Fix BasicAA's recursion detection so that it doesn't pessimize

queries in the case of a DAG, where a query reaches a node
visited earlier, but it's not on a cycle. This avoids
MayAlias results in cases where BasicAA is expected to
return MustAlias or PartialAlias in order to protect TBAA.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132609 91177308-0d34-0410-b5e6-96231b3b80d8

test/Analysis/BasicAA/dag.ll

@@ -0,0 +1,41 @@
+; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info |& FileCheck %s
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
+
+; BasicAA's guard against use-def cycles shouldn't prevent it from
+; analyzing use-def dags.
+
+; CHECK: MustAlias:  i8* %base, i8* %phi
+; CHECK: MustAlias: i8* %phi, i8* %wwa
+; CHECK: MustAlias: i8* %phi, i8* %wwb
+; CHECK: MustAlias: i16* %bigbase, i8* %phi
+define i8 @foo(i8* %base, i1 %x, i1 %w) {
+entry:
+  br i1 %w, label %wa, label %wb
+wa:
+  %wwa = bitcast i8* %base to i8*
+  br label %wc
+wb:
+  %wwb = bitcast i8* %base to i8*
+  br label %wc
+wc:
+  %first = phi i8* [ %wwa, %wa ], [ %wwb, %wb ]
+  %fc = bitcast i8* %first to i8*
+  br i1 %x, label %xa, label %xb
+xa:
+  %xxa = bitcast i8* %fc to i8*
+  br label %xc
+xb:
+  %xxb = bitcast i8* %fc to i8*
+  br label %xc
+xc:
+  %phi = phi i8* [ %xxa, %xa ], [ %xxb, %xb ]
+
+  store i8 0, i8* %phi
+
+  %bigbase = bitcast i8* %base to i16*
+  store i16 -1, i16* %bigbase
+
+  %loaded = load i8* %phi
+  ret i8 %loaded
+}