[GlobalsAA] Loosen an overly conservative bailout

Instead of bailing out when we see loads, analyze them. If we can prove that the loaded-from address must escape, then we can conclude that a load from that address must escape too and therefore cannot alias a non-addr-taken global.

When checking if a Value can alias a non-addr-taken global, if the Value is a LoadInst of a non-global, recurse instead of bailing.

If we can follow a trail of loads up to some base that is captured, we know by inference that all the loads we followed are also captured.

llvm-svn: 251017

Author: James Molloy

llvm/lib/Analysis/GlobalsModRef.cpp

@@ -595,6 +595,66 @@ void GlobalsAAResult::AnalyzeCallGraph(CallGraph &CG, Module &M) {
   }
 }
 
+// GV is a non-escaping global. V is a pointer address that has been loaded from.
+// If we can prove that V must escape, we can conclude that a load from V cannot
+// alias GV.
+static bool isNonEscapingGlobalNoAliasWithLoad(const GlobalValue *GV,
+                                               const Value *V,
+                                               int &Depth,
+                                               const DataLayout &DL) {
+  SmallPtrSet<const Value *, 8> Visited;
+  SmallVector<const Value *, 8> Inputs;
+  Visited.insert(V);
+  Inputs.push_back(V);
+  do {
+    const Value *Input = Inputs.pop_back_val();
+    
+    if (isa<GlobalValue>(Input) || isa<Argument>(Input) || isa<CallInst>(Input) ||
+        isa<InvokeInst>(Input))
+      // Arguments to functions or returns from functions are inherently
+      // escaping, so we can immediately classify those as not aliasing any
+      // non-addr-taken globals.
+      //
+      // (Transitive) loads from a global are also safe - if this aliased
+      // another global, its address would escape, so no alias.
+      continue;
+
+    // Recurse through a limited number of selects, loads and PHIs. This is an
+    // arbitrary depth of 4, lower numbers could be used to fix compile time
+    // issues if needed, but this is generally expected to be only be important
+    // for small depths.
+    if (++Depth > 4)
+      return false;
+
+    if (auto *LI = dyn_cast<LoadInst>(Input)) {
+      Inputs.push_back(GetUnderlyingObject(LI->getPointerOperand(), DL));
+      continue;
+    }  
+    if (auto *SI = dyn_cast<SelectInst>(Input)) {
+      const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
+      const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);
+      if (Visited.insert(LHS).second)
+        Inputs.push_back(LHS);
+      if (Visited.insert(RHS).second)
+        Inputs.push_back(RHS);
+      continue;
+    }
+    if (auto *PN = dyn_cast<PHINode>(Input)) {
+      for (const Value *Op : PN->incoming_values()) {
+        Op = GetUnderlyingObject(Op, DL);
+        if (Visited.insert(Op).second)
+          Inputs.push_back(Op);
+      }
+      continue;
+    }
+    
+    return false;
+  } while (!Inputs.empty());
+
+  // All inputs were known to be no-alias.
+  return true;
+}
+
 // There are particular cases where we can conclude no-alias between
 // a non-addr-taken global and some other underlying object. Specifically,
 // a non-addr-taken global is known to not be escaped from any function. It is
@@ -669,22 +729,24 @@ bool GlobalsAAResult::isNonEscapingGlobalNoAlias(const GlobalValue *GV,
       // non-addr-taken globals.
       continue;
     }
+    
+    // Recurse through a limited number of selects, loads and PHIs. This is an
+    // arbitrary depth of 4, lower numbers could be used to fix compile time
+    // issues if needed, but this is generally expected to be only be important
+    // for small depths.
+    if (++Depth > 4)
+      return false;
+
     if (auto *LI = dyn_cast<LoadInst>(Input)) {
       // A pointer loaded from a global would have been captured, and we know
       // that the global is non-escaping, so no alias.
-      if (isa<GlobalValue>(GetUnderlyingObject(LI->getPointerOperand(), DL)))
+      const Value *Ptr = GetUnderlyingObject(LI->getPointerOperand(), DL);
+      if (isNonEscapingGlobalNoAliasWithLoad(GV, Ptr, Depth, DL))
+        // The load does not alias with GV.
         continue;
-
       // Otherwise, a load could come from anywhere, so bail.
       return false;
     }
-
-    // Recurse through a limited number of selects and PHIs. This is an
-    // arbitrary depth of 4, lower numbers could be used to fix compile time
-    // issues if needed, but this is generally expected to be only be important
-    // for small depths.
-    if (++Depth > 4)
-      return false;
     if (auto *SI = dyn_cast<SelectInst>(Input)) {
       const Value *LHS = GetUnderlyingObject(SI->getTrueValue(), DL);
       const Value *RHS = GetUnderlyingObject(SI->getFalseValue(), DL);

llvm/test/Analysis/GlobalsModRef/nonescaping-noalias.ll

@@ -97,3 +97,20 @@ exit:
   %v = load i32, i32* @g1
   ret i32 %v
 }
+
+define i32 @test5(i32** %param) {
+; Ensure that we can fold a store to a load of a global across a store to
+; a parameter that has been dereferenced when the global is non-escaping.
+;
+; CHECK-LABEL: @test5(
+; CHECK: %p = load i32*
+; CHECK: store i32 42, i32* @g1
+; CHECK-NOT: load i32
+; CHECK: ret i32 42
+entry:
+  %p = load i32*, i32** %param
+  store i32 42, i32* @g1
+  store i32 7, i32* %p
+  %v = load i32, i32* @g1
+  ret i32 %v
+}