| //===- subzero/src/IceInstVarIter.h - Iterate over inst vars ----*- C++ -*-===// |
| // |
| // The Subzero Code Generator |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| /// |
| /// \file |
| /// \brief Defines a common pattern for iterating over the variables of an |
| /// instruction. |
| /// |
| //===----------------------------------------------------------------------===// |
| |
| #ifndef SUBZERO_SRC_ICEINSTVARITER_H |
| #define SUBZERO_SRC_ICEINSTVARITER_H |
| |
| /// In Subzero, an Instr may have multiple Ice::Operands, and each Operand can |
| /// have zero, one, or more Variables. |
| /// |
| /// We found that a common pattern in Subzero is to iterate over all the |
| /// Variables in an Instruction. This led to the following pattern being |
| /// repeated multiple times across the codebase: |
| /// |
| /// for (Operand Op : Instr.Operands()) |
| /// for (Variable Var : Op.Vars()) |
| /// do_my_thing(Var, Instr) |
| /// |
| /// |
| /// This code is straightforward, but one may take a couple of seconds to |
| /// identify what it is doing. We therefore introduce a macroized iterator for |
| /// hiding this common idiom behind a more explicit interface. |
| /// |
| /// FOREACH_VAR_IN_INST(Var, Instr) provides this interface. Its first argument |
| /// needs to be a valid C++ identifier currently undeclared in the current |
| /// scope; Instr can be any expression yielding a Ice::Inst&&. Even though its |
| /// definition is ugly, awful, painful-to-read, using it is fairly simple: |
| /// |
| /// FOREACH_VAR_IN_INST(Var, Instr) |
| /// do_my_thing(Var, Instr) |
| /// |
| /// If your loop body contains more than one statement, you can wrap it with a |
| /// {}, just like any other C++ statement. Note that doing |
| /// |
| /// FOREACH_VAR_IN_INST(Var0, Instr0) |
| /// FOREACH_VAR_IN_INST(Var1, Instr1) |
| /// |
| /// is perfectly safe and legal -- as long as Var0 and Var1 are different |
| /// identifiers. |
| /// |
| /// It is sometimes useful to know Var's index in Instr, which can be obtained |
| /// with |
| /// |
| /// IndexOfVarInInst(Var) |
| /// |
| /// Similarly, the current Variable's Operand index can be obtained with |
| /// |
| /// IndexOfVarOperandInInst(Var). |
| /// |
| /// And that's pretty much it. Now, if you really hate yourself, keep reading, |
| /// but beware! The iterator implementation abuses comma operators, for |
| /// statements, variable initialization and expression evaluations. You have |
| /// been warned. |
| /// |
| /// **Implementation details** |
| /// |
| /// First, let's "break" the two loops into multiple parts: |
| /// |
| /// for ( Init1; Cond1; Step1 ) |
| /// if ( CondIf ) |
| /// UnreachableThenBody |
| /// else |
| /// for ( Init2; Cond2; Step2 ) |
| /// |
| /// The hairiest, scariest, most confusing parts here are Init2 and Cond2, so |
| /// let's save them for later. |
| /// |
| /// 1) Init1 declares five integer variables: |
| /// * i --> outer loop control variable; |
| /// * Var##Index --> the current variable index |
| /// * SrcSize --> how many operands does Instr have? |
| /// * j --> the inner loop control variable |
| /// * NumVars --> how many variables does the current operand have? |
| /// |
| /// 2) Cond1 and Step1 are your typical for condition and step expressions. |
| /// |
| /// 3) CondIf is where the voodoo starts. We abuse CondIf to declare a local |
| /// Operand * variable to hold the current operand being evaluated to avoid |
| /// invoking an Instr::getOperand for each outter loop iteration -- which |
| /// caused a small performance regression. We initialize the Operand * |
| /// variable with nullptr, so UnreachableThenBody is trully unreachable, and |
