Front page | perl.cvs.parrot |
Postings from January 2009
Author: Whiteknight Date: Wed Jan 14 10:57:52 2009 New Revision: 35543 Modified: trunk/docs/book/ch03_pir_basics.pod Log: [Book] Add small bit about :unique_reg and the register allocator Modified: trunk/docs/book/ch03_pir_basics.pod ============================================================================== --- trunk/docs/book/ch03_pir_basics.pod (original) +++ trunk/docs/book/ch03_pir_basics.pod Wed Jan 14 10:57:52 2009 @@ -248,6 +248,37 @@ cause all sorts of memory allocation problems or inefficiencies in parsing. Push the limits at your own risk. +=head2 Register Allocator + +Now's a decent time to talk about Parrot's register allocator. When you use +a register like C<$P5>, you aren't necessarily talking about the fifth +register in memory. This is important since you can use a $P10000000 without +forcing Parrot to allocate an array of ten million registers. Instead Parrot's +compiler front-end uses an allocation algorithm which turns each register in +the PIR source code into a reference to an actual memory storage location. + +The allocator is a type of optimization. It performs a lifetime analysis on +the registers to determine when they are being used and when they are not. +When a register stops being used for one thing, it can be reused later for a +different purpose. Register reuse helps to keep Parrot's memory requirements +lower, because fewer unique registers need to be allocated. However, the +downside of the register allocator is that it takes more time to execute during +the compilation phase. + +In some situations it can be helpful to turn the allocator off and avoid +expensive optimizations. Such situations are subroutines where there are a +small fixed number of registers used, when variables are used throughout the +subroutine and should never be reused, or when some kind of pointer reference +needs to be made to the register N<this happens in some NCI calls that take +pointers and return values>. To turn off the register allocator for certain +variables, you can use the C<:unique_reg> modifier: + + .local pmc MyUniquePMC :unique_reg + +Notice that C<:unique_reg> shouldn't affect the behavior of Parrot, but +instead only changes the way registers are allocated. It's a trade off between +using more memory in exchange for less time spent optimizing the subroutine. + =head2 PMC variables Z<CHP-3-SECT-2.4>