From 965fefa1add8151619cc340c373a812c94342e58 Mon Sep 17 00:00:00 2001
From: Dan Gohman <gohman@apple.com>
Date: Fri, 3 Jun 2011 20:17:36 +0000
Subject: [PATCH] When merging MustAlias and PartialAlias, chose PartialAlias
 instead of conservatively choosing MayAlias.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132579 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Analysis/BasicAliasAnalysis.cpp       | 31 ++++++++++++------
 test/Analysis/BasicAA/must-and-partial.ll | 39 +++++++++++++++++++++++
 2 files changed, 60 insertions(+), 10 deletions(-)
 create mode 100644 test/Analysis/BasicAA/must-and-partial.ll

diff --git a/lib/Analysis/BasicAliasAnalysis.cpp b/lib/Analysis/BasicAliasAnalysis.cpp
index 03785fd0ebe..c292999262b 100644
--- a/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/lib/Analysis/BasicAliasAnalysis.cpp
@@ -949,6 +949,19 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
   return MayAlias;
 }
 
+static AliasAnalysis::AliasResult
+MergeAliasResults(AliasAnalysis::AliasResult A, AliasAnalysis::AliasResult B) {
+  // If the results agree, take it.
+  if (A == B)
+    return A;
+  // A mix of PartialAlias and MustAlias is PartialAlias.
+  if ((A == AliasAnalysis::PartialAlias && B == AliasAnalysis::MustAlias) ||
+      (B == AliasAnalysis::PartialAlias && A == AliasAnalysis::MustAlias))
+    return AliasAnalysis::PartialAlias;
+  // Otherwise, we don't know anything.
+  return AliasAnalysis::MayAlias;
+}
+
 /// aliasSelect - Provide a bunch of ad-hoc rules to disambiguate a Select
 /// instruction against another.
 AliasAnalysis::AliasResult
@@ -975,9 +988,7 @@ BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
       AliasResult ThisAlias =
         aliasCheck(SI->getFalseValue(), SISize, SITBAAInfo,
                    SI2->getFalseValue(), V2Size, V2TBAAInfo);
-      if (ThisAlias != Alias)
-        return MayAlias;
-      return Alias;
+      return MergeAliasResults(ThisAlias, Alias);
     }
 
   // If both arms of the Select node NoAlias or MustAlias V2, then returns
@@ -994,9 +1005,7 @@ BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
 
   AliasResult ThisAlias =
     aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
-  if (ThisAlias != Alias)
-    return MayAlias;
-  return Alias;
+  return MergeAliasResults(ThisAlias, Alias);
 }
 
 // aliasPHI - Provide a bunch of ad-hoc rules to disambiguate a PHI instruction
@@ -1026,8 +1035,9 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
           aliasCheck(PN->getIncomingValue(i), PNSize, PNTBAAInfo,
                      PN2->getIncomingValueForBlock(PN->getIncomingBlock(i)),
                      V2Size, V2TBAAInfo);
-        if (ThisAlias != Alias)
-          return MayAlias;
+        Alias = MergeAliasResults(ThisAlias, Alias);
+        if (Alias == MayAlias)
+          break;
       }
       return Alias;
     }
@@ -1065,8 +1075,9 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
 
     AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
                                        V, PNSize, PNTBAAInfo);
-    if (ThisAlias != Alias || ThisAlias == MayAlias)
-      return MayAlias;
+    Alias = MergeAliasResults(ThisAlias, Alias);
+    if (Alias == MayAlias)
+      break;
   }
 
   return Alias;
diff --git a/test/Analysis/BasicAA/must-and-partial.ll b/test/Analysis/BasicAA/must-and-partial.ll
new file mode 100644
index 00000000000..93b6184f4f4
--- /dev/null
+++ b/test/Analysis/BasicAA/must-and-partial.ll
@@ -0,0 +1,39 @@
+; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info |& FileCheck %s
+
+; When merging MustAlias and PartialAlias, merge to PartialAlias
+; instead of MayAlias.
+
+
+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"
+
+; CHECK: PartialAlias:  i16* %bigbase0, i8* %phi
+define i8 @test0(i8* %base, i1 %x) {
+entry:
+  %baseplusone = getelementptr i8* %base, i64 1
+  br i1 %x, label %red, label %green
+red:
+  br label %green
+green:
+  %phi = phi i8* [ %baseplusone, %red ], [ %base, %entry ]
+  store i8 0, i8* %phi
+
+  %bigbase0 = bitcast i8* %base to i16*
+  store i16 -1, i16* %bigbase0
+
+  %loaded = load i8* %phi
+  ret i8 %loaded
+}
+
+; CHECK: PartialAlias:  i16* %bigbase1, i8* %sel
+define i8 @test1(i8* %base, i1 %x) {
+entry:
+  %baseplusone = getelementptr i8* %base, i64 1
+  %sel = select i1 %x, i8* %baseplusone, i8* %base
+  store i8 0, i8* %sel
+
+  %bigbase1 = bitcast i8* %base to i16*
+  store i16 -1, i16* %bigbase1
+
+  %loaded = load i8* %sel
+  ret i8 %loaded
+}