Subzero: Provide a macro for iterating over instruction variables.

src/IceInstVarIter.h

+//===- 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
+/// This file 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 {
+///      }
+///
+///  to be legal. We also want to avoid warnings about "dangling else"s.

[Jim Stichnoth, 2015/08/31 21:31:11]

"from being legal" ?

[John, 2015/08/31 22:07:03]

Done.

+///
+///  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

[Jim Stichnoth, 2015/08/31 21:31:11]

I think this last paragraph should be de-indented one space, because it's not
part of step 6), right?

[John, 2015/08/31 22:07:03]

Done.

+///  -- 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_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_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)
+#undef OnlyValidIn_FOREACH_VAR_IN_INSTS
+
+#endif // SUBZERO_SRC_ICEINSTVARITER_H