llvm dev - Apr 2010 - [LLVMdev] Why the same code is much slower in JIT compared to separate executable?

Török Edwin wrote:
> Are you using 2.6 or 2.7, 32-bit or 64-bit?
>   
I use 2.7 on i386. lli has debug asserts enabled, but I guess this 
shouldn't matter for JIT code speed.

jit: 11.32 real
exe: 7.64 user

Both have -O3 option. Speed should be the same.

Yuri

On 04/29/2010 08:21 PM, Yuri wrote:
> Török Edwin wrote:
>> Are you using 2.6 or 2.7, 32-bit or 64-bit?
>>   
> 
> I use 2.7 on i386. lli has debug
try a release build (ENABLE_OPTIMIZED=1 DISABLE_ASSERTIONS=1)
> asserts enabled, but I guess this
> shouldn't matter for JIT code speed.
> 
> jit: 11.32 real
> exe: 7.64 user
> 
> Both have -O3 option. Speed should be the same.
> 
> Yuri

Yuri <yuri at tsoft.com> writes:
> Török Edwin wrote:
>> Are you using 2.6 or 2.7, 32-bit or 64-bit?
>>   
>
> I use 2.7 on i386. lli has debug asserts enabled, but I guess this 
> shouldn't matter for JIT code speed.
>
> jit: 11.32 real
> exe: 7.64 user
>
> Both have -O3 option. Speed should be the same.
With

time lli -O3 fib.bc 45

you are measuring the time lli takes optimizing the LLVM code,
generating the native code and, finally, executing it. If you add to
this the debug asserts, it is not surprising that lli ends being quite a
bit slower than directly executing the native code.

As Török suggests, using a Release build with asserts off will make a
difference on lli's speed.

Yuri <yuri at tsoft.com> writes:
>> With
>>
>> time lli -O3 fib.bc 45
>>
>> you are measuring the time lli takes optimizing the LLVM code,
>> generating the native code and, finally, executing it. If you add to
>> this the debug asserts, it is not surprising that lli ends being quite
a
>> bit slower than directly executing the native code
>
> You can see that this is not true running 'time lli -O3 fib.bc 4',
> which has exactly the same code. Compiler still compiles the same way,
> and it takes 0.00 user seconds.
You are right. The code is small enough to have any impact on the total
required time.

Try passing -print-machineinstrs to lli and compare the output with the
assembler generated by llc.