I noticed that GCC >= 3.3 (not tried the ones before) automatically
branches out the "if" code (and follow-through the "else" code, if there).
Is that a coincidence or a rule we can rely on going forward?
- Davide
Davide Libenzi wrote:
> I noticed that GCC >= 3.3 (not tried the ones before) automatically
> branches out the "if" code (and follow-through the "else" code, if there).
> Is that a coincidence or a rule we can rely on going forward?
That's the default behavior, but there are lots of things that can cause it to
behave differently. Also, not all branch predictors behave the same way, and
some architectures use things like conditional instructions to fill their
pipeline bubbles, so it's still generally useful to have a real compiler hint in
fast-path code, even if it ends up being a no-op most of the time.
Most kernel code isn't so clock-cycle-critical that it needs these annotations.
If you're working on code that already has them, that's a good indication you
should probably use them too, but otherwise you don't need to worry about it
unless your code starts chewing up a lot of CPU time.
-- Chris
On Wed, 28 Jan 2009, Chris Snook wrote:
> Davide Libenzi wrote:
> > I noticed that GCC >= 3.3 (not tried the ones before) automatically branches
> > out the "if" code (and follow-through the "else" code, if there). Is that a
> > coincidence or a rule we can rely on going forward?
>
> That's the default behavior, but there are lots of things that can cause it to
> behave differently.
Please don't keep me hanging. What are they (just a few of the "lots"
that makes GCC follow-through "if" code)?
- Davide
Davide Libenzi writes:
> I noticed that GCC >= 3.3 (not tried the ones before) automatically
> branches out the "if" code (and follow-through the "else" code, if there).
> Is that a coincidence or a rule we can rely on going forward?
Coincidence.
Why on earth would you want to rely on an purely private implementation
detail like that?
On Wed, 28 Jan 2009, Mikael Pettersson wrote:
> Davide Libenzi writes:
> > I noticed that GCC >= 3.3 (not tried the ones before) automatically
> > branches out the "if" code (and follow-through the "else" code, if there).
> > Is that a coincidence or a rule we can rely on going forward?
>
> Coincidence.
>
> Why on earth would you want to rely on an purely private implementation
> detail like that?
I didn't want to. I was just curious if anyone that actually followed GCC
developments in the last few years could shed some light on it.
- Davide
Davide Libenzi wrote:
> On Wed, 28 Jan 2009, Chris Snook wrote:
>
>> Davide Libenzi wrote:
>>> I noticed that GCC >= 3.3 (not tried the ones before) automatically branches
>>> out the "if" code (and follow-through the "else" code, if there). Is that a
>>> coincidence or a rule we can rely on going forward?
>> That's the default behavior, but there are lots of things that can cause it to
>> behave differently.
>
> Please don't keep me hanging. What are they (just a few of the "lots"
> that makes GCC follow-through "if" code)?
>
>
> - Davide
>
>
When you turn on optimizations, gcc will try to avoid branching just to execute
a few instructions, since the cache miss and page fault penalties greatly exceed
the cost of a branch mispredict. The thresholds and heuristics vary, but in
general, if you stick something like this:
if (condition) foo++;
else if (complex condition) {do lots of stuff}
In the middle of a long function body and compile with optimizations enabled,
gcc will try to put the foo++ right after the evaluation. Some ISAs support
conditional instructions to let the compiler help fill pipeline bubbles, and
some superscalar processors will speculatively execute it in parallel with their
evaluation of the second condition, and proceed with whichever execution path is
chosen when they retire the instruction evaluating the first conditional.
-- Chris
On Wed, 28 Jan 2009, Chris Snook wrote:
> When you turn on optimizations, gcc will try to avoid branching just to
> execute a few instructions, since the cache miss and page fault penalties
> greatly exceed the cost of a branch mispredict. The thresholds and heuristics
> vary, but in general, if you stick something like this:
>
> if (condition) foo++;
> else if (complex condition) {do lots of stuff}
>
> In the middle of a long function body and compile with optimizations enabled,
> gcc will try to put the foo++ right after the evaluation. Some ISAs support
> conditional instructions to let the compiler help fill pipeline bubbles, and
> some superscalar processors will speculatively execute it in parallel with
> their evaluation of the second condition, and proceed with whichever execution
> path is chosen when they retire the instruction evaluating the first
> conditional.
OK, been finally able to trigger a different behavior. I thought that
became a somehow rule, after quite a few trials yesterday all leading to
the same results.
- Davide