[llvm] 5d5b087 - [DependenceAnalysis] Guard analysis using getPointerBase().

[Eli Friedman via llvm-commits]

Author: Eli Friedman
Date: 2021-07-15T14:57:32-07:00
New Revision: 5d5b08761f944d5b9822d582378333cc4b36a0a7

URL: https://github.com/llvm/llvm-project/commit/5d5b08761f944d5b9822d582378333cc4b36a0a7
DIFF: https://github.com/llvm/llvm-project/commit/5d5b08761f944d5b9822d582378333cc4b36a0a7.diff

LOG: [DependenceAnalysis] Guard analysis using getPointerBase().

D104806 broke some uses of getMinusSCEV() in DependenceAnalysis:
subtraction with different pointer bases returns a SCEVCouldNotCompute.
Make sure we avoid cases involving such subtractions.

Differential Revision: https://reviews.llvm.org/D106099

Added: 
    llvm/test/Analysis/DependenceAnalysis/lcssa.ll

Modified: 
    llvm/lib/Analysis/DependenceAnalysis.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Analysis/DependenceAnalysis.cpp b/llvm/lib/Analysis/DependenceAnalysis.cpp
index f11cce38406ad..9564cfb2aa454 100644
--- a/llvm/lib/Analysis/DependenceAnalysis.cpp
+++ b/llvm/lib/Analysis/DependenceAnalysis.cpp
@@ -3553,6 +3553,16 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
   const SCEV *DstSCEV = SE->getSCEV(DstPtr);
   LLVM_DEBUG(dbgs() << "    SrcSCEV = " << *SrcSCEV << "\n");
   LLVM_DEBUG(dbgs() << "    DstSCEV = " << *DstSCEV << "\n");
+  if (SE->getPointerBase(SrcSCEV) != SE->getPointerBase(DstSCEV)) {
+    // If two pointers have 
diff erent bases, trying to analyze indexes won't
+    // work; we can't compare them to each other. This can happen, for example,
+    // if one is produced by an LCSSA PHI node.
+    //
+    // We check this upfront so we don't crash in cases where getMinusSCEV()
+    // returns a SCEVCouldNotCompute.
+    LLVM_DEBUG(dbgs() << "can't analyze SCEV with 
diff erent pointer base\n");
+    return std::make_unique<Dependence>(Src, Dst);
+  }
   Pair[0].Src = SrcSCEV;
   Pair[0].Dst = DstSCEV;
 

diff  --git a/llvm/test/Analysis/DependenceAnalysis/lcssa.ll b/llvm/test/Analysis/DependenceAnalysis/lcssa.ll
new file mode 100644
index 0000000000000..a42295cbe30eb
--- /dev/null
+++ b/llvm/test/Analysis/DependenceAnalysis/lcssa.ll
@@ -0,0 +1,38 @@
+; RUN: opt < %s -disable-output "-passes=print<da>"                            \
+; RUN: "-aa-pipeline=basic-aa,tbaa" 2>&1 | FileCheck %s
+; RUN: opt < %s -analyze -enable-new-pm=0 -basic-aa -tbaa -da | FileCheck %s
+
+; CHECK:      Src:  %v = load i32, i32* %arrayidx1, align 4 --> Dst:  store i32 %add, i32* %a.lcssa, align 4
+; CHECK-NEXT: da analyze - confused!
+
+define void @f(i32 *%a, i32 %n, i64 %n2) {
+entry:
+  br label %while.body
+
+while.body:
+  %n.addr = phi i32 [ %mul, %while.body ], [ %n, %entry ]
+  %inc.phi = phi i32 [ %inc, %while.body ], [ 0, %entry ]
+  %mul = mul i32 %n.addr, 3
+  %div = udiv i32 %mul, 2
+  %inc = add i32 %inc.phi, 1
+  %incdec.ptr = getelementptr inbounds i32, i32* %a, i32 %inc
+  %cmp.not = icmp eq i32 %div, 1
+  br i1 %cmp.not, label %while.end, label %while.body
+
+while.end:
+  %a.lcssa = phi i32* [ %incdec.ptr, %while.body ]
+  br label %for.body
+
+for.body:
+  %indvars.iv = phi i64 [ 1, %while.end ], [ %indvars.iv.next, %for.body ]
+  %arrayidx1 = getelementptr inbounds i32, i32* %a, i64 %indvars.iv
+  %v = load i32, i32* %arrayidx1, align 4
+  %add = add nsw i32 %v, 1
+  store i32 %add, i32* %a.lcssa, align 4
+  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
+  %exitcond.not = icmp eq i64 %indvars.iv.next, %n2
+  br i1 %exitcond.not, label %ret, label %for.body
+
+ret:
+  ret void
+}