From: Manfred Spraul and Brian Gerst

The patch performs the kmalloc cache lookup for constant kmalloc calls at
compile time.  The idea is that the loop in kmalloc takes a significant
amount of time, and for kmalloc(4096,GFP_KERNEL), that lookup can happen
entirely at compile time.

A problem has been seen with gcc-3.2.2-5 from RedHat.  This code:

    if(__builtin_constant_t(size)) {
          if(size < 32) return kmem_cache_alloc(...);
          if(size < 64) return kmem_cache_alloc(...);
          if(size < 96) return kmem_cache_alloc(...);
          if(size < 128) return kmem_cache_alloc(...);
          ...
    }

doesn't work, because gcc only optimizes the first two or three comparisons,
and then suddenly generates code.

But we did it that way anyway.  Apparently it's fixed in later compilers.