Add, and infer, a nofree function attribute

This patch adds a function attribute, nofree, to indicate that a function does
not, directly or indirectly, call a memory-deallocation function (e.g., free,
C++'s operator delete).

Reviewers: jdoerfert

Differential Revision: https://reviews.llvm.org/D49165

llvm-svn: 365336

1 files changed, 55 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
index 9e95e796da6..a3fb7d1d57a 100644
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -27,6 +27,7 @@
 #include "llvm/Analysis/CallGraphSCCPass.h"
 #include "llvm/Analysis/CaptureTracking.h"
 #include "llvm/Analysis/LazyCallGraph.h"
+#include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/MemoryLocation.h"
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/Argument.h"
@@ -75,6 +76,7 @@ STATISTIC(NumNoAlias, "Number of function returns marked noalias");
 STATISTIC(NumNonNullReturn, "Number of function returns marked nonnull");
 STATISTIC(NumNoRecurse, "Number of functions marked as norecurse");
 STATISTIC(NumNoUnwind, "Number of functions marked as nounwind");
+STATISTIC(NumNoFree, "Number of functions marked as nofree");
 
 // FIXME: This is disabled by default to avoid exposing security vulnerabilities
 // in C/C++ code compiled by clang:
@@ -88,6 +90,10 @@ static cl::opt<bool> DisableNoUnwindInference(
     "disable-nounwind-inference", cl::Hidden,
     cl::desc("Stop inferring nounwind attribute during function-attrs pass"));
 
+static cl::opt<bool> DisableNoFreeInference(
+    "disable-nofree-inference", cl::Hidden,
+    cl::desc("Stop inferring nofree attribute during function-attrs pass"));
+
 namespace {
 
 using SCCNodeSet = SmallSetVector<Function *, 8>;
@@ -1227,6 +1233,25 @@ static bool InstrBreaksNonThrowing(Instruction &I, const SCCNodeSet &SCCNodes) {
   return true;
 }
 
+/// Helper for NoFree inference predicate InstrBreaksAttribute.
+static bool InstrBreaksNoFree(Instruction &I, const SCCNodeSet &SCCNodes) {
+  CallSite CS(&I);
+  if (!CS)
+    return false;
+
+  Function *Callee = CS.getCalledFunction();
+  if (!Callee)
+    return true;
+
+  if (Callee->doesNotFreeMemory())
+    return false;
+
+  if (SCCNodes.count(Callee) > 0)
+    return false;
+
+  return true;
+}
+
 /// Infer attributes from all functions in the SCC by scanning every
 /// instruction for compliance to the attribute assumptions. Currently it
 /// does:
@@ -1280,6 +1305,29 @@ static bool inferAttrsFromFunctionBodies(const SCCNodeSet &SCCNodes) {
         },
         /* RequiresExactDefinition= */ true});
 
+  if (!DisableNoFreeInference)
+    // Request to infer nofree attribute for all the functions in the SCC if
+    // every callsite within the SCC does not directly or indirectly free
+    // memory (except for calls to functions within the SCC). Note that nofree
+    // attribute suffers from derefinement - results may change depending on
+    // how functions are optimized. Thus it can be inferred only from exact
+    // definitions.
+    AI.registerAttrInference(AttributeInferer::InferenceDescriptor{
+        Attribute::NoFree,
+        // Skip functions known not to free memory.
+        [](const Function &F) { return F.doesNotFreeMemory(); },
+        // Instructions that break non-deallocating assumption.
+        [SCCNodes](Instruction &I) {
+          return InstrBreaksNoFree(I, SCCNodes);
+        },
+        [](Function &F) {
+          LLVM_DEBUG(dbgs()
+                     << "Adding nofree attr to fn " << F.getName() << "\n");
+          F.setDoesNotFreeMemory();
+          ++NumNoFree;
+        },
+        /* RequiresExactDefinition= */ true});
+
   // Perform all the requested attribute inference actions.
   return AI.run(SCCNodes);
 }
@@ -1322,7 +1370,8 @@ static bool addNoRecurseAttrs(const SCCNodeSet &SCCNodes) {
 }
 
 template <typename AARGetterT>
-static bool deriveAttrsInPostOrder(SCCNodeSet &SCCNodes, AARGetterT &&AARGetter,
+static bool deriveAttrsInPostOrder(SCCNodeSet &SCCNodes,
+                                   AARGetterT &&AARGetter,
                                    bool HasUnknownCall) {
   bool Changed = false;
 
@@ -1353,6 +1402,11 @@ PreservedAnalyses PostOrderFunctionAttrsPass::run(LazyCallGraph::SCC &C,
   FunctionAnalysisManager &FAM =
       AM.getResult<FunctionAnalysisManagerCGSCCProxy>(C, CG).getManager();
 
+  const ModuleAnalysisManager &MAM =
+      AM.getResult<ModuleAnalysisManagerCGSCCProxy>(C, CG).getManager();
+  assert(C.size() > 0 && "Cannot handle an empty SCC!");
+  Module &M = *C.begin()->getFunction().getParent();
+
   // We pass a lambda into functions to wire them up to the analysis manager
   // for getting function analyses.
   auto AARGetter = [&](Function &F) -> AAResults & {