[PATCH] D119261: [DependenceAnalysis][PR52170] Conservative crash on overflowed loop backedge

[Artem Radzikhovskyy via Phabricator via llvm-commits]

artemrad created this revision.
artemrad added reviewers: uabelho, fhahn, philip.pfaffe, pfaffe, bmahjour, Meinersbur.
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The following commit (https://github.com/llvm/llvm-project/commit/7086025d6567562d31fadbaccf08b4fd72ec2100) enabled delinearization of fixed sized arrays. It is now possible to propagate SIV constraints to coupled subscripts in more cases, resulting in more accurate dependence analysis. When a SIV constraint is propagated we re-classify the other non-SIV subscripts. It is possible that the re-classified subscript is now marked as non-linear, in cases where we cannot prove that the taken back-edge of the loop may not overflow.

Changes:

- Removed overly conservative code (that is causing the crash reported in the bug), in checkSubscript. SCEVs already do the overflow checks that the code was targeting.
- Added a new LIT test to test that DA reports conservative dependencies when it cannot be determined if overflow will happen in the AddRec


https://reviews.llvm.org/D119261

Files:
  llvm/lib/Analysis/DependenceAnalysis.cpp
  llvm/test/Analysis/DependenceAnalysis/TripCountOverflow.ll


Index: llvm/test/Analysis/DependenceAnalysis/TripCountOverflow.ll
===================================================================
--- /dev/null
+++ llvm/test/Analysis/DependenceAnalysis/TripCountOverflow.ll
@@ -0,0 +1,40 @@
+; RUN: opt %s -disable-output "-passes=print<da>" 2>&1 | FileCheck %s
+
+; [PR52170]
+; Test that in cases where SIV propagation does not produce a linear equation
+; (e.g. the trip count of the post propagation SCEV can overflow) produce a
+; conservative answer - don't crash.
+; CHECK-LABEL: dynamic_trip_count
+; CHECK: da analyze - output [* S]!
+; CHECK: da analyze - output [* *|<]!
+; CHECK: da analyze - output [* *]!
+
+define void @dynamic_trip_count(i32 %n) {
+entry:
+  %array = alloca [2 x [5 x i32]], align 1
+  br label %for.outer
+
+for.outer:
+  %i = phi i32 [ 0, %entry ], [ %add1140.i.i, %cleanup ]
+  %i.trunc = trunc i32 %i to i16
+  %i.trunc.3 = add i16 %i.trunc, 3
+  %i.ptr = getelementptr inbounds [2 x [5 x i32]], [2 x [5 x i32]]* %array, i16 0, i16 %i.trunc, i16 %i.trunc.3
+  br label %for.inner
+
+for.inner:
+  %j = phi i32 [ 0, %for.outer ], [ %j.1, %for.inner ]
+  store i32 undef, i32* %i.ptr, align 1
+  %j.trunc = trunc i32 %j to i16
+  %j.ptr = getelementptr inbounds [2 x [5 x i32]], [2 x [5 x i32]]* %array, i16 0, i16 %j.trunc, i16 %i.trunc.3
+  store i32 0, i32* %j.ptr, align 1
+  %j.1 = add nuw nsw i32 %j, 1
+  br i1 false, label %for.inner, label %cleanup
+
+cleanup:
+  %add1140.i.i = add nuw nsw i32 %i, 1
+  %cmp602.i.i = icmp slt i32 %i, %n
+  br i1 %cmp602.i.i, label %for.outer, label %other
+
+other:
+  ret void
+}
\ No newline at end of file
Index: llvm/lib/Analysis/DependenceAnalysis.cpp
===================================================================
--- llvm/lib/Analysis/DependenceAnalysis.cpp
+++ llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -899,14 +899,7 @@
     return isLoopInvariant(Expr, LoopNest);
   const SCEV *Start = AddRec->getStart();
   const SCEV *Step = AddRec->getStepRecurrence(*SE);
-  const SCEV *UB = SE->getBackedgeTakenCount(AddRec->getLoop());
-  if (!isa<SCEVCouldNotCompute>(UB)) {
-    if (SE->getTypeSizeInBits(Start->getType()) <
-        SE->getTypeSizeInBits(UB->getType())) {
-      if (!AddRec->getNoWrapFlags())
-        return false;
-    }
-  }
+
   if (!isLoopInvariant(Step, LoopNest))
     return false;
   if (IsSrc)
@@ -3296,12 +3289,12 @@
 
   int Size = SrcSubscripts.size();
   LLVM_DEBUG({
-    dbgs() << "\nSrcSubscripts: ";
+    dbgs() << "SrcSubscripts:\n";
     for (int I = 0; I < Size; I++)
-      dbgs() << *SrcSubscripts[I];
-    dbgs() << "\nDstSubscripts: ";
+      dbgs() << "\t" << *SrcSubscripts[I] << "\n";
+    dbgs() << "DstSubscripts:\n";
     for (int I = 0; I < Size; I++)
-      dbgs() << *DstSubscripts[I];
+      dbgs() << "\t" << *DstSubscripts[I] << "\n";
   });
 
   // The delinearization transforms a single-subscript MIV dependence test into


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