[llvm] r258184 - [SCEV] Fix PR26207

[Sanjoy Das via llvm-commits]

Author: sanjoy
Date: Tue Jan 19 14:53:51 2016
New Revision: 258184

URL: http://llvm.org/viewvc/llvm-project?rev=258184&view=rev
Log:
[SCEV] Fix PR26207

In some cases, the max backedge taken count can be more conservative
than the exact backedge taken count (for instance, because
ScalarEvolution::getRange is not control-flow sensitive whereas
computeExitLimitFromICmp can be).  In these cases,
computeExitLimitFromCond (specifically the bit that deals with `and` and
`or` instructions) can create an ExitLimit instance with a
`SCEVCouldNotCompute` max backedge count expression, but a computable
exact backedge count expression.  This violates an implicit SCEV
assumption: a computable exact BE count should imply a computable max BE
count.

This change

 - Makes the above implicit invariant explicit by adding an assert to
   ExitLimit's constructor

 - Changes `computeExitLimitFromCond` to be more robust around
   conservative max backedge counts

Added:
    llvm/trunk/test/Transforms/IndVarSimplify/pr26207.ll
Modified:
    llvm/trunk/include/llvm/Analysis/ScalarEvolution.h
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/include/llvm/Analysis/ScalarEvolution.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ScalarEvolution.h?rev=258184&r1=258183&r2=258184&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/ScalarEvolution.h (original)
+++ llvm/trunk/include/llvm/Analysis/ScalarEvolution.h Tue Jan 19 14:53:51 2016
@@ -412,7 +412,11 @@ namespace llvm {
 
       /*implicit*/ ExitLimit(const SCEV *E) : Exact(E), Max(E) {}
 
-      ExitLimit(const SCEV *E, const SCEV *M) : Exact(E), Max(M) {}
+      ExitLimit(const SCEV *E, const SCEV *M) : Exact(E), Max(M) {
+        assert((isa<SCEVCouldNotCompute>(Exact) ||
+                !isa<SCEVCouldNotCompute>(Max)) &&
+               "Exact is not allowed to be less precise than Max");
+      }
 
       /// Test whether this ExitLimit contains any computed information, or
       /// whether it's all SCEVCouldNotCompute values.

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=258184&r1=258183&r2=258184&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Tue Jan 19 14:53:51 2016
@@ -5367,6 +5367,14 @@ ScalarEvolution::computeExitLimitFromCon
           BECount = EL0.Exact;
       }
 
+      // There are cases (e.g. PR26207) where computeExitLimitFromCond is able
+      // to be more aggressive when computing BECount than when computing
+      // MaxBECount.  In these cases it is possible for EL0.Exact and EL1.Exact
+      // to match, but for EL0.Max and EL1.Max to not.
+      if (isa<SCEVCouldNotCompute>(MaxBECount) &&
+          !isa<SCEVCouldNotCompute>(BECount))
+        MaxBECount = BECount;
+
       return ExitLimit(BECount, MaxBECount);
     }
     if (BO->getOpcode() == Instruction::Or) {

Added: llvm/trunk/test/Transforms/IndVarSimplify/pr26207.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/IndVarSimplify/pr26207.ll?rev=258184&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/IndVarSimplify/pr26207.ll (added)
+++ llvm/trunk/test/Transforms/IndVarSimplify/pr26207.ll Tue Jan 19 14:53:51 2016
@@ -0,0 +1,20 @@
+; RUN: opt -S -indvars < %s | FileCheck %s
+
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @main(i16 %in) {
+; CHECK-LABEL: @main(
+  br label %bb2
+
+bb2:                                              ; preds = %bb1.i, %bb2, %0
+  %_tmp44.i = icmp slt i16 %in, 2
+  br i1 %_tmp44.i, label %bb1.i, label %bb2
+
+bb1.i:                                            ; preds = %bb1.i, %bb2
+  %_tmp25.i = phi i16 [ %in, %bb2 ], [ %_tmp6.i, %bb1.i ]
+  %_tmp6.i = add nsw i16 %_tmp25.i, 1
+  %_tmp10.i = icmp sge i16 %_tmp6.i, 2
+  %exitcond.i = icmp eq i16 %_tmp6.i, 2
+  %or.cond = and i1 %_tmp10.i, %exitcond.i
+  br i1 %or.cond, label %bb2, label %bb1.i
+}