Greatest Software Ever Written 4261

the abstraction had code blocks that were sequential sequence of instructions ... a code block could be bracketed by control flow leaving and-or code flow entering (from unconditional or conditional branch instructions). code blocks could be sequentially aggregated if they were connected by a single unconditional branch ... i.e. the branched-to code block was entered by only a single unconditional branch.

conditional end was a pair of condition setting instruction followed by a conditional branch. a sequence of such instruction pairs could be collapsed into a single conditional end statement abstraction ... as long as the conditional "to" location for all conditional branches was the same ... and there were no other intervening instructions between the conditional branching operations.

the issue with some highly optimized kernel paths was that they actually did a complex set of condition testing-branching ... with various intervening instructions. it wasn't just a sequence of multiple condition testing with no other intervening instructions and all the conditional branches having the same target. some kernel paths might be possible 1-3rd instructions condition testing-conditional branching with a mixed of target branch locations and arbitrary intervening instructions (both between sequences of conditional end instructions and between branch target locations).

so a possible instruction sequence

test condition1 cond branch A inst1 inst2 inst3 branch B A test condition2 cond branch B inst4 inst5 inst6 B ....

so w-o elseif it would be

if not condition1 then inst1 inst2 inst3 else if not condition2 then inst4 inst5 inst6 end end

so that can collapses into

if not condition1 then inst1 inst2 inst3 elseif not condition2 then inst4 inst5 inst6 end

however if the sequence at A was

A inst9 testing condition2 cond branch B inst4 inst5 inst6 B .... it really becomes nested ... the subsequent conditional testing-branch has to be at code block boundary of the previous code block ... w-o any other intervening instructions. the problem was that there was some amount of highly optimized kernel pathlengths that exhibited that characteristic ... potentially to the nested depth of 20. the collapsing of condition boundaries like multiple conditions for a single if statement and-or constructs like elseif ... had to have specific semantics-boundaries for the conditional executed code blocks.