[llvm] r274199 - [SCEV] Compute max be count from shift operator only if all else fails

[Sanjoy Das via llvm-commits]

Author: sanjoy
Date: Wed Jun 29 21:47:28 2016
New Revision: 274199

URL: http://llvm.org/viewvc/llvm-project?rev=274199&view=rev
Log:
[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.

Added:
    llvm/trunk/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll
Modified:
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=274199&r1=274198&r2=274199&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Wed Jun 29 21:47:28 2016
@@ -5948,11 +5948,6 @@ ScalarEvolution::computeExitLimitFromICm
         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::computeExitLimitFromICm
   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

Added: llvm/trunk/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll?rev=274199&view=auto
==============================================================================
--- llvm/trunk/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll (added)
+++ llvm/trunk/test/Analysis/ScalarEvolution/exhaustive-trip-counts.ll Wed Jun 29 21:47:28 2016
@@ -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()