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    March 7, 2008

Flush register windows

Guest Author

The SPARC architecture has an interesting feature called Register Windows. The idea is that the processor should contain multiple sets of registers on chip. When a new routine is called, the processor can give a fresh set of registers to the new routine, preserving the value of the old registers. When the new routine completes and control returns to the calling routine, the register values for the old routine are also restored. The idea is for the chip not to have to save and load the values held in registers whenever a routine is called; this reduces memory traffic and should improve performance.

The trouble with register windows, is that each chip can only hold a finite number of them. Once all the register windows are full, the processor has to spill a complete set of registers to memory. This is in contrast with the situation where the program is responsible for spilling and filling registers - the program only need spill a single register if that is all that the routine requires.

Most SPARC processors have about seven sets of register windows, so if the program remains in a call stack depth of about seven, there is no register spill/fill cost associated with calls of other routines. Beyond this stack depth, there is a cost for the spills and fills of the register windows.

The SPARC architecture book contains a more detailed description of register windows in section 5.2.2.

Most of the time software is completely unaware of this architectural decision, in fact user code should never have to be aware of it. There are two situations where software does need to know about register windows, these really only impact virtual machines or operating systems:

  • Context switches. In a context switch the processor changes to executing another software thread, so all the state from that thread needs to be saved for the thread to later resume execution. Note that setjmp and longjmp which are sometimes used as part of code to implement context switching already have the appropriate flushes in them.
  • Garbage collection. Garbage collection involves inspecting the state of the objects held in memory and determining whether each object is live or dead. Live objects are identified by having other live objects point to them. So all the registers need to be stored in memory so that they can be inspected to check whether they point to any objects that should be considered live.

The SPARC V9 instruction flushw will cause the processor to store all the register windows in a thread to memory. For SPARC V8, the same effect is attained through trap x03. Either way, the cost can be quite high since the processor needs to store up to about 7 sets of register windows to memory Each set is 16 8-byte registers, which results in potentially a lot of memory traffic and cycles.

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