[PlaceSafepoints] Make the width of a counted loop settable.

Summary:
This change lets a `PlaceSafepoints` client change how wide the trip
count of a loop has to be for the loop to be considerd "counted", via
`CountedLoopTripWidth`.  It also removes the boolean `SkipCounted` flag
and the `upperTripBound` constant -- we can get the old behavior of
`SkipCounted` == `false` by setting `CountedLoopTripWidth` to `13` (2 ^
13 == 8192).

Reviewers: reames

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D12789

llvm-svn: 247656

2 files changed, 68 insertions, 19 deletions

diff --git a/llvm/lib/Transforms/Scalar/PlaceSafepoints.cpp b/llvm/lib/Transforms/Scalar/PlaceSafepoints.cpp
index eb1fe48f42d..1473e3db7c7 100644
--- a/llvm/lib/Transforms/Scalar/PlaceSafepoints.cpp
+++ b/llvm/lib/Transforms/Scalar/PlaceSafepoints.cpp
@@ -96,8 +96,10 @@ using namespace llvm;
 static cl::opt<bool> AllBackedges("spp-all-backedges", cl::Hidden,
                                   cl::init(false));
 
-/// If true, do not place backedge safepoints in counted loops.
-static cl::opt<bool> SkipCounted("spp-counted", cl::Hidden, cl::init(true));
+/// How narrow does the trip count of a loop have to be to have to be considered
+/// "counted"?  Counted loops do not get safepoints at backedges.
+static cl::opt<int> CountedLoopTripWidth("spp-counted-loop-trip-width",
+                                         cl::Hidden, cl::init(32));
 
 // If true, split the backedge of a loop when placing the safepoint, otherwise
 // split the latch block itself.  Both are useful to support for
@@ -255,18 +257,12 @@ static bool containsUnconditionalCallSafepoint(Loop *L, BasicBlock *Header,
 /// conservatism in the analysis.
 static bool mustBeFiniteCountedLoop(Loop *L, ScalarEvolution *SE,
                                     BasicBlock *Pred) {
-  // Only used when SkipCounted is off
-  const unsigned upperTripBound = 8192;
-
   // A conservative bound on the loop as a whole.
   const SCEV *MaxTrips = SE->getMaxBackedgeTakenCount(L);
-  if (MaxTrips != SE->getCouldNotCompute()) {
-    if (SE->getUnsignedRange(MaxTrips).getUnsignedMax().ult(upperTripBound))
-      return true;
-    if (SkipCounted &&
-        SE->getUnsignedRange(MaxTrips).getUnsignedMax().isIntN(32))
-      return true;
-  }
+  if (MaxTrips != SE->getCouldNotCompute() &&
+      SE->getUnsignedRange(MaxTrips).getUnsignedMax().isIntN(
+          CountedLoopTripWidth))
+    return true;
 
   // If this is a conditional branch to the header with the alternate path
   // being outside the loop, we can ask questions about the execution frequency
@@ -275,13 +271,10 @@ static bool mustBeFiniteCountedLoop(Loop *L, ScalarEvolution *SE,
     // This returns an exact expression only.  TODO: We really only need an
     // upper bound here, but SE doesn't expose that.
     const SCEV *MaxExec = SE->getExitCount(L, Pred);
-    if (MaxExec != SE->getCouldNotCompute()) {
-      if (SE->getUnsignedRange(MaxExec).getUnsignedMax().ult(upperTripBound))
-        return true;
-      if (SkipCounted &&
-          SE->getUnsignedRange(MaxExec).getUnsignedMax().isIntN(32))
+    if (MaxExec != SE->getCouldNotCompute() &&
+        SE->getUnsignedRange(MaxExec).getUnsignedMax().isIntN(
+            CountedLoopTripWidth))
         return true;
-    }
   }
 
   return /* not finite */ false;
diff --git a/llvm/test/Transforms/PlaceSafepoints/finite-loops.ll b/llvm/test/Transforms/PlaceSafepoints/finite-loops.ll
index 3cc7158afcf..9121e92896c 100644
--- a/llvm/test/Transforms/PlaceSafepoints/finite-loops.ll
+++ b/llvm/test/Transforms/PlaceSafepoints/finite-loops.ll
@@ -1,6 +1,7 @@
 ; Tests to ensure that we are not placing backedge safepoints in
 ; loops which are clearly finite.
-;; RUN: opt %s -place-safepoints -S | FileCheck %s
+;; RUN: opt %s -place-safepoints -spp-counted-loop-trip-width=32 -S | FileCheck %s
+;; RUN: opt %s -place-safepoints -spp-counted-loop-trip-width=64 -S | FileCheck %s -check-prefix=COUNTED-64
 
 
 ; A simple counted loop with trivially known range
@@ -69,6 +70,61 @@ exit:
   ret void
 }
 
+; The range is a 64 bit value
+define void @test4(i64 %upper) gc "statepoint-example" {
+; CHECK-LABEL: test4
+; CHECK-LABEL: entry
+; CHECK: statepoint
+; CHECK-LABEL: loop
+; CHECK: statepoint
+
+; COUNTED-64-LABEL: test4
+; COUNTED-64-LABEL: entry
+; COUNTED-64: statepoint
+; COUNTED-64-LABEL: loop
+; COUNTED-64-NOT: statepoint
+
+entry:
+  br label %loop
+
+loop:
+  %counter = phi i64 [ 0 , %entry ], [ %counter.inc , %loop ]
+  %counter.inc = add i64 %counter, 1
+  %counter.cmp = icmp slt i64 %counter.inc, %upper
+  br i1 %counter.cmp, label %loop, label %exit
+
+exit:
+  ret void
+}
+
+; This loop can run infinitely (for %upper == INT64_MAX) so it needs a
+; safepoint.
+define void @test5(i64 %upper) gc "statepoint-example" {
+; CHECK-LABEL: test5
+; CHECK-LABEL: entry
+; CHECK: statepoint
+; CHECK-LABEL: loop
+; CHECK: statepoint
+
+; COUNTED-64-LABEL: test5
+; COUNTED-64-LABEL: entry
+; COUNTED-64: statepoint
+; COUNTED-64-LABEL: loop
+; COUNTED-64: statepoint
+
+entry:
+  br label %loop
+
+loop:
+  %counter = phi i64 [ 0 , %entry ], [ %counter.inc , %loop ]
+  %counter.inc = add i64 %counter, 1
+  %counter.cmp = icmp sle i64 %counter.inc, %upper
+  br i1 %counter.cmp, label %loop, label %exit
+
+exit:
+  ret void
+}
+
 
 ; This function is inlined when inserting a poll.
 declare void @do_safepoint()