PNaCl: Add ExpandConstantExpr pass for converting ConstantExprs to Instructions

lib/Transforms/NaCl/ExpandConstantExpr.cpp

Index: lib/Transforms/NaCl/ExpandConstantExpr.cpp
diff --git a/lib/Transforms/NaCl/ExpandConstantExpr.cpp b/lib/Transforms/NaCl/ExpandConstantExpr.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..5a936f07d2e7a87bb51120f304a325e753b89f0e
--- /dev/null
+++ b/lib/Transforms/NaCl/ExpandConstantExpr.cpp
@@ -0,0 +1,86 @@
+//===- ExpandConstantExpr.cpp - Convert ConstantExprs to Instructions------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass expands out ConstantExprs into Instructions.
+//
+// Note that this only converts ConstantExprs that are referenced by
+// Instructions.  It does not convert ConstantExprs that are used as
+// initializers for global variables.
+//
+// This simplifies the language so that the PNaCl translator does not
+// need to handle ConstantExprs as part of a stable wire format for
+// PNaCl.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/Pass.h"
+#include "llvm/Transforms/NaCl.h"
+
+using namespace llvm;
+
+namespace {
+  // This is a FunctionPass because our handling of PHI nodes means
+  // that our modifications may cross BasicBlocks.
+  struct ExpandConstantExpr : public FunctionPass {
+    static char ID; // Pass identification, replacement for typeid
+    ExpandConstantExpr() : FunctionPass(ID) {
+      initializeExpandConstantExprPass(*PassRegistry::getPassRegistry());
+    }
+
+    virtual bool runOnFunction(Function &Func);
+  };
+}
+
+char ExpandConstantExpr::ID = 0;
+INITIALIZE_PASS(ExpandConstantExpr, "expand-constant-expr",

[Derek Schuff, 2013/03/14 21:23:50]

if you use a static RegisterPass<ExpandConstantExprPass> Foo(...) in the pass's
cpp file, you don't need to have this, or to manually call
initializeExpandConstantExprPass() in the constructor or in opt.cpp

[Mark Seaborn, 2013/03/14 21:44:18]

But then what causes ExpandConstantExpr.cpp to get linked into 'opt'?  You would
need icky linker tricks such as --whole-archive to do that.

Basically RegisterPass isn't used in the LLVM codebase for registering passes. 
All the passes are registered via the INITIALIZE_PASS pattern instead (including
those I added before in lib/Transforms/NaCl).  Grep the codebase to see
examples.

RegisterPass is basically only used for lib/Transforms/Hello/Hello.cpp.  It's
recommended by the doc http://llvm.org/docs/WritingAnLLVMPass.html for building
a pass into a .so library, but it doesn't work well for other linking use cases.

+                "Expand out ConstantExprs into Instructions",
+                false, false)
+
+static bool expandInstruction(Instruction *Inst) {
+  bool Modified = false;
+  for (unsigned OpNum = 0; OpNum < Inst->getNumOperands(); OpNum++) {
+    if (ConstantExpr *Expr =
+        dyn_cast<ConstantExpr>(Inst->getOperand(OpNum))) {
+      Modified = true;
+      Instruction *InsertPt = Inst;
+      if (PHINode *PN = dyn_cast<PHINode>(InsertPt)) {
+        // We cannot insert instructions before a PHI node, so insert
+        // before the incoming block's terminator.  This could be
+        // suboptimal if the terminator is a conditional.
+        InsertPt = PN->getIncomingBlock(OpNum)->getTerminator();
+      }
+      Instruction *NewInst = Expr->getAsInstruction();
+      NewInst->insertBefore(InsertPt);
+      NewInst->setName("expanded");
+      Inst->setOperand(OpNum, NewInst);
+      expandInstruction(NewInst);
+    }
+  }
+  return Modified;
+}
+
+bool ExpandConstantExpr::runOnFunction(Function &Func) {
+  bool Modified = false;
+  for (llvm::Function::iterator BB = Func.begin(), E = Func.end();
+       BB != E;
+       ++BB) {
+    for (BasicBlock::InstListType::iterator Inst = BB->begin(), E = BB->end();
+         Inst != E;
+         ++Inst) {
+      Modified |= expandInstruction(Inst);
+    }
+  }
+  return Modified;
+}
+
+FunctionPass *llvm::createExpandConstantExprPass() {
+  return new ExpandConstantExpr();
+}