On 05/27/2014 08:39 AM, Jeff Law wrote:
> But the value we want may be sitting around in a convenient register (such as
> r11). So if we force the sibcall to use %rax, then we have to generate a
> copy. Yet if we have a constraint for the set of registers allowed here, then
> we give the register allocator the opportunity to coalesce away the copies.
We do have an existing call-clobbered register class that we currently do use
for indirect sibcalls.
The problem that Kai is trying to work around is that he doesn't want to load
the value into a register, but its address. But the register allocator only
has BASE_REG_CLASS and INDEX_REG_CLASS, and has no possibility to force the
address components into registers that are call clobbered.
My thinking is that, with few exceptions, this doesn't help anything. If we
have to load the full address into a register:
lea ofs(base, index, scale), %eax
...
call *0(%eax)
we might as well include the memory load
mov ofs(base, index, scale), %eax
...
call *%eax
As far as I can see, the only time we can actually save an instruction is when
the address is fully symbolic:
mov foo, %eax
...
call *%eax
becomes
call *foo
r~
