For each basic-block in the cfg, local stack analysis is performed. The analysis is started right after the block is created with begin-local-analysis and finished with end-local-analysis, when the construction of the block is complete. During the analysis, three sets containing stack locations are built:
This is done while constructing the CFG. These sets are then used by the end-stack-analysis word to emit optimal sequences of ##peek and ##replace instructions to the cfg.
For example, the code [ dup dup dup ] will only execute ##peek once, instead of three time which a 'non-lazy' method would.
Words for reading the stack state:
Words for writing the stack state:
Beginning and ending analysis:
Temporary variables that keeps track of the block's read and written stack locations:
| peeks | all stack locations that the block reads before writing |
| replaces | all stack locations that the block writes |
| kills | all stack locations which become unavailable after the block ends because of the stack height being decremented. For example, if the block contains drop, then D: 0 will be contained in kills because that stack location will not be live anymore. |
This is done while constructing the CFG. These sets are then used by the end-stack-analysis word to emit optimal sequences of ##peek and ##replace instructions to the cfg.
For example, the code [ dup dup dup ] will only execute ##peek once, instead of three time which a 'non-lazy' method would.
Words for reading the stack state:
peek-loc ( loc -- vreg )
global-loc>local ( loc height-state -- loc' )
local-loc>global ( loc height-state -- loc' )
Words for writing the stack state:
inc-stack ( loc -- )
replace-loc ( vreg loc -- )
Beginning and ending analysis:
begin-local-analysis ( basic-block -- )
end-local-analysis ( basic-block -- )
Temporary variables that keeps track of the block's read and written stack locations:
local-peek-set
replaces