[llvm] r353779 - [LoopReroll] Fix reroll root legality checking.

[Eli Friedman via llvm-commits]

Author: efriedma
Date: Mon Feb 11 16:33:25 2019
New Revision: 353779

URL: http://llvm.org/viewvc/llvm-project?rev=353779&view=rev
Log:
[LoopReroll] Fix reroll root legality checking.

The code checked that the first root was an appropriate distance from
the base value, but skipped checking the other roots. This could lead to
rerolling a loop that can't be legally rerolled (at least, not without
rewriting the loop in a non-trivial way).

Differential Revision: https://reviews.llvm.org/D56812


Modified:
    llvm/trunk/lib/Transforms/Scalar/LoopRerollPass.cpp
    llvm/trunk/test/Transforms/LoopReroll/basic.ll

Modified: llvm/trunk/lib/Transforms/Scalar/LoopRerollPass.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopRerollPass.cpp?rev=353779&r1=353778&r2=353779&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/LoopRerollPass.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/LoopRerollPass.cpp Mon Feb 11 16:33:25 2019
@@ -891,12 +891,22 @@ bool LoopReroll::DAGRootTracker::validat
   const auto *ADR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(DRS.BaseInst));
   if (!ADR)
     return false;
+
+  // Check that the first root is evenly spaced.
   unsigned N = DRS.Roots.size() + 1;
   const SCEV *StepSCEV = SE->getMinusSCEV(SE->getSCEV(DRS.Roots[0]), ADR);
   const SCEV *ScaleSCEV = SE->getConstant(StepSCEV->getType(), N);
   if (ADR->getStepRecurrence(*SE) != SE->getMulExpr(StepSCEV, ScaleSCEV))
     return false;
 
+  // Check that the remainling roots are evenly spaced.
+  for (unsigned i = 1; i < N - 1; ++i) {
+    const SCEV *NewStepSCEV = SE->getMinusSCEV(SE->getSCEV(DRS.Roots[i]),
+                                               SE->getSCEV(DRS.Roots[i-1]));
+    if (NewStepSCEV != StepSCEV)
+      return false;
+  }
+
   return true;
 }
 

Modified: llvm/trunk/test/Transforms/LoopReroll/basic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopReroll/basic.ll?rev=353779&r1=353778&r2=353779&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/LoopReroll/basic.ll (original)
+++ llvm/trunk/test/Transforms/LoopReroll/basic.ll Mon Feb 11 16:33:25 2019
@@ -785,6 +785,30 @@ bb19:
   ret void
 }
 
+define void @bad_step(i32* nocapture readnone %x) #0 {
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %i.08 = phi i32 [ 0, %entry ], [ %add3, %for.body ]
+  %call = tail call i32 @foo(i32 %i.08) #1
+  %add = add nsw i32 %i.08, 2
+  %call1 = tail call i32 @foo(i32 %add) #1
+  %add2 = add nsw i32 %i.08, 3
+  %call3 = tail call i32 @foo(i32 %add2) #1
+  %add3 = add nsw i32 %i.08, 6
+  %exitcond = icmp sge i32 %add3, 500
+  br i1 %exitcond, label %for.end, label %for.body
+
+; CHECK-LABEL: @bad_step
+; CHECK: %add = add nsw i32 %i.08, 2
+; CHECK: %add2 = add nsw i32 %i.08, 3
+; CHECK: %add3 = add nsw i32 %i.08, 6
+
+for.end:                                          ; preds = %for.body
+  ret void
+}
+
 attributes #0 = { nounwind uwtable }
 attributes #1 = { nounwind }