I am one of the authors of the "highly-optimized implementation of Cytron et al.'s algorithm" that is used in LLVM.
That is a bit of a misattribution; I changed LLVM to use the DJ graph algorithm by Sreedhar and Gao before they took their measurements for the paper. What: The IR (internal represention) is in SSA form is if every variable is defined once, and every use of a variable refers to exactly one definition.
You can write compilers without, but for most optimizations you'll need to convert to SSA. what makes SSA so special for compiler construction It makes doing immutability and loop invariant reasoning a ton easier, along with constant propagation: https://en.wikipedia.org/wiki/Static_single_assignment_form More generally, SSA means implicit use/def information.
Use/def information and traversal are critical components of many static analyses and transformations: having the IR in SSA form greatly simplifies reasoning about and implementing these analyses and transformations.
In practice, this occurs in the presence of switch-statements.
SSA factors the def-use chain over a φ-node, avoiding this pathological case.OK, english version: in program analysis, you want to know the value of a variable, so you create a "def-use chain" which connects a definition of a variable to all of its uses.In some cases, you might have very many uses and definitions, esp of the same variable. In SSA form, each variable is only defined once, so you dont get pathological cases here.In English: When storing the results of your analysis, you can save them much more cheaply in SSA form, from a CPU and memory perspective.Without SSA, you need a hashtable of variables, with a list of values each.I would have expected their approach to be even faster in comparison than what they measured, but it could just be that they didn't do serious performance analysis and tuning. Think of const-only or functional programming, without any assignment statements.In SSA you can have only one lvalue, and multiple writes to the same variable in different branches are split into so-called phi merges, with a renamed final variable. It's the basis for most compiler optimizations. People who like to optimize their programs to get better performance. taylor simpson Tera Computer Company, 2815 Eastlake Ave.Does anyone have thoughts/opinions on the algorithm presented in "Simple and Efficient Construction of Static Single Assignment Form" by Braun, et al? Main MS 132, Houston, TX 77005, USA (email: keith,harv Krice.edu) Trilogy Development Group, 6034 W. East, Seattle, WA 98102, USA (email: preston Ktera.com) Department of Computer Science, Rice University, 6100 S.