Add a new beta option for critical edge splitting, to avoid a problem that

Nate noticed in yacr2 (and I know occurs in other places as well).

This is still rough, as the critical edge blocks are not intelligently placed
but is added to get some idea to see if this improves performance.

llvm-svn: 22825

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
index 7303ee2add2..6ba00bccdf8 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
@@ -29,12 +29,18 @@
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetLowering.h"
 #include "llvm/Target/TargetMachine.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include <map>
 #include <iostream>
 using namespace llvm;
 
+static cl::opt<bool>
+SplitPHICritEdges("split-phi-constant-crit-edges", cl::Hidden,
+         cl::desc("Split critical edges for PHI values that are constants"));
+
+
 #ifndef _NDEBUG
 static cl::opt<bool>
 ViewDAGs("view-isel-dags", cl::Hidden,
@@ -959,6 +965,10 @@ unsigned SelectionDAGISel::MakeReg(MVT::ValueType VT) {
   return RegMap->createVirtualRegister(TLI.getRegClassFor(VT));
 }
 
+void SelectionDAGISel::getAnalysisUsage(AnalysisUsage &AU) const {
+  if (!SplitPHICritEdges)
+    AU.setPreservesAll();
+}
 
 
 bool SelectionDAGISel::runOnFunction(Function &Fn) {
@@ -966,6 +976,19 @@ bool SelectionDAGISel::runOnFunction(Function &Fn) {
   RegMap = MF.getSSARegMap();
   DEBUG(std::cerr << "\n\n\n=== " << Fn.getName() << "\n");
 
+  // First pass, split all critical edges for PHI nodes with incoming values
+  // that are constants, this way the load of the constant into a vreg will not
+  // be placed into MBBs that are used some other way.
+  if (SplitPHICritEdges)
+    for (Function::iterator BB = Fn.begin(), E = Fn.end(); BB != E; ++BB) {
+      PHINode *PN;
+      for (BasicBlock::iterator BBI = BB->begin();
+           (PN = dyn_cast<PHINode>(BBI)); ++BBI)
+        for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+          if (isa<Constant>(PN->getIncomingValue(i)))
+            SplitCriticalEdge(PN->getIncomingBlock(i), BB);
+    }
+
   FunctionLoweringInfo FuncInfo(TLI, Fn, MF);
 
   for (Function::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)