| /** |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you under the Apache License, Version 2.0 (the |
| * "License"); you may not use this file except in compliance |
| * with the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, |
| * software distributed under the License is distributed on an |
| * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY |
| * KIND, either express or implied. See the License for the |
| * specific language governing permissions and limitations |
| * under the License. |
| */ |
| #pragma once |
| |
| #if ENABLE(DFG_JIT) |
| |
| namespace JSC { namespace DFG { |
| |
| class Graph; |
| |
| // Convert ThreadedCPS form into SSA form. This results in a form that has: |
| // |
| // - Minimal Phi's. We use the the Cytron et al (TOPLAS'91) algorithm for |
| // Phi insertion. Most of the algorithm is implemented in SSACalculator |
| // and Dominators. |
| // |
| // - No uses of GetLocal/SetLocal except for captured variables and flushes. |
| // After this, any remaining SetLocal means Flush. PhantomLocals become |
| // Phantoms. Nodes may have children that are in another basic block. |
| // |
| // - MovHints are used for OSR information, and are themselves minimal. |
| // A MovHint will occur at some point after the assigning, and at Phi |
| // points. |
| // |
| // - Unlike conventional SSA in which Phi functions refer to predecessors |
| // and values, our SSA uses Upsilon functions to indicate values in |
| // predecessors. A merge will look like: |
| // |
| // labelA: |
| // a: Thingy(...) |
| // b: Upsilon(^e, @a) |
| // Jump(labelC) |
| // |
| // labelB: |
| // c: OtherThingy(...) |
| // d: Upsilon(^e, @c) |
| // Jump(labelC) |
| // |
| // labelC: |
| // e: Phi() |
| // |
| // Note that the Phi has no children, but the predecessors have Upsilons |
| // that have a weak reference to the Phi (^e instead of @e; we store it |
| // in the OpInfo rather than the AdjacencyList). Think of the Upsilon |
| // as "assigning" to the "variable" associated with the Phi, and that |
| // this is the one place in SSA form where you can have multiple |
| // assignments. |
| // |
| // This implies some other loosenings of SSA. For example, an Upsilon |
| // may precede a Phi in the same basic block; this may arise after CFG |
| // simplification. Although it's profitable for CFG simplification (or |
| // some other phase) to remove these, it's not strictly necessary. As |
| // well, this form allows the Upsilon to be in any block that dominates |
| // the predecessor block of the Phi, which allows for block splitting to |
| // ignore the possibility of introducing an extra edge between the Phi |
| // and the predecessor (though normal SSA would allow this, also, with |
| // the caveat that the Phi predecessor block lists would have to be |
| // updated). |
| // |
| // Fun fact: Upsilon is so named because it comes before Phi in the |
| // alphabet. It can be written as "Y". |
| |
| bool performSSAConversion(Graph&); |
| |
| } } // namespace JSC::DFG |
| |
| #endif // ENABLE(DFG_JIT) |
