[LLVMdev] Loss of precision with very large branch weights

[Smith, Kevin B]

FWIW.  Intel compiler's profile instrumentation uses 64 bit integer counters.
We wrestled with similar problems for a long time before biting the bullet and switching to 64 bit counters.

For 32 bit architectures this is definitely not ideal, as it now the code must to use multi-instruction sequences
to perform the counter increments.

With 64 bits, even with 1 billion counter increments/second, the range still wouldn't cause counter overflow
for something like 800 years.  So I think 64 bit scale of values is definitely enough.

Kevin Smith

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Diego Novillo
Sent: Friday, April 24, 2015 11:47 AM
To: Xinliang David Li
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Loss of precision with very large branch weights



On 04/24/15 14:44, Xinliang David Li wrote:
>
> Isn't that the direct result of  the branch weights not being scaled
> (after reaching the cap) -- thus leading to wrong branch probability
> (computed from weights)? Wrong probability leads to wrong Frequency
> propagation.

Yup, I'm trying to see if we couldn't just use 64bit values all over to 
make things easier. The drawback would be that we are just punting the 
problem to a different scale of values (but, it may be enough).


Diego.
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