David Kanter from RealWorldTech.com in his “PhysX87: Software Deficiency” article has hypothesized that origin of slow execution of PhysX content on CPU is fact that PhysX SDK is mostly based on x87 rather than faster SSE instruction set.
“On modern CPUs, SSE can easily run 1.3-2X faster than similar x87 code” – stated Kanter.
However, TGDaily has managed to recieve commentaries from Bryan Del Rizzo, Nvidia spokesperson
[And although] our SDK does [include] some SSE code, we found [that] non-SSE code can result in higher performance than SSE in many situations. [Nevertheless], we will continue to use SSE and plan to enable it by default in future releases. That being said, not all developers want SSE enabled by default, because they still want support for older CPUs for their SW versions.
Update: official responce from Nvidia – We’re not hobbling CPU PhysX
Update #2: some more Nvidia statements at this ars technica article
Update #3: and more at Hothardware.com article “NVIDIA Sheds Light On Lack Of PhysX CPU Optimizations”
But lets get back to original article. According to David, sole reason for PhysX SDK to rely on outdated x87 instruction is to make “Nvidia GPUs looks a lot better than the CPU“. This idea was inherited other websites, like TechReport.com
The PhysX logo is intended as a selling point for games taking full advantage of Nvidia hardware, but it now may take on a stronger meaning: intentionally slow on everything else.
and Semi Accurate
In the end, there is one thing that is unquestionably clear, if you remove the de-optimizations that Nvidia inflicts only on the PC CPU version of PhysX, the GPU version would unquestionably be slower than a modern CPU.
Unfortunately, previous authors are missing few vital points: PhysX SDK is used in many games running on CPU, and physics level in those titles can be easily compared to physics content in games based on other “non crippled” physics engines, like Havok; nor there are any games, that can offer content, similar to GPU PhysX effects, but running on CPU with stable framerate.
And, most important, GPU can accelerate only few parts of PhysX code – rigid bodies, joints, raycasts, forcefields, broadphase, etc – rely purely on CPU, so what is the reason not to optimize those at the full potential, to make PhysX SDK more attractive for developers (and thus increase number of games with GPU PhysX support) ?! Something is telling us that reason “just to make GPUs look better over CPU” is not so obvious.
And what do you think ? Tell us in comments.