don't unroll loops with zero trip count; fix for PR21409

[spatel]

Summary

In PR21409 ( http://llvm.org/bugs/show_bug.cgi?id=21409 ), the loop unroller tries to transform a loop with a backedge count of i32 4294967295. Adding 1 to that causes an overflow of the SCEV value for the trip count. This patch adds a check to prevent unrolling that type of loop.

[hfinkel]

So this solves the constant-trip-count case, do we have the same problem at runtime if the backedge count dynamically takes the maximum value?

[spatel]

This change actually is in the runtime unroller...why we're getting here for a compile-time constant loop is another bug?

In the compile-time unroller, we do this check:

TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);

And getSmallConstantTripCount() has this:

// In case of integer overflow, this returns 0, which is correct.
return ((unsigned)ExitConst->getZExtValue()) + 1;

[hfinkel]

In D6200#5, @spatel wrote:

This change actually is in the runtime unroller...why we're getting here for a compile-time constant loop is another bug?

The runtime unroller also does partial unrolling of large-trip-count loops. So we're probably in the right place (UINT_MAX is certainly above the full-unrolling limit).

[spatel]

Ping.

[spatel]

Abandoning - better patch checked in at:
http://llvm.org/viewvc/llvm-project?view=revision&revision=222451

lib/Transforms/Utils/LoopUnrollRuntime.cpp

if (!SE)
     return false;
 
   // Only unroll loops with a computable trip count and the trip count needs
   // to be an int value (allowing a pointer type is a TODO item)
   const SCEV *BECount = SE->getBackedgeTakenCount(L);
   if (isa<SCEVCouldNotCompute>(BECount) || !BECount->getType()->isIntegerTy())
     return false;
 
-  // Add 1 since the backedge count doesn't include the first loop iteration
+  // Add 1 since the backedge count doesn't include the first loop iteration.
   const SCEV *TripCountSC =
     SE->getAddExpr(BECount, SE->getConstant(BECount->getType(), 1));
-  if (isa<SCEVCouldNotCompute>(TripCountSC))
+
+  // If the trip count is not computable, zero, or the preceding add caused an
+  // overflow of the SCEV (causing a zero trip count), then don't unroll.
+  if (isa<SCEVCouldNotCompute>(TripCountSC) || TripCountSC->isZero())
     return false;
 
   // We only handle cases when the unroll factor is a power of 2.
   // Count is the loop unroll factor, the number of extra copies added + 1.
   if ((Count & (Count-1)) != 0)
     return false;
 
   // If this loop is nested, then the loop unroller changes the code in

test/Transforms/LoopUnroll/runtime-zero-tripcount.ll

+; RUN: opt < %s -S -loop-unroll -unroll-runtime | FileCheck %s
+
+; PR21409 ( http://llvm.org/bugs/show_bug.cgi?id=21409 )
+; Don't unroll a loop with a zero trip count.
+; Either the loop is never entered, or the calculation of the
+; trip count overflowed.
+
+define i32 @zero_trip_count() {
+entry:
+  br label %while.body
+
+while.body:
+  %d = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+  %inc = add i32 %d, 1
+  %cmp = icmp eq i32 %inc, 0
+  br i1 %cmp, label %while.end, label %while.body
+
+while.end:
+  ret i32 0
+
+; CHECK-LABEL: zero_trip_count(
+; CHECK: entry:
+; CHECK-NEXT: br label %while.body
+; CHECK: while.body:
+; CHECK-NEXT: %d = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+; CHECK-NEXT: %inc = add i32 %d, 1
+; CHECK-NEXT: %cmp = icmp eq i32 %inc, 0
+; CHECK-NEXT: br i1 %cmp, label %while.end, label %while.body
+; CHECK: while.end:
+; CHECK-NEXT: ret i32 0
+}
+
+