Subzero: Improve handling of alloca instructions of constant size.

src/IceCfg.cpp

 //===- subzero/src/IceCfg.cpp - Control flow graph implementation ---------===//
 //
 //                        The Subzero Code Generator
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 ///
 /// \file
@@ skipping 183 matching lines @@
       addCallToProfileSummary();
     }
     dump("Profiled CFG");
   }
 
   // Create the Hi and Lo variables where a split was needed
   for (Variable *Var : Variables)
     if (auto Var64On32 = llvm::dyn_cast<Variable64On32>(Var))
       Var64On32->initHiLo(this);
 
+  // Figure out which alloca instructions result in storage at known stack frame
+  // offsets.  If this is true for all alloca instructions, then a stack pointer
+  // can still be used instead of a frame pointer, freeing up the frame pointer
+  // for normal register allocation.  Additionally, for each such alloca, its
+  // address could be rematerialized at each use in terms of the stack/frame
+  // pointer, saving a stack slot and a load from that stack slot.
+  //
+  // This simple implementation is limited to alloca instructions at the start
+  // of the entry node.
+  for (Inst &Instr : getEntryNode()->getInsts()) {
+    if (auto *Alloca = llvm::dyn_cast<InstAlloca>(&Instr)) {
+      if (llvm::isa<Constant>(Alloca->getSizeInBytes())) {
+        Alloca->setKnownFrameOffset();
+        continue;
+      }
+    }
+    // The first instruction that is not an alloca with a constant size stops
+    // the search.
+    break;
+  }
+
   // The set of translation passes and their order are determined by the
   // target.
   getTarget()->translate();
 
   dump("Final output");
   if (getFocusedTiming())
     getContext()->dumpTimers();
 }
 
 void Cfg::computeInOutEdges() {
@@ skipping 573 matching lines @@
     }
   }
   // Print each basic block
   for (CfgNode *Node : Nodes)
     Node->dump(this);
   if (isVerbose(IceV_Instructions))
     Str << "}\n";
 }
 
 } // end of namespace Ice