[Gcl-devel] Re: sibling call optimization
Sat May 8 01:30:00 GMT 2004
On Fri, 2004-05-07 at 14:39, Camm Maguire wrote:
> OK. The full cpp output providing a testcase is included below. I
> will endeavor to file a bugzilla report on it (never done this before)
I took a quick look.
In the -O2 case, LI2 has two functions calls, both of which are
converted to sibcalls.
In the -O3 case, LI2 has two functions calls. The second one gets
inlined, so now we have 3 function calls, 1 from the original function,
and two from the inlined function. The call from the original function
gets converted to a sihcall. The two calls from the inlined function
are not converted to a sibcall because of a technicality.
The code that creates a sibcall only matches a simple pattern, a call
followed by a branch to the prologue. In the inlined case, the call
sequence has been optimized a bit, to commonize some code, so we end up
These are both still sibcalls, but the simple pattern matching code in
sibcall.c can't recognize this.
Overall, the -O3 code isn't really much worse than the -O2 code. In the
O2 code, calling from LI2->LI1->LI2 involves two sibcalls. In the -O3
case, the same sequence involves one regular function call. Note that
LI2 is calling itself, not LI1. This is because LI1 was inlined into
LI2. So it is possible that the -O3 code may actually be faster, even
though it has calls instead of sibcalls.
Still, we could do even better if we were able to handle these
sibcalls. I didn't check to see where the inlined calls got optimized.
It might be possible to avoid this so that we still get the sibcalls
created. I am unlikely to look into this further. I'd suggest filing a
bug report if you really care.
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com
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