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The short trick might be a bit fragile.  For example, the current patch would incorrectly use it on machines where unaligned accesses are forbidden.

I isolated the other issue I talked about (making stack_pointer a register variable) in a separate patch.  This patch alone is clearly safe.  It should give a bit of speed-up on any machine but it definitely gives 5% on PCs with gcc by forcing the two most important local variables into specific registers.  (If someone knows the corresponding syntax for other compilers, it can be added in the #if.)