Every time I use an application requiring the use of the radeonsi driver from Mesa, I get a continuous stream of the following  message :

radeon: Failed to allocate a buffer:
radeon:    size      : 0 bytes
radeon:    alignment : 4096 bytes
radeon:    domains   : 4

I never get this kind of message when running apps using the 'normal' xf86-video-ati driver, so I'm pretty sure radeonsi is the one sending it. 
I've made some basic tests with a self-made program and it seems like the message is sent at least once on every frame. 

If it were just for that, I wouldn't mind that much about it; most apps that spawn it don't show any performance problems. However, it might be linked to another error message I sometimes get : 
LLVM ERROR: ran out of registers during register allocation 

This LLVM error always turns up as a report of a recent crash. So far, I've seen two situations where it happened : trying to start a game on Xonotic with video effects settings on ultra, and reaching some triggers/areas in the Warcraft III campaign on Wine with Warcraft III being in Direct3D mode. I could probably search and/or file reports about that LLVM error, so I won't digress any further; this post is mostly meant to be about the 'Failed to allocate a buffer' error messages.

To end this report, I'll provide the relevant hardware and software components from my setup:
- CPU : Intel Core i5 4460
- Motherboard : Asus H87 Plus
- RAM : GSkill Ripjaws 2 x 4gb
- GPU : Gigabyte Radeon R9 280
- OS : Arch Linux with kernel version 3.16.1-1
- Xorg 1.16.0-6 
- GPU drivers : all open-source; xf86-video-ati 1:7.4.0-3, 
radeonsi from xf86-video-ati package 10.2.5-1

That's pretty much it, I guess.