llvm dev - Sep 2009 - [LLVMdev] FYI: Phoronix GCC vs. LLVM-GCC benchmarks

Chris Lattner:
> Comparing -O3 (and even -O4) is interesting, but we want all  
> optimization levels to perform better than GCC :).  Lots of people use  
> -O2 and -Os, so comparing against other compiler's -O2 and -Os levels  
> is just as interesting as comparing -O3 vs -O3.
> 
My thinking was that, for instance, -02 for GCC and -02 for LLVM(-GCC) 
do not necessarily mean the same thing, they may be not /commensurable/.
But perhaps, my ignorance, you are saying that they _are_, that LLVM 
assigns the same types of optimizations as GCC to the different levels.

Regards,
Stefano

On Sep 16, 2009, at 10:05 AM, Stefano Delli Ponti wrote:
> Chris Lattner:
>> Comparing -O3 (and even -O4) is interesting, but we want all   
>> optimization levels to perform better than GCC :).  Lots of people  
>> use  -O2 and -Os, so comparing against other compiler's -O2 and -Os
>> levels  is just as interesting as comparing -O3 vs -O3.
>
> My thinking was that, for instance, -02 for GCC and -02 for LLVM(- 
> GCC) do not necessarily mean the same thing, they may be not / 
> commensurable/.
> But perhaps, my ignorance, you are saying that they _are_, that LLVM  
> assigns the same types of optimizations as GCC to the different  
> levels.
Right, we want them to be roughly comparable.

O0 -> best debug experience, fastest compile times.
O2 -> optimize without bloating the code too much and without burning  
*too* many cycles.
O3 -> take more time and produce fatter code to get faster code.

O1 is something of a wasteland with no clear purpose :)

-Chris

Since we are in the area, what *should* O1 do?

It's basically good for nothing, since it doesn't tune for size or
performance. The only good I personally ever have for it is once in a
while there is a miscompile at -O1 which narrows the problem.

Would it be crazy to make -O1 equivalent to -Os?

 - Daniel

On Wednesday, September 16, 2009, Chris Lattner <clattner at apple.com>
wrote:
>
> On Sep 16, 2009, at 10:05 AM, Stefano Delli Ponti wrote:
>
>> Chris Lattner:
>>> Comparing -O3 (and even -O4) is interesting, but we want all
>>> optimization levels to perform better than GCC :).  Lots of people
>>> use  -O2 and -Os, so comparing against other compiler's -O2 and
-Os
>>> levels  is just as interesting as comparing -O3 vs -O3.
>>
>> My thinking was that, for instance, -02 for GCC and -02 for LLVM(-
>> GCC) do not necessarily mean the same thing, they may be not /
>> commensurable/.
>> But perhaps, my ignorance, you are saying that they _are_, that LLVM
>> assigns the same types of optimizations as GCC to the different
>> levels.
>
> Right, we want them to be roughly comparable.
>
> O0 -> best debug experience, fastest compile times.
> O2 -> optimize without bloating the code too much and without burning
> *too* many cycles.
> O3 -> take more time and produce fatter code to get faster code.
>
> O1 is something of a wasteland with no clear purpose :)
>
> -Chris
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