[DependenceAnalysis] Make sure base objects are the same when compari…

…ng GEPs

The dependence analysis was returning incorrect information when using the GEPs
to compute dependences. The analysis uses the GEP indices under certain
conditions, but was doing it incorrectly when the base objects of the GEP are
aliases, but pointing to different locations in the same array.

This patch adds another check for the base objects. If the base pointer SCEVs
are not equal, then the dependence analysis should fall back on the path
that uses the whole SCEV for the dependence check. This fixes PR33567.

Differential Revision: https://reviews.llvm.org/D34702

llvm-svn: 307203

llvm/lib/Analysis/DependenceAnalysis.cpp

@@ -3342,7 +3342,8 @@ DependenceInfo::depends(Instruction *Src, Instruction *Dst,
 
     UsefulGEP = isLoopInvariant(SrcPtrSCEV, LI->getLoopFor(Src->getParent())) &&
                 isLoopInvariant(DstPtrSCEV, LI->getLoopFor(Dst->getParent())) &&
-                (SrcGEP->getNumOperands() == DstGEP->getNumOperands());
+                (SrcGEP->getNumOperands() == DstGEP->getNumOperands()) &&
+                isKnownPredicate(CmpInst::ICMP_EQ, SrcPtrSCEV, DstPtrSCEV);
   }
   unsigned Pairs = UsefulGEP ? SrcGEP->idx_end() - SrcGEP->idx_begin() : 1;
   SmallVector<Subscript, 4> Pair(Pairs);

llvm/test/Analysis/DependenceAnalysis/BasePtrBug.ll

@@ -0,0 +1,80 @@
+; RUN: opt < %s -analyze -basicaa -da  | FileCheck %s
+
+; Test that the dependence analysis generates the correct results when using
+; an aliased object that points to a different element in the same array.
+; PR33567 - https://bugs.llvm.org/show_bug.cgi?id=33567
+
+; void test1(int *A, int *B, int N) {
+;   int *top = A;
+;   int *bot = A + N/2;
+;   for (int i = 0; i < N; i++)
+;     B[i] = top[i] + bot[i];
+; }
+
+; CHECK-LABEL: test1
+; CHECK: da analyze - input [*|<]!
+
+define void @test1(i32* nocapture %A, i32* nocapture %B, i32 %N) #0 {
+entry:
+  %cmp9 = icmp sgt i32 %N, 0
+  br i1 %cmp9, label %for.body.lr.ph, label %for.end
+
+for.body.lr.ph:
+  %div = sdiv i32 %N, 2
+  %bot.gep = getelementptr i32, i32* %A, i32 %div
+  br label %for.body
+
+for.body:
+  %i = phi i32 [ 0, %for.body.lr.ph ], [ %inc, %for.body ]
+  %gep.0 = getelementptr i32, i32* %A, i32 %i
+  %gep.1 = getelementptr i32, i32* %bot.gep, i32 %i
+  %gep.B = getelementptr i32, i32* %B, i32 %i
+  %0 = load i32, i32* %gep.0, align 4
+  %1 = load i32, i32* %gep.1, align 4
+  %add = add nsw i32 %1, %0
+  store i32 %add, i32* %gep.B, align 4
+  %inc = add nsw i32 %i, 1
+  %exitcond = icmp eq i32 %inc, %N
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+  ret void
+}
+
+
+; void test2(int *A, unsigned n) {
+;   int *B = A + 1;
+;   for (unsigned i = 0; i < n; ++i) {
+;     A[i] = B[i];
+;   }
+; }
+
+; CHECK-LABEL: test2
+; CHECK: da analyze - consistent anti [1]!
+
+define void @test2(i32*, i32) #3 {
+  %3 = getelementptr inbounds i32, i32* %0, i64 1
+  br label %4
+
+; <label>:4:
+  %.0 = phi i32 [ 0, %2 ], [ %14, %13 ]
+  %5 = sub i32 %1, 1
+  %6 = icmp ult i32 %.0, %5
+  br i1 %6, label %7, label %15
+
+; <label>:7:
+  %8 = zext i32 %.0 to i64
+  %9 = getelementptr inbounds i32, i32* %3, i64 %8
+  %10 = load i32, i32* %9, align 4
+  %11 = zext i32 %.0 to i64
+  %12 = getelementptr inbounds i32, i32* %0, i64 %11
+  store i32 %10, i32* %12, align 4
+  br label %13
+
+; <label>:13:
+  %14 = add i32 %.0, 1
+  br label %4
+
+; <label>:15:
+  ret void
+}