function attribute is used at
the call site:
===
void f(void);
void g(void) __attribute__((longcall));
void h(void)
{
f();
g();
}
static void (*i)(void) = f;
static void (*j)(void) __attribute__((longcall)) = f;
void k(void)
{
i();
j();
}
===
Note that in "k" the call via "i" is done as a direct call, and that via
"j" uses a bctr.
> My understanding is that the longcall attribute is intended to issue
> call sequences that assume the target of the call will be out of range
> for available PC-relative addressing modes in call instructions.
Yes.
> One use I'm familiar with is when different modules are known to be
> going to be linked separately, and dynamically loaded at disparate
> memory locations, so that PC-relative branches across modules won't do,
> and PLTs are not available.
And you need to call via function pointers *anyway*, so you do not need
this attribute at all? It could matter if you do cross-module IPA
optimisations, but you don't (do you? That will cause no end of fresh
new problems!)
> Indirect calls are pretty much long calls by definition,
It's the other way around. Longcalls are implemented as indirect calls.
> but I suppose
> the goal of retaining the long-call requests has to do with the
> possibility of sometimes resolving an indirect call back to a direct
> call.
You will save yourself a lot of problems by simply making that
impossible. Makes such function pointers explicit in your code.
> If it wasn't associated with the type,
It isn't. It is a function attribute.
> > but it does look better than just a global flag.
The flag is just for setting a default, which often is handy (esp. for
code that is too humonguous to compile without it: it is a simple flag
to add, and your program was probably slow to begin with, being huge and
all).
Segher
