Analyze recursive PHI nodes in BasicAA

Summary:
This patch allows phi nodes like
  %x = phi [ %incptr, ... ] [ %var, ... ]
  %incptr = getelementptr %x, 1
to be analyzed by BasicAliasAnalysis.

In aliasPHI, we can detect incoming values that are recursive GEPs with a
constant offset. Instead of trying to analyze a recursive GEP (and failing), 
we now ignore it and instead set the size of the memory referenced by
the PHINode to UnknownSize. This represents all the possible memory
locations the pointer represented by the PHINode could be advanced to
by the GEP.

For now, this new behavior is turned off by default to allow debugging of
performance degradations seen with SPEC/x86 and Hexagon benchmarks.
The flag -basicaa-recphi turns it on.


Reviewers: hfinkel, sanjoy

Subscribers: tobiasvk_caf, sanjoy, llvm-commits

Differential Revision: http://reviews.llvm.org/D10368

llvm-svn: 242320

1 files changed, 26 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 68f766edb30..06e600e8835 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -42,6 +42,10 @@
 #include <algorithm>
 using namespace llvm;
 
+/// Enable analysis of recursive PHI nodes.
+static cl::opt<bool> EnableRecPhiAnalysis("basicaa-recphi",
+                                          cl::Hidden, cl::init(false));
+
 /// Cutoff after which to stop analysing a set of phi nodes potentially involved
 /// in a cycle. Because we are analysing 'through' phi nodes we need to be
 /// careful with value equivalence. We use reachability to make sure a value
@@ -1297,6 +1301,7 @@ AliasResult BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
 
   SmallPtrSet<Value*, 4> UniqueSrc;
   SmallVector<Value*, 4> V1Srcs;
+  bool isRecursive = false;
   for (Value *PV1 : PN->incoming_values()) {
     if (isa<PHINode>(PV1))
       // If any of the source itself is a PHI, return MayAlias conservatively
@@ -1304,12 +1309,33 @@ AliasResult BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
       // sides are PHI nodes. In which case, this is O(m x n) time where 'm'
       // and 'n' are the number of PHI sources.
       return MayAlias;
+
+    if (EnableRecPhiAnalysis)
+      if (GEPOperator *PV1GEP = dyn_cast<GEPOperator>(PV1)) {
+        // Check whether the incoming value is a GEP that advances the pointer
+        // result of this PHI node (e.g. in a loop). If this is the case, we
+        // would recurse and always get a MayAlias. Handle this case specially
+        // below.
+        if (PV1GEP->getPointerOperand() == PN && PV1GEP->getNumIndices() == 1 &&
+            isa<ConstantInt>(PV1GEP->idx_begin())) {
+          isRecursive = true;
+          continue;
+        }
+      }
+
     if (UniqueSrc.insert(PV1).second)
       V1Srcs.push_back(PV1);
   }
 
+  // If this PHI node is recursive, set the size of the accessed memory to
+  // unknown to represent all the possible values the GEP could advance the
+  // pointer to.
+  if (isRecursive)
+    PNSize = MemoryLocation::UnknownSize;
+
   AliasResult Alias = aliasCheck(V2, V2Size, V2AAInfo,
                                  V1Srcs[0], PNSize, PNAAInfo);
+
   // Early exit if the check of the first PHI source against V2 is MayAlias.
   // Other results are not possible.
   if (Alias == MayAlias)