Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -894,6 +894,26 @@
       Offset = EmitGEPOffset(GEPLHS);
     return new ICmpInst(ICmpInst::getSignedPredicate(Cond), Offset,
                         Constant::getNullValue(Offset->getType()));
+
+
+  } else if (GEPLHS->isInBounds() && ICmpInst::isEquality(Cond) &&
+             GEPLHS->getType()->isPointerTy() && // TODO: extend to vector geps
+             isa<Constant>(RHS) && cast<Constant>(RHS)->isNullValue() &&
+             !NullPointerIsDefined(I.getFunction(),
+                                   RHS->getType()->getPointerAddressSpace())) {
+    // For most address spaces, an allocation can't be placed at null, but null
+    // itself is treated as a 0 size allocation in the in bounds rules.  Thus,
+    // the only valid inbounds address derived from null, is null itself.
+    // Thus, we have three cases to consider:
+    // 1) Base == nullptr, Index == 0 -> inbounds, null
+    // 2) Base == nullptr, Index != 0 -> poison as the result is out of bounds
+    // 3) Base != nullptr, Index == (-base) -> poison (crossing allocations)
+    // 4) Base != nullptr, Index != (-base) -> nonnull (and possibly poison)
+    //
+    // In general, we're allowed to make values less poison (i.e. remove
+    //   sources of full UB), so in this case, we just select between the two
+    //   non-poison cases (1 and 4 above).
+    return new ICmpInst(Cond, GEPLHS->getPointerOperand(), RHS);
   } else if (GEPOperator *GEPRHS = dyn_cast<GEPOperator>(RHS)) {
     // If the base pointers are different, but the indices are the same, just
     // compare the base pointer.
Index: test/Transforms/InstCombine/gep-inbounds-null.ll
===================================================================
--- test/Transforms/InstCombine/gep-inbounds-null.ll
+++ test/Transforms/InstCombine/gep-inbounds-null.ll
@@ -0,0 +1,156 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S < %s -instcombine | FileCheck %s
+
+;; Start by showing the results of constant folding (which doesn't use
+;; the poison implied by gep for the nonnull cases).
+
+define i1 @test_ne_constants_null() {
+; CHECK-LABEL: @test_ne_constants_null(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* null, i64 0
+  %cnd = icmp ne i8* %gep, null
+  ret i1 %cnd
+}
+
+define i1 @test_ne_constants_nonnull() {
+; CHECK-LABEL: @test_ne_constants_nonnull(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 true
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* null, i64 1
+  %cnd = icmp ne i8* %gep, null
+  ret i1 %cnd
+}
+
+define i1 @test_eq_constants_null() {
+; CHECK-LABEL: @test_eq_constants_null(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* null, i64 0
+  %cnd = icmp ne i8* %gep, null
+  ret i1 %cnd
+}
+
+define i1 @test_eq_constants_nonnull() {
+; CHECK-LABEL: @test_eq_constants_nonnull(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* null, i64 1
+  %cnd = icmp eq i8* %gep, null
+  ret i1 %cnd
+}
+
+;; Then show the results for non-constants.  These use the inbounds provided
+;; UB fact to ignore the possible overflow cases.
+
+define i1 @test_ne(i8* %base, i64 %idx) {
+; CHECK-LABEL: @test_ne(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CND:%.*]] = icmp ne i8* [[BASE:%.*]], null
+; CHECK-NEXT:    ret i1 [[CND]]
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* %base, i64 %idx
+  %cnd = icmp ne i8* %gep, null
+  ret i1 %cnd
+}
+
+define i1 @test_eq(i8* %base, i64 %idx) {
+; CHECK-LABEL: @test_eq(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CND:%.*]] = icmp eq i8* [[BASE:%.*]], null
+; CHECK-NEXT:    ret i1 [[CND]]
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* %base, i64 %idx
+  %cnd = icmp eq i8* %gep, null
+  ret i1 %cnd
+}
+
+;; TODO: vectors not yet handled
+define <2 x i1> @test_vector_base(<2 x i8*> %base, i64 %idx) {
+; CHECK-LABEL: @test_vector_base(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, <2 x i8*> [[BASE:%.*]], i64 [[IDX:%.*]]
+; CHECK-NEXT:    [[CND:%.*]] = icmp eq <2 x i8*> [[GEP]], zeroinitializer
+; CHECK-NEXT:    ret <2 x i1> [[CND]]
+;
+entry:
+  %gep = getelementptr inbounds i8, <2 x i8*> %base, i64 %idx
+  %cnd = icmp eq <2 x i8*> %gep, zeroinitializer
+  ret <2 x i1> %cnd
+}
+
+define <2 x i1> @test_vector_index(i8* %base, <2 x i64> %idx) {
+; CHECK-LABEL: @test_vector_index(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, i8* [[BASE:%.*]], <2 x i64> [[IDX:%.*]]
+; CHECK-NEXT:    [[CND:%.*]] = icmp eq <2 x i8*> [[GEP]], zeroinitializer
+; CHECK-NEXT:    ret <2 x i1> [[CND]]
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* %base, <2 x i64> %idx
+  %cnd = icmp eq <2 x i8*> %gep, zeroinitializer
+  ret <2 x i1> %cnd
+}
+
+;; These two show instsimplify's reasoning getting to the non-zero offsets
+;; before instcombine does.
+
+define i1 @test_eq_pos_idx(i8* %base) {
+; CHECK-LABEL: @test_eq_pos_idx(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* %base, i64 1
+  %cnd = icmp eq i8* %gep, null
+  ret i1 %cnd
+}
+
+define i1 @test_eq_neg_idx(i8* %base) {
+; CHECK-LABEL: @test_eq_neg_idx(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret i1 false
+;
+entry:
+  %gep = getelementptr inbounds i8, i8* %base, i64 -1
+  %cnd = icmp eq i8* %gep, null
+  ret i1 %cnd
+}
+
+;; Finally, some negative tests for sanity checking.
+
+define i1 @neq_noinbounds(i8* %base, i64 %idx) {
+; CHECK-LABEL: @neq_noinbounds(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, i8* [[BASE:%.*]], i64 [[IDX:%.*]]
+; CHECK-NEXT:    [[CND:%.*]] = icmp ne i8* [[GEP]], null
+; CHECK-NEXT:    ret i1 [[CND]]
+;
+entry:
+  %gep = getelementptr i8, i8* %base, i64 %idx
+  %cnd = icmp ne i8* %gep, null
+  ret i1 %cnd
+}
+
+define i1 @neg_objectatnull(i8 addrspace(2)* %base, i64 %idx) {
+; CHECK-LABEL: @neg_objectatnull(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i8, i8 addrspace(2)* [[BASE:%.*]], i64 [[IDX:%.*]]
+; CHECK-NEXT:    [[CND:%.*]] = icmp eq i8 addrspace(2)* [[GEP]], null
+; CHECK-NEXT:    ret i1 [[CND]]
+;
+entry:
+  %gep = getelementptr inbounds i8, i8 addrspace(2)* %base, i64 %idx
+  %cnd = icmp eq i8 addrspace(2)* %gep, null
+  ret i1 %cnd
+}