Improve what can be promoted in LICM.

Summary:
In case of non-alloca pointers, we check for whether it is a pointer
from malloc-like calls and it is not captured. In such case, we can
promote the pointer, as the caller will have no way to access this pointer
even if there is unwinding in middle of the loop.

Reviewers: hfinkel, sanjoy, reames, eli.friedman

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28834

llvm-svn: 292510

1 files changed, 31 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index be576404add..20459aa79c2 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -999,14 +999,34 @@ bool llvm::promoteLoopAccessesToScalars(
 
   const DataLayout &MDL = Preheader->getModule()->getDataLayout();
 
+  // Do we know this object does not escape ?
+  bool IsKnownNonEscapingObject = false;
   if (SafetyInfo->MayThrow) {
     // If a loop can throw, we have to insert a store along each unwind edge.
     // That said, we can't actually make the unwind edge explicit. Therefore,
     // we have to prove that the store is dead along the unwind edge.
     //
-    // Currently, this code just special-cases alloca instructions.
-    if (!isa<AllocaInst>(GetUnderlyingObject(SomePtr, MDL)))
-      return false;
+    // If the underlying object is not an alloca, nor a pointer that does not
+    // escape, then we can not effectively prove that the store is dead along
+    // the unwind edge. i.e. the caller of this function could have ways to
+    // access the pointed object.
+    Value *Object = GetUnderlyingObject(SomePtr, MDL);
+    // If this is a base pointer we do not understand, simply bail.
+    // We only handle alloca and return value from alloc-like fn right now.
+    if (!isa<AllocaInst>(Object)) {
+        if (!isAllocLikeFn(Object, TLI))
+          return false;
+      // If this is an alloc like fn. There are more constraints we need to verify.
+      // More specifically, we must make sure that the pointer can not escape.
+      //
+      // NOTE: PointerMayBeCaptured is not enough as the pointer may have escaped
+      // even though its not captured by the enclosing function. Standard allocation
+      // functions like malloc, calloc, and operator new return values which can
+      // be assumed not to have previously escaped.
+      if (PointerMayBeCaptured(Object, true, true))
+        return false;
+      IsKnownNonEscapingObject = true;
+    }
   }
 
   // Check that all of the pointers in the alias set have the same type.  We
@@ -1109,10 +1129,15 @@ bool llvm::promoteLoopAccessesToScalars(
   // stores along paths which originally didn't have them without violating the
   // memory model.
   if (!SafeToInsertStore) {
-    Value *Object = GetUnderlyingObject(SomePtr, MDL);
-    SafeToInsertStore =
-        (isAllocLikeFn(Object, TLI) || isa<AllocaInst>(Object)) &&
+    // If this is a known non-escaping object, it is safe to insert the stores.
+    if (IsKnownNonEscapingObject)
+      SafeToInsertStore = true;
+    else {
+      Value *Object = GetUnderlyingObject(SomePtr, MDL);
+      SafeToInsertStore =
+        (isAllocLikeFn(Object, TLI) || isa<AllocaInst>(Object)) && 
         !PointerMayBeCaptured(Object, true, true);
+    }
   }
 
   // If we've still failed to prove we can sink the store, give up.