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

[sanjoy]

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).

[reames]

LGTM

lib/Transforms/Scalar/PlaceSafepoints.cpp

 
 using namespace llvm;
 
 // Ignore opportunities to avoid placing safepoints on backedges, useful for
 // validation
 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
 // experimentation, but in practice, it looks like splitting the backedge
 // optimizes better.
 static cl::opt<bool> SplitBackedge("spp-split-backedge", cl::Hidden,
                                    cl::init(false));
 

 }
 
 /// Returns true if this loop is known to terminate in a finite number of
 /// iterations.  Note that this function may return false for a loop which
 /// does actual terminate in a finite constant number of iterations due to
 /// 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))
 
   // 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
   // of the exit block.
   if (L->isLoopExiting(Pred)) {
     // 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))
+    if (MaxExec != SE->getCouldNotCompute() &&
+        SE->getUnsignedRange(MaxExec).getUnsignedMax().isIntN(
+            CountedLoopTripWidth))
         return true;
-      if (SkipCounted &&
-          SE->getUnsignedRange(MaxExec).getUnsignedMax().isIntN(32))
-        return true;
-    }
   }
 
   return /* not finite */ false;
 }
 
 static void scanOneBB(Instruction *start, Instruction *end,
                       std::vector<CallInst *> &calls,
                       std::set<BasicBlock *> &seen,

test/Transforms/PlaceSafepoints/finite-loops.ll

 ; 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
 define void @test1(i32) gc "statepoint-example" {
 ; CHECK-LABEL: test1
 ; CHECK-LABEL: entry
 ; CHECK: statepoint
 ; CHECK-LABEL: loop
loop:
   %counter.inc = add nsw i8 %counter, 1
   %counter.cmp = icmp slt i8 %counter.inc, %upper
   br i1 %counter.cmp, label %loop, label %exit
 
 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()
 define void @gc.safepoint_poll() {
 ; CHECK-LABEL: gc.safepoint_poll
 entry:
   call void @do_safepoint()
   ret void
 }
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