[llvm] r265337 - Enable unroll for constant bound loops when TripCount is not modulo of unroll factor, reducing it to maximum power-of-2 that satisfies threshold limit.

[Zia Ansari via llvm-commits]

Author: zansari
Date: Mon Apr  4 14:24:46 2016
New Revision: 265337

URL: http://llvm.org/viewvc/llvm-project?rev=265337&view=rev
Log:
Enable unroll for constant bound loops when TripCount is not modulo of unroll factor, reducing it to maximum power-of-2 that satisfies threshold limit.

Commit for Evgeny Stupachenko (evstupac at gmail.com)

Differential Revision: http://reviews.llvm.org/D18290


Added:
    llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp?rev=265337&r1=265336&r2=265337&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopUnrollPass.cpp Mon Apr  4 14:24:46 2016
@@ -639,6 +639,16 @@ static bool tryToUnrollLoop(Loop *L, Dom
       Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
       while (Count != 0 && TripCount % Count != 0)
         Count--;
+      if (Count <= 1) {
+        // If there is no Count that is modulo of TripCount, set Count to
+        // largest power-of-two factor that satisfies the threshold limit.
+        Count = (std::max(UP.PartialThreshold, 3u) - 2) / (LoopSize - 2);
+        UnrolledSize = (LoopSize - 2) * Count + 2;
+        while (Count != 0 && UnrolledSize > UP.PartialThreshold) {
+          Count >>= 1;
+          UnrolledSize = (LoopSize - 2) * Count + 2;
+        }
+      }
     }
   } else if (Unrolling == Runtime) {
     if (!AllowRuntime && !CountSetExplicitly) {

Added: llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll?rev=265337&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll (added)
+++ llvm/trunk/test/Transforms/LoopUnroll/partial-unroll-const-bounds.ll Mon Apr  4 14:24:46 2016
@@ -0,0 +1,29 @@
+; RUN: opt < %s -S -unroll-threshold=20 -loop-unroll -unroll-allow-partial | FileCheck %s
+
+; The Loop TripCount is 9. However unroll factors 3 or 9 exceed given threshold.
+; The test checks that we choose a smaller, power-of-two, unroll count and do not give up on unrolling.
+
+; CHECK: for.body:
+; CHECK: store
+; CHECK: for.body.1:
+; CHECK: store
+
+define void @foo(i32* nocapture %a, i32* nocapture readonly %b) nounwind uwtable {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %indvars.iv = phi i64 [ 1, %entry ], [ %indvars.iv.next, %for.body ]
+  %arrayidx = getelementptr inbounds i32, i32* %b, i64 %indvars.iv
+  %ld = load i32, i32* %arrayidx, align 4
+  %idxprom1 = sext i32 %ld to i64
+  %arrayidx2 = getelementptr inbounds i32, i32* %a, i64 %idxprom1
+  %st = trunc i64 %indvars.iv to i32
+  store i32 %st, i32* %arrayidx2, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond = icmp eq i64 %indvars.iv.next, 10
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}