Add -unroll-runtime for unrolling loops with run-time trip counts.

Patch by Brendon Cahoon!

This extends the existing LoopUnroll and LoopUnrollPass. Brendon
measured no regressions in the llvm test suite with -unroll-runtime
enabled. This implementation works by using the existing loop
unrolling code to unroll the loop by a power-of-two (default 8). It
generates an if-then-else sequence of code prior to the loop to
execute the extra iterations before entering the unrolled loop.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146245 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 7 deletions

diff --git a/lib/Transforms/Scalar/LoopUnrollPass.cpp b/lib/Transforms/Scalar/LoopUnrollPass.cpp
index f3fdd4f3c5..22dbfe326c 100644
--- a/lib/Transforms/Scalar/LoopUnrollPass.cpp
+++ b/lib/Transforms/Scalar/LoopUnrollPass.cpp
@@ -40,6 +40,10 @@ UnrollAllowPartial("unroll-allow-partial", cl::init(false), cl::Hidden,
   cl::desc("Allows loops to be partially unrolled until "
            "-unroll-threshold loop size is reached."));
 
+static cl::opt<bool>
+UnrollRuntime("unroll-runtime", cl::ZeroOrMore, cl::init(false), cl::Hidden,
+  cl::desc("Unroll loops with run-time trip counts"));
+
 namespace {
   class LoopUnroll : public LoopPass {
   public:
@@ -63,6 +67,10 @@ namespace {
     // explicit -unroll-threshold).
     static const unsigned OptSizeUnrollThreshold = 50;
 
+    // Default unroll count for loops with run-time trip count if
+    // -unroll-count is not set
+    static const unsigned UnrollRuntimeCount = 8;
+
     unsigned CurrentCount;
     unsigned CurrentThreshold;
     bool     CurrentAllowPartial;
@@ -151,8 +159,13 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
     TripCount = SE->getSmallConstantTripCount(L, LatchBlock);
     TripMultiple = SE->getSmallConstantTripMultiple(L, LatchBlock);
   }
-  // Automatically select an unroll count.
+  // Use a default unroll-count if the user doesn't specify a value
+  // and the trip count is a run-time value.  The default is different
+  // for run-time or compile-time trip count loops.
   unsigned Count = CurrentCount;
+  if (UnrollRuntime && CurrentCount == 0 && TripCount == 0)
+    Count = UnrollRuntimeCount;
+
   if (Count == 0) {
     // Conservative heuristic: if we know the trip count, see if we can
     // completely unroll (subject to the threshold, checked below); otherwise
@@ -177,15 +190,23 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
     if (TripCount != 1 && Size > Threshold) {
       DEBUG(dbgs() << "  Too large to fully unroll with count: " << Count
             << " because size: " << Size << ">" << Threshold << "\n");
-      if (!CurrentAllowPartial) {
+      if (!CurrentAllowPartial && !(UnrollRuntime && TripCount == 0)) {
         DEBUG(dbgs() << "  will not try to unroll partially because "
               << "-unroll-allow-partial not given\n");
         return false;
       }
-      // Reduce unroll count to be modulo of TripCount for partial unrolling
-      Count = Threshold / LoopSize;
-      while (Count != 0 && TripCount%Count != 0) {
-        Count--;
+      if (TripCount) {
+        // Reduce unroll count to be modulo of TripCount for partial unrolling
+        Count = CurrentThreshold / LoopSize;
+        while (Count != 0 && TripCount%Count != 0)
+          Count--;
+      }
+      else if (UnrollRuntime) {
+        // Reduce unroll count to be a lower power-of-two value
+        while (Count != 0 && Size > CurrentThreshold) {
+          Count >>= 1;
+          Size = LoopSize*Count;
+        }
       }
       if (Count < 2) {
         DEBUG(dbgs() << "  could not unroll partially\n");
@@ -196,7 +217,7 @@ bool LoopUnroll::runOnLoop(Loop *L, LPPassManager &LPM) {
   }
 
   // Unroll the loop.
-  if (!UnrollLoop(L, Count, TripCount, TripMultiple, LI, &LPM))
+  if (!UnrollLoop(L, Count, TripCount, UnrollRuntime, TripMultiple, LI, &LPM))
     return false;
 
   return true;