Samsung's Galaxy S6 was powered by the Exynos 7420 last year. It was the first time, in a few years, that Samsung had relied solely on their own in-house chip for every variant of their flagship device. Typically, Samsung sticks with Exynos in some markets and uses Qualcomm's Snapdragon chips in many 4G LTE markets like the US - this is what they did with the Galaxy S7, Galaxy S7 Edge and Galaxy Note 7 this year. Samsung's Exynos processors perform impressively, and some have even said that they are better than Qualcomm's own chipsets that are used in many of Samsung's larger markets.
Looking ahead to the Galaxy S8, it looks like the next flagship will sport an improved processor from Samsung. According to some leaks, it looks like the Exynos 8895 - which is a 10nm chipset - will get the nod this time around. And it'll be paired with the Mali-G71 GPU, which is a huge upgrade over the Mali-T880 that's being used in most smartphones today (that don't have a Snapdragon chipset, as those use Qualcomm's Adreno GPUs). This new GPU that will be used with the Exynos 8895 is said to be faster than the one coming in Qualcomm's new chipset, the Snapdragon 830. It's important to note that like the Exynos 8895, the Snapdragon 830 is not official either, so there's very little information available for either chip right now.
According to these leaked documents, the Mali-G71 is going to support API's like the OpenGL ES 3.2, GPU Computer, Vulkan and the Android RenderScript. This is thanks to it using the new Bitfrost architecture. This chip was designed with virtual reality and 4K content in mind. This could be a big reason why Samsung is going with the Mali-G71, as they are already heavily into virtual reality through their partnership with Oculus for the Gear VR, and there has been plenty of talk about the Galaxy S8 sporting a 4K display this time around. Instead of a QHD or 2K display.
These leaked documents are coming from the Samsung executive who was arrested - and currently being sued by Samsung - for allegedly selling confidential information regarding the South Korean company's fabrication process for 14nm and 10nm chips, to competitors in China. So it's worth taking all of this with a pretty big grain of salt. As it's unclear how true this information will be. It's also worth noting that we are still pretty far out from Samsung's unveiling of the Galaxy S8. And things do change. However the processor likely won't change, as that's something that the smartphone is built around, typically.