Certainly check the assembly, but loop invariant code motion and strength reduction are basic optimizations. C compilers tend to be good at optimizing indexing patterns even at -O1.
Take a look, GCC and Clang go further than these suggestions by adding screenRect.w to the pointer each iteration to avoid the multiplication: https://godbolt.org/z/YfroqK7T6
Writing anything but pixels[y*screenRect.w + x] in an attempt to be faster, without checking the assembly first, is obfuscation.
(For what it's worth, you can beat the compiler by using *pixels++. I didn't profile the code to check it actually was faster in practice however.)
Possibly, but always check the assembly.
The even faster version, opts aside, would be to initialize the pointer at y*screenRect.w and ++ at every loop to avoid the addressing arithmetic.
Certainly check the assembly, but loop invariant code motion and strength reduction are basic optimizations. C compilers tend to be good at optimizing indexing patterns even at -O1.
Take a look, GCC and Clang go further than these suggestions by adding screenRect.w to the pointer each iteration to avoid the multiplication: https://godbolt.org/z/YfroqK7T6
Writing anything but pixels[y*screenRect.w + x] in an attempt to be faster, without checking the assembly first, is obfuscation.
(For what it's worth, you can beat the compiler by using *pixels++. I didn't profile the code to check it actually was faster in practice however.)