Simplify pointer comparisons involving memory allocation functions

System memory allocation functions, which are identified at the IR level by the
noalias attribute on the return value, must return a pointer into a memory region
disjoint from any other memory accessible to the caller. We can use this
property to simplify pointer comparisons between allocated memory and local
stack addresses and the addresses of global variables. Neither the stack nor
global variables can overlap with the region used by the memory allocator.

Fixes PR21556.

llvm-svn: 223093

2 files changed, 143 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 3c61cd16cd5..85ebf694e69 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -20,6 +20,7 @@
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/ADT/SetVector.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
 #include "llvm/Analysis/ConstantFolding.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Analysis/ValueTracking.h"
@@ -31,6 +32,7 @@
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/ValueHandle.h"
+#include <algorithm>
 using namespace llvm;
 using namespace llvm::PatternMatch;
 
@@ -2007,6 +2009,39 @@ static Constant *computePointerICmp(const DataLayout *DL,
       return ConstantExpr::getICmp(Pred,
                                    ConstantExpr::getAdd(LHSOffset, LHSNoBound),
                                    ConstantExpr::getAdd(RHSOffset, RHSNoBound));
+
+    // If one side of the equality comparison must come from a noalias call
+    // (meaning a system memory allocation function), and the other side must
+    // come from a pointer that cannot overlap with dynamically-allocated
+    // memory within the lifetime of the current function (allocas, byval
+    // arguments, globals), then determine the comparison result here.
+    SmallVector<Value *, 8> LHSUObjs, RHSUObjs;
+    GetUnderlyingObjects(LHS, LHSUObjs, DL);
+    GetUnderlyingObjects(RHS, RHSUObjs, DL);
+
+    // Is the set of underlying objects all noalias calls?
+    auto IsNAC = [](SmallVectorImpl<Value *> &Objects) {
+      return std::all_of(Objects.begin(), Objects.end(),
+                         [](Value *V){ return isNoAliasCall(V); });
+    };
+
+    // Is the set of underlying objects all things which must be disjoint from
+    // noalias calls.
+    auto IsAllocDisjoint = [](SmallVectorImpl<Value *> &Objects) {
+      return std::all_of(Objects.begin(), Objects.end(),
+                         [](Value *V){
+                           if (isa<AllocaInst>(V) || isa<GlobalValue>(V))
+                             return true;
+                           if (const Argument *A = dyn_cast<Argument>(V))
+                             return A->hasByValAttr();
+                           return false;
+                         });
+    };
+
+    if ((IsNAC(LHSUObjs) && IsAllocDisjoint(RHSUObjs)) ||
+        (IsNAC(RHSUObjs) && IsAllocDisjoint(LHSUObjs)))
+        return ConstantInt::get(GetCompareTy(LHS),
+                                !CmpInst::isTrueWhenEqual(Pred));
   }
 
   // Otherwise, fail.
diff --git a/llvm/test/Transforms/InstSimplify/noalias-ptr.ll b/llvm/test/Transforms/InstSimplify/noalias-ptr.ll
new file mode 100644
index 00000000000..e3435097880
--- /dev/null
+++ b/llvm/test/Transforms/InstSimplify/noalias-ptr.ll
@@ -0,0 +1,108 @@
+; RUN: opt -instsimplify -S < %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@g1 = global i32 0, align 4
+
+; Make sure we can simplify away a pointer comparison between
+; dynamically-allocated memory and a local stack allocation.
+;   void p()
+;   {
+;     int *mData;
+;     int mStackData[10];
+;     mData = new int[12];
+;     if (mData != mStackData) {
+;       delete[] mData;
+;     }
+;   }
+
+define void @_Z2p1v() #0 {
+  %mStackData = alloca [10 x i32], align 16
+  %1 = bitcast [10 x i32]* %mStackData to i8*
+  %2 = tail call noalias i8* @_Znam(i64 48) #4
+  %3 = bitcast i8* %2 to i32*
+  %4 = getelementptr inbounds [10 x i32]* %mStackData, i64 0, i64 0
+  %5 = icmp eq i32* %3, %4
+  br i1 %5, label %7, label %6
+
+; CHECK-LABEL: @_Z2p1v
+; CHECK-NOT: icmp
+; CHECK: ret void
+
+; <label>:6                                       ; preds = %0
+  call void @_ZdaPv(i8* %2) #5
+  br label %7
+
+; <label>:7                                       ; preds = %0, %6
+  ret void
+}
+
+; Also check a more-complicated case with multiple underlying objects.
+
+define void @_Z2p2bb(i1 zeroext %b1, i1 zeroext %b2) #0 {
+  %mStackData = alloca [10 x i32], align 16
+  %1 = bitcast [10 x i32]* %mStackData to i8*
+  %2 = getelementptr inbounds [10 x i32]* %mStackData, i64 0, i64 0
+  %3 = select i1 %b1, i32* %2, i32* @g1
+  %4 = tail call noalias i8* @_Znam(i64 48) #4
+  %5 = tail call noalias i8* @_Znam(i64 48) #4
+  %.v = select i1 %b2, i8* %4, i8* %5
+  %6 = bitcast i8* %.v to i32*
+  %7 = icmp eq i32* %6, %3
+  br i1 %7, label %9, label %8
+
+; CHECK-LABEL: @_Z2p2bb
+; CHECK-NOT: icmp
+; CHECK: ret void
+
+; <label>:8                                       ; preds = %0
+  call void @_ZdaPv(i8* %4) #5
+  call void @_ZdaPv(i8* %5) #5
+  br label %9
+
+; <label>:9                                       ; preds = %0, %8
+  ret void
+}
+
+; Here's another case involving multiple underlying objects, but this time we
+; must keep the comparison (it might involve a regular pointer-typed function
+; argument).
+
+define void @_Z4nopebbPi(i1 zeroext %b1, i1 zeroext %b2, i32* readnone %q) #0 {
+  %mStackData = alloca [10 x i32], align 16
+  %1 = bitcast [10 x i32]* %mStackData to i8*
+  %2 = getelementptr inbounds [10 x i32]* %mStackData, i64 0, i64 0
+  %3 = select i1 %b1, i32* %2, i32* %q
+  %4 = tail call noalias i8* @_Znam(i64 48) #4
+  %5 = tail call noalias i8* @_Znam(i64 48) #4
+  %.v = select i1 %b2, i8* %4, i8* %5
+  %6 = bitcast i8* %.v to i32*
+  %7 = icmp eq i32* %6, %3
+  br i1 %7, label %9, label %8
+
+; CHECK-LABEL: @_Z4nopebbPi
+; CHECK: icmp
+; CHECK: ret void
+
+; <label>:8                                       ; preds = %0
+  call void @_ZdaPv(i8* %4) #5
+  call void @_ZdaPv(i8* %5) #5
+  br label %9
+
+; <label>:9                                       ; preds = %0, %8
+  ret void
+}
+
+; Function Attrs: nobuiltin
+declare noalias i8* @_Znam(i64) #2
+
+; Function Attrs: nobuiltin nounwind
+declare void @_ZdaPv(i8*) #3
+
+attributes #0 = { uwtable }
+attributes #1 = { nounwind }
+attributes #2 = { nobuiltin }
+attributes #3 = { nobuiltin nounwind }
+attributes #4 = { builtin }
+attributes #5 = { builtin nounwind }
+