14.3.2 TOS Optimization

An important optimization for stack machine emulators, e.g., Forth engines, is keeping one or more of the top stack items in registers. If a word has the stack effect in1...inx -- out1...outy, keeping the top n items in registers

• is better than keeping n-1 items, if x>=n and y>=n, due to fewer loads from and stores to the stack.
• is slower than keeping n-1 items, if x<>y and x<n and y<n, due to additional moves between registers.

In particular, keeping one item in a register is never a disadvantage, if there are enough registers. Keeping two items in registers is a disadvantage for frequent words like ?branch, constants, variables, literals and i. Therefore our generator only produces code that keeps zero or one items in registers. The generated C code covers both cases; the selection between these alternatives is made at C-compile time using the switch -DUSE_TOS. TOS in the C code for + is just a simple variable name in the one-item case, otherwise it is a macro that expands into sp[0]. Note that the GNU C compiler tries to keep simple variables like TOS in registers, and it usually succeeds, if there are enough registers.

The primitive generator performs the TOS optimization for the floating-point stack, too (-DUSE_FTOS). For floating-point operations the benefit of this optimization is even larger: floating-point operations take quite long on most processors, but can be performed in parallel with other operations as long as their results are not used. If the FP-TOS is kept in a register, this works. If it is kept on the stack, i.e., in memory, the store into memory has to wait for the result of the floating-point operation, lengthening the execution time of the primitive considerably.

The TOS optimization makes the automatic generation of primitives a bit more complicated. Just replacing all occurrences of sp[0] by TOS is not sufficient. There are some special cases to consider:

• In the case of dup ( w -- w w ) the generator must not eliminate the store to the original location of the item on the stack, if the TOS optimization is turned on.
• Primitives with stack effects of the form -- out1...outy must store the TOS to the stack at the start. Likewise, primitives with the stack effect in1...inx -- must load the TOS from the stack at the end. But for the null stack effect -- no stores or loads should be generated.