[msan] Implement exact shadow propagation for relational ICmp.

Only for integers, pointers, and vectors of those. No floats.
Instrumentation seems very heavy, and may need to be replaced
with some approximation in the future.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173452 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Instrumentation/MemorySanitizer.cpp

@@ -127,6 +127,10 @@ static cl::opt<bool> ClHandleICmp("msan-handle-icmp",
        cl::desc("propagate shadow through ICmpEQ and ICmpNE"),
        cl::Hidden, cl::init(true));
 
+static cl::opt<bool> ClHandleICmpExact("msan-handle-icmp-exact",
+       cl::desc("exact handling of relational integer ICmp"),
+       cl::Hidden, cl::init(true));
+
 static cl::opt<bool> ClStoreCleanOrigin("msan-store-clean-origin",
        cl::desc("store origin for clean (fully initialized) values"),
        cl::Hidden, cl::init(false));
@@ -1155,6 +1159,70 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
     setOriginForNaryOp(I);
   }
 
+  /// \brief Build the lowest possible value of V, taking into account V's
+  ///        uninitialized bits.
+  Value *getLowestPossibleValue(IRBuilder<> &IRB, Value *A, Value *Sa,
+                                bool isSigned) {
+    if (isSigned) {
+      // Split shadow into sign bit and other bits.
+      Value *SaOtherBits = IRB.CreateLShr(IRB.CreateShl(Sa, 1), 1);
+      Value *SaSignBit = IRB.CreateXor(Sa, SaOtherBits);
+      // Maximise the undefined shadow bit, minimize other undefined bits.
+      return
+        IRB.CreateOr(IRB.CreateAnd(A, IRB.CreateNot(SaOtherBits)), SaSignBit);
+    } else {
+      // Minimize undefined bits.
+      return IRB.CreateAnd(A, IRB.CreateNot(Sa));
+    }
+  }
+
+  /// \brief Build the highest possible value of V, taking into account V's
+  ///        uninitialized bits.
+  Value *getHighestPossibleValue(IRBuilder<> &IRB, Value *A, Value *Sa,
+                                bool isSigned) {
+    if (isSigned) {
+      // Split shadow into sign bit and other bits.
+      Value *SaOtherBits = IRB.CreateLShr(IRB.CreateShl(Sa, 1), 1);
+      Value *SaSignBit = IRB.CreateXor(Sa, SaOtherBits);
+      // Minimise the undefined shadow bit, maximise other undefined bits.
+      return
+        IRB.CreateOr(IRB.CreateAnd(A, IRB.CreateNot(SaSignBit)), SaOtherBits);
+    } else {
+      // Maximize undefined bits.
+      return IRB.CreateOr(A, Sa);
+    }
+  }
+
+  /// \brief Instrument relational comparisons.
+  ///
+  /// This function does exact shadow propagation for all relational
+  /// comparisons of integers, pointers and vectors of those.
+  /// FIXME: output seems suboptimal when one of the operands is a constant
+  void handleRelationalComparisonExact(ICmpInst &I) {
+    IRBuilder<> IRB(&I);
+    Value *A = I.getOperand(0);
+    Value *B = I.getOperand(1);
+    Value *Sa = getShadow(A);
+    Value *Sb = getShadow(B);
+
+    // Get rid of pointers and vectors of pointers.
+    // For ints (and vectors of ints), types of A and Sa match,
+    // and this is a no-op.
+    A = IRB.CreatePointerCast(A, Sa->getType());
+    B = IRB.CreatePointerCast(B, Sb->getType());
+
+    bool IsSigned = I.isSigned();
+    Value *S1 = IRB.CreateICmp(I.getPredicate(),
+                               getLowestPossibleValue(IRB, A, Sa, IsSigned),
+                               getHighestPossibleValue(IRB, B, Sb, IsSigned));
+    Value *S2 = IRB.CreateICmp(I.getPredicate(),
+                               getHighestPossibleValue(IRB, A, Sa, IsSigned),
+                               getLowestPossibleValue(IRB, B, Sb, IsSigned));
+    Value *Si = IRB.CreateXor(S1, S2);
+    setShadow(&I, Si);
+    setOriginForNaryOp(I);
+  }
+
   /// \brief Instrument signed relational comparisons.
   ///
   /// Handle (x<0) and (x>=0) comparisons (essentially, sign bit tests) by
@@ -1186,6 +1254,8 @@ struct MemorySanitizerVisitor : public InstVisitor<MemorySanitizerVisitor> {
   void visitICmpInst(ICmpInst &I) {
     if (ClHandleICmp && I.isEquality())
       handleEqualityComparison(I);
+    else if (ClHandleICmp && ClHandleICmpExact && I.isRelational())
+      handleRelationalComparisonExact(I);
     else if (ClHandleICmp && I.isSigned() && I.isRelational())
       handleSignedRelationalComparison(I);
     else