InstSimplify: Simplify trivial pointer expressions like b + (e - b)

consider:
long long *f(long long *b, long long *e) {
  return b + (e - b);
}

we would lower this to something like:
define i64* @f(i64* %b, i64* %e) {
  %1 = ptrtoint i64* %e to i64
  %2 = ptrtoint i64* %b to i64
  %3 = sub i64 %1, %2
  %4 = ashr exact i64 %3, 3
  %5 = getelementptr inbounds i64* %b, i64 %4
  ret i64* %5
}

This should fold away to just 'e'.

N.B.  This adds m_SpecificInt as a convenient way to match against a
particular 64-bit integer when using LLVM's match interface.

llvm-svn: 216439

1 files changed, 36 insertions, 5 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 459fc92bce1..c9c9eede435 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -2783,6 +2783,7 @@ Value *llvm::SimplifySelectInst(Value *Cond, Value *TrueVal, Value *FalseVal,
 static Value *SimplifyGEPInst(ArrayRef<Value *> Ops, const Query &Q, unsigned) {
   // The type of the GEP pointer operand.
   PointerType *PtrTy = cast<PointerType>(Ops[0]->getType()->getScalarType());
+  unsigned AS = PtrTy->getAddressSpace();
 
   // getelementptr P -> P.
   if (Ops.size() == 1)
@@ -2791,7 +2792,7 @@ static Value *SimplifyGEPInst(ArrayRef<Value *> Ops, const Query &Q, unsigned) {
   if (isa<UndefValue>(Ops[0])) {
     // Compute the (pointer) type returned by the GEP instruction.
     Type *LastType = GetElementPtrInst::getIndexedType(PtrTy, Ops.slice(1));
-    Type *GEPTy = PointerType::get(LastType, PtrTy->getAddressSpace());
+    Type *GEPTy = PointerType::get(LastType, AS);
     if (VectorType *VT = dyn_cast<VectorType>(Ops[0]->getType()))
       GEPTy = VectorType::get(GEPTy, VT->getNumElements());
     return UndefValue::get(GEPTy);
@@ -2801,11 +2802,41 @@ static Value *SimplifyGEPInst(ArrayRef<Value *> Ops, const Query &Q, unsigned) {
     // getelementptr P, 0 -> P.
     if (match(Ops[1], m_Zero()))
       return Ops[0];
-    // getelementptr P, N -> P if P points to a type of zero size.
-    if (Q.DL) {
-      Type *Ty = PtrTy->getElementType();
-      if (Ty->isSized() && Q.DL->getTypeAllocSize(Ty) == 0)
+
+    Type *Ty = PtrTy->getElementType();
+    if (Q.DL && Ty->isSized()) {
+      Value *P;
+      uint64_t C;
+      uint64_t TyAllocSize = Q.DL->getTypeAllocSize(Ty);
+      // getelementptr P, N -> P if P points to a type of zero size.
+      if (TyAllocSize == 0)
         return Ops[0];
+
+      // The following transforms are only safe if the ptrtoint cast
+      // doesn't truncate the pointers.
+      if (Ops[1]->getType()->getScalarSizeInBits() ==
+          Q.DL->getPointerSizeInBits(AS)) {
+        // getelementptr P, (sub Q, P) -> Q if P points to a type of size 1.
+        if (TyAllocSize == 1 &&
+            match(Ops[1], m_Sub(m_PtrToInt(m_Value(P)),
+                                m_PtrToInt(m_Specific(Ops[0])))))
+          return P;
+
+        // getelementptr P, (ashr (sub Q, P), C) -> Q
+        // if P points to a type of size 1 << C.
+        if (match(Ops[1], m_AShr(m_Sub(m_PtrToInt(m_Value(P)),
+                                       m_PtrToInt(m_Specific(Ops[0]))),
+                                 m_ConstantInt(C))) &&
+            TyAllocSize == 1ULL << C)
+          return P;
+
+        // getelementptr P, (sdiv (sub Q, P), C) -> Q
+        // if P points to a type of size C.
+        if (match(Ops[1], m_SDiv(m_Sub(m_PtrToInt(m_Value(P)),
+                                       m_PtrToInt(m_Specific(Ops[0]))),
+                                 m_SpecificInt(TyAllocSize))))
+          return P;
+      }
     }
   }