[LLVMdev] [cfe-dev] Branches &,&&

[Quentin Colombet]

Hi Fisnik,

> On Jun 11, 2015, at 9:23 AM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> +llvmdev
> 
>> On 2015-Jun-11, at 06:06, Fisnik Kastrati <kastrati at informatik.uni-mannheim.de> wrote:
>> 
>> I'm turning to you with regards to an unwanted optimization/un-optimization that clang++ (all versions that I tested, in the following link) is generating, see the code in the following link:
>> http://goo.gl/oiTPX5
>> 
>> 
>> 
>> The assembly code generated for both methods "amp", "ampamp" is the practically the same, when using the optimization       flag "-O3". However, I'm interested to have a single jump for the code in the method "amp", as branch misprediction penalty is very high otherwise. It should generate a single jump, when using single ampersand (&), something that icc13 is already doing (try it in the link above).
>> 
>> Is there any optimization flag that I should set, in order to avoid this feature when using "-O3"?
>> I made a similar question to the g++ community, however this seems to be a bug (performance bug) with g++.
>> 
>> 
>> Thank you in advance
>> F. 
> 
> For reference, here's the (slightly reduced) source code:
> ---
> $ cat t.c
> void foo(int x, int y);
> void ampamp(int x, int y) {
>  if (x < 3 && y > 10)
>    foo(x, y);
> }
> void amp(int x, int y) {
>  if ((x < 3) & (y > 10))
>    foo(x, y);
> }
> --
> 
> This looks like an instruction selection issue.
> 
> The LLVM optimization passes reduce the `&&` case down to `&` --
> neither case needs a branch here, since `y > 10` has no side effects
> -- but then the instruction selector chooses a branchy instruction
> sequence for some reason.
> 
> Note that -O0 and -O3 use completely different instruction selectors:
> FastISel and SelectionDAG, respectively.  It looks like FastISel is
> doing the obvious thing, but for some reason SelectionDAG reintroduces
> a branch.
> 
> To answer the question you actually asked, I don't think there's a way
> to choose FastISel at other optimization levels.

Actually, there is a way to use FastISel at other optimization level: (-mllvm) -fast-isel.
That being said, this is a backend option and thus, there is no guarantee that the option will be stable or supported in future release. I do not expect it to change anytime soon, but something to be aware of.

Cheers,
-Quentin

> 
> Perhaps you could file a PR?
>  http://llvm.org/bugs/ <http://llvm.org/bugs/>
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