| /// use the else statement to declare the inner loop. We want to use an else |
| /// here to prevent code like |
| /// |
| /// FOREACH_VAR_IN_INST(Var, Instr) { |
| /// } else { |
| /// } |
| /// |
| /// from being legal. We also want to avoid warnings about "dangling else"s. |
| /// |
| /// 4) Init2 is where the voodoo starts. It declares a Variable * local |
| /// variable name 'Var' (i.e., whatever identifier the first parameter to |
| /// FOREACH_VAR_IN_INST is), and initializes it with nullptr. Why nullptr? |
| /// Because as stated above, some operands have zero Variables, and therefore |
| /// initializing Var = CurrentOperand->Variable(0) would lead to an assertion. |
| /// Init2 is also required to initialize the control variables used in Cond2, |
| /// as well as the current Operand * holder, Therefore, we use the obscure |
| /// comma operator to initialize Var, and the control variables. The |
| /// declaration |
| /// |
| /// Variable *Var = (j = 0, CurrentOperand = Instr.Operand[i], |
| /// NumVars = CurrentOperand.NumVars, nullptr) |
| /// |
| /// achieves that. |
| /// |
| /// 5) Cond2 is where we lose all hopes of having a self-documenting |
| /// implementation. The stop condition for the inner loop is simply |
| /// |
| /// j < NumVars |
| /// |
| /// But there is one more thing we need to do before jumping to the iterator's |
| /// body: we need to initialize Var with the current variable, but only if the |
| /// loop has not terminated. So we implemented Cond2 in a way that it would |
| /// make Var point to the current Variable, but only if there were more |
| /// variables. So Cond2 became: |
| /// |
| /// j < NumVars && (Var = CurrentOperand.Var[j]) |
| /// |
| /// which is not quite right. Cond2 would evaluate to false if |
| /// CurrentOperand.Var[j] == nullptr. Even though that should never happen in |
| /// Subzero, assuming this is always true is dangerous and could lead to |
| /// problems in the future. So we abused the comma operator one more time here: |
| /// |
| /// j < NumVars && ((Var = CurrentOperand.Var[j]), true) |
| /// |
| /// this expression will evaluate to true if, and only if, j < NumVars. |
| /// |
| /// 6) Step2 increments the inner loop's control variable, as well as the |
| /// current variable index. |
| /// |
| /// We use Var -- which should be a valid C++ identifier -- to uniquify names |
| /// -- e.g., i##Var instead of simply i because we want users to be able to use |
| /// the iterator for cross-products involving instructions' variables. |
| #define FOREACH_VAR_IN_INST(Var, Instr) \ |
| for (SizeT Sz_I##Var##_ = 0, Sz_##Var##Index_ = 0, \ |
| Sz_SrcSize##Var##_ = (Instr).getSrcSize(), Sz_J##Var##_ = 0, \ |
| Sz_NumVars##Var##_ = 0, Sz_Foreach_Break = 0; \ |
| !Sz_Foreach_Break && Sz_I##Var##_ < Sz_SrcSize##Var##_; ++Sz_I##Var##_) \ |
| if (Operand *Sz_Op##Var##_ = nullptr) \ |
| /*nothing*/; \ |
| else \ |
| for (Variable *Var = \ |
| (Sz_J##Var##_ = 0, \ |
| Sz_Op##Var##_ = (Instr).getSrc(Sz_I##Var##_), \ |
| Sz_NumVars##Var##_ = Sz_Op##Var##_->getNumVars(), nullptr); \ |
| !Sz_Foreach_Break && Sz_J##Var##_ < Sz_NumVars##Var##_ && \ |
| ((Var = Sz_Op##Var##_->getVar(Sz_J##Var##_)), true); \ |
| ++Sz_J##Var##_, ++Sz_##Var##Index_) |
| |
| #define IsOnlyValidInFOREACH_VAR_IN_INST(V) \ |
| (static_cast<const SizeT>(Sz_##V##_)) |
| #define IndexOfVarInInst(Var) IsOnlyValidInFOREACH_VAR_IN_INST(Var##Index) |
| #define IndexOfVarOperandInInst(Var) IsOnlyValidInFOREACH_VAR_IN_INST(I##Var) |
| #define FOREACH_VAR_IN_INST_BREAK \ |
| if (true) { \ |
| Sz_Foreach_Break = 1; \ |
| continue; \ |
| } else { \ |
| } |
| #undef OnlyValidIn_FOREACH_VAR_IN_INSTS |
| |
| #endif // SUBZERO_SRC_ICEINSTVARITER_H |
