Merge fixed alloca stack adjustments into the prolog

src/IceTargetLowering.cpp

 //===- subzero/src/IceTargetLowering.cpp - Basic lowering implementation --===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 345 matching lines @@
   // function. In that case, basic blocks may be revisited, and variables local
   // to those basic blocks are actually live until after the called function
   // returns a second time.
   const bool SimpleCoalescing = !callsReturnsTwice();
 
   std::vector<size_t> LocalsSize(Func->getNumNodes());
   const VarList &Variables = Func->getVariables();
   VarList SpilledVariables;
   for (Variable *Var : Variables) {
     if (Var->hasReg()) {
-      RegsUsed[Var->getRegNum()] = true;
+      // Don't consider a rematerializable variable to be an actual register use
+      // (specifically of the frame pointer).  Otherwise, the prolog may decide
+      // to save the frame pointer twice - once because of the explicit need for
+      // a frame pointer, and once because of an active use of a callee-save
+      // register.
+      if (!Var->isRematerializable())
+        RegsUsed[Var->getRegNum()] = true;
       continue;
     }
     // An argument either does not need a stack slot (if passed in a register)
     // or already has one (if passed on the stack).
     if (Var->getIsArg())
       continue;
     // An unreferenced variable doesn't need a stack slot.
     if (!IsVarReferenced[Var->getIndex()])
       continue;
     // Check a target-specific variable (it may end up sharing stack slots) and
@@ skipping 266 matching lines @@
   if (Target == Target_##X)                                                    \
     return TargetHeader##X::create(Ctx);
 #include "llvm/Config/SZTargets.def"
 
   llvm::report_fatal_error("Unsupported target header lowering");
 }
 
 TargetHeaderLowering::~TargetHeaderLowering() = default;
 
 } // end of namespace Ice