[SCEV] Compute max be count from shift operator only if all else fails

In particular, check to see if we can compute a precise trip count by
exhaustively simulating the loop first.

llvm-svn: 274199

2 files changed, 35 insertions, 6 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 40274f77c6f..e28587e67d3 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -5948,11 +5948,6 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
         return ItCnt;
     }
 
-  ExitLimit ShiftEL = computeShiftCompareExitLimit(
-      ExitCond->getOperand(0), ExitCond->getOperand(1), L, Cond);
-  if (ShiftEL.hasAnyInfo())
-    return ShiftEL;
-
   const SCEV *LHS = getSCEV(ExitCond->getOperand(0));
   const SCEV *RHS = getSCEV(ExitCond->getOperand(1));
 
@@ -6018,7 +6013,15 @@ ScalarEvolution::computeExitLimitFromICmp(const Loop *L,
   default:
     break;
   }
-  return computeExitCountExhaustively(L, ExitCond, !L->contains(TBB));
+
+  auto *ExhaustiveCount =
+      computeExitCountExhaustively(L, ExitCond, !L->contains(TBB));
+
+  if (!isa<SCEVCouldNotCompute>(ExhaustiveCount))
+    return ExhaustiveCount;
+
+  return computeShiftCompareExitLimit(ExitCond->getOperand(0),
+                                      ExitCond->getOperand(1), L, Cond);
 }
 
 ScalarEvolution::ExitLimit
diff --git a/llvm/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll b/llvm/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll
new file mode 100644
index 00000000000..16d9cc54438
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll
@@ -0,0 +1,26 @@
+; RUN: opt -analyze -scalar-evolution < %s | FileCheck %s
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @f_0() {
+; CHECK-LABEL: Printing analysis 'Scalar Evolution Analysis' for function 'f_0':
+; CHECK: Loop %for.body: backedge-taken count is 5
+; CHECK: Loop %for.body: max backedge-taken count is 5
+; CHECK: Loop %for.body: Predicated backedge-taken count is 5
+
+entry:
+  br label %for.body
+
+for.body:
+  %i.05 = phi i32 [ 32, %entry ], [ %div4, %for.body ]
+  tail call void @dummy()
+  %div4 = lshr i32 %i.05, 1
+  %cmp = icmp eq i32 %div4, 0
+  br i1 %cmp, label %for.cond.cleanup, label %for.body
+
+for.cond.cleanup:
+  ret void
+}
+
+declare void @dummy()