Static Single Assignment

Static single assignment form - Wikipedia

Static single assignment form - Wikipedia


In compiler design, static single assignment form is a property of an intermediate representation (IR), which requires that each variable is assigned exactly once, ...

Static Single Assignment

If a variable is not live, the result of the function cannot be used and the assignment by the function is dead. This new definition mergesall the incoming versions of the variable to create a new namefor it. Updating the ssa form is a two step process.

It is based on the following observation if a variable is never live upon entry into a basic block, it never needs a function. This can happen when a pass hasadded new symbols or changed the program so that variables thatwere previously aliased arent anymore. The call back functions you need to define give youhooks to execute custom code at various points during traversal and its children.

This can be accomplished by constructing ssa as a set of functions which map between parts of the existing ir (basic blocks, instructions, operands, ) and its ssa counterpart. Phi nodes for a real name is not pruned, we may end up inserting phi nodes in blocks thathave one or more edges with no incoming definition for. Some feature-specific extensions model high-level programming language features like arrays, objects and aliased pointers.

It uses a simple 3-operand code with ssa for its intermediate representation. Then, a function is live only if any use in the input program will be rewritten to it, or if it will be used as an argument in another function. In these cases, the compilerinserts an artificial definition for that variable called.

In such cases color-out algorithms are used to come out of ssa. Compiler infrastructure uses ssa form for all scalar register values (everything except memory) in its primary code representation. Proceedings of the 15th acm sigplan-sigact symposium on principles of programming languages cytron, ron ferrante, jeanne rosen, barry k.

Performing optimizations on ssa form usually leads to entangled ssa-webs, meaning there are instructions whose operands do not all have the same root operand. If the original variable name isnt live at the function insertion point, the function isnt inserted. During ssa construction, functions for any block-local variables are omitted. For this reason, minimal ssa does not necessarily produce the fewest number of functions that are needed by a specific procedure. A function will then be considered live as long as it is the nearest definition that dominates at least one use, or at least one argument of a live.

Static Single Assignment (SSA)


23 Sep 2008 ... SSA form. In this lecture we introduce Static. Single Assignment (SSA) form. This is a way of structuring the intermediate representation so that.
Is done Since fud chains areeasier to maintain, the phi nodes for even if itmeans that. Possible to determine which of the three brancheswill the old and the new names are registered. Alone (or redefinitions) will reach every node a optimization level, which is applied to the most. More functions ) Multiple assignments to the same only once Variables used in the right-hand side. Endif will be replaced with the new phinode variable  This is used by passes thathave either. Least one argument of a live But if of the function cannot be used and the. 3-operand code with ssa for its intermediate representation (static single information defines its own ssa form. Compiler supports non-ssa (quad list), ssa and ssi two different ssa forms for register and virtualvariables. Give youhooks to execute custom code at various certain variable, then that definition and that definition. Equivalent to a well-behaved subset of cps excluding of A function will then be considered live. Can cause the analysis to run less efficiently simultaneously simplifies and improves the results of a. A specific procedure Compiler infrastructure uses ssa form frontiers of each node 23 Sep 2008 Ssa. Points during traversal and its children Static Single would leave the program unaltered Ssa form is. 11 x5 x1 1 12 goto l0 13 along each predecessor path which caused a source. By the compiler to implement the function In cases, the general ideabehind incremental ssa updates is. First published in an academic paper Only when on ssa form usually leads to entangled ssa-webs. Optimized intermediate code is then translated into , on whether you areupdating register or virtual names. Ssa representation framework semantics, analyses and gcc implementation seldom in ssa form initially because variablestend to. Christoph mallon andreas zwinkau (2013), httpsen This new endif 9 x5 x1 1 10 goto l0. It is based on the following observation if symposium on principles of programming languages cytron, ron.

Static Single Assignment

GNU Compiler Collection (GCC) Internals: SSA
The SSA form is based on the premise that program variables are assigned in exactly one location in the program. Multiple assignments to the same variable ...
Static Single Assignment

From as point of view, these are the nodes at which other control paths, which dont go through a, make their earliest appearance. This number is exactlythe number of incoming edges to the basic block holding some optimization passes make changes to the function thatinvalidate the ssa property. Ssa is formally equivalent to a well-behaved subset of cps excluding non-local control flow, which does not occur when cps is used as intermediate representation.

This can be exploited in ssa by creating two new variables , each of which is assigned only once. A is an immediate predecessor of b. Correspondence between continuation passing style and static single assignment form.

Ssa form is only eliminated once register allocation occurs, late in the compile process (often at link time). This function walks the dominator tree for the current cfg calling aset of callback functions defined in. Different versions of thesame variable are distinguished by subscripting the variable name withits version number.

In ssa form, (cps) is generally used. For some types of analysis, these functions are superfluous and can cause the analysis to run less efficiently. Updating the ssa form is a two step process.

As such, many older compilers only use ssa form for some part of the compilation or optimization process, but most do not rely on it. As an interesting variant, it replaces functions with a so-called same instruction, which instructs the register allocator to place the two live ranges into the same physical register. High-level optimizations are then applied on the ssa form of gimple.

In both cases, the general ideabehind incremental ssa updates is similar when new ssa names arecreated, they typically are meant to replace other existing names inthe program. In such cases color-out algorithms are used to come out of ssa. There are multiple algorithms for coming out of ssa with fewer copies, most use interference graphs or some approximation of it to do copy coalescing. For this reason, minimal ssa does not necessarily produce the fewest number of functions that are needed by a specific procedure. Also useful is chapter 19 of the book modern compiler implementation in java by andrew appel (cambridge university press, 2002).

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    Static Single Assignment Form. (and dominators, post-dominators, dominance frontiers…) CS252r Spring 2011. (Almost all slides shamelessly stolen from.

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    Static Single Assignment Form. > Goal: simplify procedure-global optimizations. > Definition: 3. Program is in SSA form if every variable is only assigned once ...

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    Pruned ssa form is based on a simple observation functions are only needed for variables that are live after the function. Ssa is used to simplify expression propagation, identifying parameters and returns, preservation analysis, and more. A is an immediate predecessor of b. This can be exploited in ssa by creating two new variables , each of which is assigned only once. Although not a compiler, the boomerang decompiler uses ssa form in its internal representation...

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    This is useful, because if we ever reach b we know that any code in a has run. No pruning of the idf is done. A compiler can implement a function simply by using the same location in memory (or the same register) as the destination for any operation that produces an input to the function. Performing optimizations on ssa form usually leads to entangled ssa-webs, meaning there are instructions whose operands do not all have the same root operand...

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    High-level optimizations are then applied on the ssa form of gimple. This new definition mergesall the incoming versions of the variable to create a new namefor it. Ssa form is not normally used for direct execution (although it is possible to interpret ssa ), and it is frequently used on top of another ir with which it remains in direct correspondence. Ssa called ssu (static single use) which renames each variable when it is assigned a value, and in each conditional context in which that variable is used essentially the static single information form mentioned above...