These functions deal with the formation of CMP;CCMP;...
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These functions deal with the formation of CMP;CCMP;...
sequences. The CCMP/CCMN/FCCMP/FCCMPE instructions allow the conditional execution of a comparison. They set the NZCV flags to a predefined value if their predicate is false. This allows to express arbitrary conjunctions, for example "cmp 0 (and (setCA (cmp A)) (setCB (cmp B)))" expressed as: cmp A ccmp B, inv(CB), CA check for CB flags
This naturally lets us implement chains of AND operations with SETCC operands. And we can even implement some other situations by transforming them:
- We can implement (NEG SETCC) i.e. negating a single comparison by negating the flags used in a CCMP/FCCMP operations.
- We can negate the result of a whole chain of CMP/CCMP/FCCMP operations by negating the flags we test for afterwards. i.e. NEG (CMP CCMP CCCMP ...) can be implemented.
- Note that we can only ever negate all previously processed results. What we can not implement by flipping the flags to test is a negation of two sub-trees (because the negation affects all sub-trees emitted so far, so the 2nd sub-tree we emit would also affect the first). With those tools we can implement some OR operations:
- (OR (SETCC A) (SETCC B)) can be implemented via: NEG (AND (NEG (SETCC A)) (NEG (SETCC B)))
- After transforming OR to NEG/AND combinations we may be able to use NEG elimination rules from earlier to implement the whole thing as a CCMP/FCCMP chain.
As complete example: or (or (setCA (cmp A)) (setCB (cmp B))) (and (setCC (cmp C)) (setCD (cmp D)))" can be reassociated to: or (and (setCC (cmp C)) setCD (cmp D))