[Hexagon] Always generate mux out of predicated transfers if possible

HexagonGenMux would collapse pairs of predicated transfers if it assumed
that the predicated .new forms cannot be created. Turns out that generating
mux is preferable in almost all cases.
Introduce an option -hexagon-gen-mux-threshold that controls the minimum
distance between the instruction defining the predicate and the later of
the two transfers. If the distance is closer than the threshold, mux will
not be generated. Set the threshold to 0 by default.

llvm-svn: 328346

1 files changed, 10 insertions, 2 deletions

diff --git a/llvm/lib/Target/Hexagon/HexagonGenMux.cpp b/llvm/lib/Target/Hexagon/HexagonGenMux.cpp
index 5a001d6ed9c..7c91412c250 100644
--- a/llvm/lib/Target/Hexagon/HexagonGenMux.cpp
+++ b/llvm/lib/Target/Hexagon/HexagonGenMux.cpp
@@ -40,6 +40,7 @@
 #include "llvm/MC/MCInstrDesc.h"
 #include "llvm/MC/MCRegisterInfo.h"
 #include "llvm/Pass.h"
+#include "llvm/Support/CommandLine.h"
 #include "llvm/Support/MathExtras.h"
 #include <algorithm>
 #include <cassert>
@@ -56,6 +57,11 @@ namespace llvm {
 
 } // end namespace llvm
 
+// Initialize this to 0 to always prefer generating mux by default.
+static cl::opt<unsigned> MinPredDist("hexagon-gen-mux-threshold", cl::Hidden,
+  cl::init(0), cl::desc("Minimum distance between predicate definition and "
+  "farther of the two predicated uses"));
+
 namespace {
 
   class HexagonGenMux : public MachineFunctionPass {
@@ -269,11 +275,13 @@ bool HexagonGenMux::genMuxInBlock(MachineBasicBlock &B) {
     // There is now a complete definition of DR, i.e. we have the predicate
     // register, the definition if-true, and definition if-false.
 
-    // First, check if both definitions are far enough from the definition
+    // First, check if the definitions are far enough from the definition
     // of the predicate register.
     unsigned MinX = std::min(CI.TrueX, CI.FalseX);
     unsigned MaxX = std::max(CI.TrueX, CI.FalseX);
-    unsigned SearchX = (MaxX > 4) ? MaxX-4 : 0;
+    // Specifically, check if the predicate definition is within a prescribed
+    // distance from the farther of the two predicated instructions.
+    unsigned SearchX = (MaxX >= MinPredDist) ? MaxX-MinPredDist : 0;
     bool NearDef = false;
     for (unsigned X = SearchX; X < MaxX; ++X) {
       const DefUseInfo &DU = DUM.lookup(X);