Optimistically analyse Phi cycles

Analyse Phis under the starting assumption that they are NoAlias. Recursively
look at their inputs.
If they MayAlias/MustAlias there must be an input that makes them so.

Addresses bug 14351.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169788 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 76 insertions, 43 deletions

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 279ff0603f..b0578b9c42 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -1064,48 +1064,20 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
                    Location(V2, V2Size, V2TBAAInfo));
       if (PN > V2)
         std::swap(Locs.first, Locs.second);
-
-      // Find the first incoming phi value not from its parent.
-      unsigned f = 0;
-      while (PN->getIncomingBlock(f) == PN->getParent() &&
-             f < PN->getNumIncomingValues()-1)
-        ++f;
-
-      AliasResult Alias =
-        aliasCheck(PN->getIncomingValue(f), PNSize, PNTBAAInfo,
-                   PN2->getIncomingValueForBlock(PN->getIncomingBlock(f)),
-                   V2Size, V2TBAAInfo);
-      if (Alias == MayAlias)
-        return MayAlias;
-
-      // If the first source of the PHI nodes NoAlias and the other inputs are
-      // the PHI node itself through some amount of recursion this does not add
-      // any new information so just return NoAlias.
-      // bb:
-      //    ptr = ptr2 + 1
-      // loop:
-      //    ptr_phi = phi [bb, ptr], [loop, ptr_plus_one]
-      //    ptr2_phi = phi [bb, ptr2], [loop, ptr2_plus_one]
-      //    ...
-      //    ptr_plus_one = gep ptr_phi, 1
-      //    ptr2_plus_one = gep ptr2_phi, 1
-      // We assume for the recursion that the the phis (ptr_phi, ptr2_phi) do
-      // not alias each other.
-      bool ArePhisAssumedNoAlias = false;
-      AliasResult OrigAliasResult = NoAlias;
-      if (Alias == NoAlias) {
-        // Pretend the phis do not alias.
-        assert(AliasCache.count(Locs) &&
-               "There must exist an entry for the phi node");
-        OrigAliasResult = AliasCache[Locs];
-        AliasCache[Locs] = NoAlias;
-        ArePhisAssumedNoAlias = true;
-      }
+      // Analyse the PHIs' inputs under the assumption that the PHIs are
+      // NoAlias.
+      // If the PHIs are May/MustAlias there must be (recursively) an input
+      // operand from outside the PHIs' cycle that is MayAlias/MustAlias or
+      // there must be an operation on the PHIs within the PHIs' value cycle
+      // that causes a MayAlias.
+      // Pretend the phis do not alias.
+      AliasResult Alias = NoAlias;
+      assert(AliasCache.count(Locs) &&
+             "There must exist an entry for the phi node");
+      AliasResult OrigAliasResult = AliasCache[Locs];
+      AliasCache[Locs] = NoAlias;
 
       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
-        if (i == f)
-          continue;
-
         AliasResult ThisAlias =
           aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
                      PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
@@ -1116,7 +1088,7 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
       }
 
       // Reset if speculation failed.
-      if (ArePhisAssumedNoAlias && Alias != NoAlias)
+      if (Alias != NoAlias)
         AliasCache[Locs] = OrigAliasResult;
 
       return Alias;
diff --git a/test/Analysis/BasicAA/phi-speculation.ll b/test/Analysis/BasicAA/phi-speculation.ll
index 21c6592986..5e1e118d98 100644
--- a/test/Analysis/BasicAA/phi-speculation.ll
+++ b/test/Analysis/BasicAA/phi-speculation.ll
@@ -4,9 +4,9 @@ target datalayout =
 ; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
 
 ; ptr_phi and ptr2_phi do not alias.
+; CHECK: test_noalias_1
 ; CHECK: NoAlias: i32* %ptr2_phi, i32* %ptr_phi
-
-define i32 @test_noalias(i32* %ptr2, i32 %count, i32* %coeff) {
+define i32 @test_noalias_1(i32* %ptr2, i32 %count, i32* %coeff) {
 entry:
   %ptr = getelementptr inbounds i32* %ptr2, i64 1
   br label %while.body
@@ -31,3 +31,64 @@ while.body:
 the_exit:
   ret i32 %add
 }
+
+; CHECK: test_noalias_2
+; CHECK: NoAlias: i32* %ptr_outer_phi, i32* %ptr_outer_phi2
+; CHECK: NoAlias: i32* %ptr2_phi, i32* %ptr_phi
+define i32 @test_noalias_2(i32* %ptr2, i32 %count, i32* %coeff) {
+entry:
+  %ptr = getelementptr inbounds i32* %ptr2, i64 1
+  br label %outer.while.header
+
+outer.while.header:
+  %ptr_outer_phi = phi i32* [%ptr_inc_outer, %outer.while.backedge], [ %ptr, %entry]
+  %ptr_outer_phi2 = phi i32* [%ptr2_inc_outer, %outer.while.backedge], [ %ptr2, %entry]
+  %num.outer = phi i32 [ %count, %entry ], [ %dec.outer, %outer.while.backedge ]
+  br label %while.body
+
+while.body:
+  %num = phi i32 [ %count, %outer.while.header ], [ %dec, %while.body ]
+  %ptr_phi = phi i32* [ %ptr_outer_phi, %outer.while.header ], [ %ptr_inc, %while.body ]
+  %ptr2_phi = phi i32* [ %ptr_outer_phi2, %outer.while.header ], [ %ptr2_inc, %while.body ]
+  %result.09 = phi i32 [ 0 , %outer.while.header ], [ %add, %while.body ]
+  %dec = add nsw i32 %num, -1
+  %0 = load i32* %ptr_phi, align 4
+  store i32 %0, i32* %ptr2_phi, align 4
+  %1 = load i32* %coeff, align 4
+  %2 = load i32* %ptr_phi, align 4
+  %mul = mul nsw i32 %1, %2
+  %add = add nsw i32 %mul, %result.09
+  %tobool = icmp eq i32 %dec, 0
+  %ptr_inc = getelementptr inbounds i32* %ptr_phi, i64 1
+  %ptr2_inc = getelementptr inbounds i32* %ptr2_phi, i64 1
+  br i1 %tobool, label %outer.while.backedge, label %while.body
+
+outer.while.backedge:
+  %ptr_inc_outer = getelementptr inbounds i32* %ptr_phi, i64 1
+  %ptr2_inc_outer = getelementptr inbounds i32* %ptr2_phi, i64 1
+  %dec.outer = add nsw i32 %num.outer, -1
+  %br.cond = icmp eq i32 %dec.outer, 0
+  br i1 %br.cond, label %the_exit, label %outer.while.header
+
+the_exit:
+  ret i32 %add
+}
+
+; CHECK: test_noalias_3
+; CHECK: MayAlias: i8* %ptr2_phi, i8* %ptr_phi
+define i32 @test_noalias_3(i8* noalias %x, i8* noalias %y, i8* noalias %z,
+                           i32 %count) {
+entry:
+  br label %while.body
+
+while.body:
+  %num = phi i32 [ %count, %entry ], [ %dec, %while.body ]
+  %ptr_phi = phi i8* [ %x, %entry ], [ %z, %while.body ]
+  %ptr2_phi = phi i8* [ %y, %entry ], [ %ptr_phi, %while.body ]
+  %dec = add nsw i32 %num, -1
+  %tobool = icmp eq i32 %dec, 0
+  br i1 %tobool, label %the_exit, label %while.body
+
+the_exit:
+  ret i32 1
+}