"human readable" optimized code

[Reza Roboubi]

Andrew Haley wrote:
> There are two distinct phases of optimization, one on high-level
> code and on low-level code.  So -- to cut a long story short -- we first
> optimize the C, then we generate assembler, and then we optimize that.
> You're probably seeing one of the later optimizations.

I was just interested why that is?  I could think of things like 
aliasing issues, but it still seems possible to do that in a single 
"Universal Machine Instruction Set" pass.

Thank you.

Reza.