Check in initial version of ipcp

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9423 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/IPO/IPConstantPropagation.cpp

@@ -0,0 +1,110 @@
+//===-- IPConstantPropagation.cpp - Propagate constants through calls -----===//
+// 
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by the LLVM research group and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+// 
+//===----------------------------------------------------------------------===//
+//
+// This pass implements an _extremely_ simple interprocedural constant
+// propagation pass.  It could certainly be improved in many different ways,
+// like using a worklist.  This pass makes arguments dead, but does not remove
+// them.  The existing dead argument elimination pass should be run after this
+// to clean up the mess.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO.h"
+#include "llvm/Module.h"
+#include "llvm/Pass.h"
+#include "llvm/Constants.h"
+#include "llvm/Support/CallSite.h"
+#include "Support/Statistic.h"
+
+namespace {
+  Statistic<> NumArgumentsProped("ipconstprop",
+                                 "Number of args turned into constants");
+
+  /// IPCP - The interprocedural constant propagation pass
+  ///
+  struct IPCP : public Pass {
+    bool run(Module &M);
+  private:
+    bool processFunction(Function &F);
+  };
+  RegisterOpt<IPCP> X("ipconstprop", "Interprocedural constant propagation");
+}
+
+Pass *createIPConstantPropagationPass() { return new IPCP(); }
+
+bool IPCP::run(Module &M) {
+  bool Changed = false;
+  for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I)
+    if (!I->isExternal() && I->hasInternalLinkage())
+      Changed |= processFunction(*I);
+  return Changed;
+}
+
+/// processFunction - Look at all uses of the specified function.  If all uses
+/// are direct call sites, and all pass a particular constant in for an
+/// argument, propagate that constant in as the argument.
+///
+bool IPCP::processFunction(Function &F) {
+  if (F.aempty() || F.use_empty()) return false;  // No arguments?  Early exit.
+
+  std::vector<std::pair<Constant*, bool> > ArgumentConstants;
+  ArgumentConstants.resize(F.asize());
+
+  unsigned NumNonconstant = 0;
+
+  for (Value::use_iterator I = F.use_begin(), E = F.use_end(); I != E; ++I)
+    if (!isa<Instruction>(*I))
+      return false;  // Used by a non-instruction, do not transform
+    else {
+      CallSite CS = CallSite::get(cast<Instruction>(*I));
+      if (CS.getInstruction() == 0 || 
+          CS.getCalledFunction() != &F)
+        return false;  // Not a direct call site?
+      
+      // Check out all of the potentially constant arguments
+      CallSite::arg_iterator AI = CS.arg_begin();
+      for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI) {
+        if (*AI == &F) return false;  // Passes the function into itself
+
+        if (!ArgumentConstants[i].second) {
+          if (isa<Constant>(*AI) || isa<GlobalValue>(*AI)) {
+            Constant *C = dyn_cast<Constant>(*AI);
+            if (!C) C = ConstantPointerRef::get(cast<GlobalValue>(*AI));
+            
+            if (!ArgumentConstants[i].first)
+              ArgumentConstants[i].first = C;
+            else if (ArgumentConstants[i].first != C) {
+              // Became non-constant
+              ArgumentConstants[i].second = true;
+              ++NumNonconstant;
+              if (NumNonconstant == ArgumentConstants.size()) return false;
+            }
+          } else {
+            // This is not a constant argument.  Mark the argument as
+            // non-constant.
+            ArgumentConstants[i].second = true;
+            ++NumNonconstant;
+            if (NumNonconstant == ArgumentConstants.size()) return false;
+          }
+        }
+      }
+    }
+
+  // If we got to this point, there is a constant argument!
+  assert(NumNonconstant != ArgumentConstants.size());
+  Function::aiterator AI = F.abegin();
+  for (unsigned i = 0, e = ArgumentConstants.size(); i != e; ++i, ++AI)
+    // Do we have a constant argument!?
+    if (!ArgumentConstants[i].second) {
+      assert(ArgumentConstants[i].first && "Unknown constant value!");
+      AI->replaceAllUsesWith(ArgumentConstants[i].first);
+      ++NumArgumentsProped;
+    }
+  return true;
+}