diff --git a/llvm/include/llvm/Analysis/MemoryBuiltins.h b/llvm/include/llvm/Analysis/MemoryBuiltins.h
--- a/llvm/include/llvm/Analysis/MemoryBuiltins.h
+++ b/llvm/include/llvm/Analysis/MemoryBuiltins.h
@@ -134,17 +134,21 @@
 struct ObjectSizeOpts {
   /// Controls how we handle conditional statements with unknown conditions.
   enum class Mode : uint8_t {
-    /// Fail to evaluate an unknown condition.
-    Exact,
+    /// All branches must be known and have the same size, starting from the
+    /// offset, to be merged.
+    ExactSizeFromOffset,
+    /// All branches must be known and have the same underlying size and offset
+    /// to be merged.
+    ExactUnderlyingSizeAndOffset,
     /// Evaluate all branches of an unknown condition. If all evaluations
     /// succeed, pick the minimum size.
     Min,
     /// Same as Min, except we pick the maximum size of all of the branches.
-    Max
+    Max,
   };
 
   /// How we want to evaluate this object's size.
-  Mode EvalMode = Mode::Exact;
+  Mode EvalMode = Mode::ExactSizeFromOffset;
   /// Whether to round the result up to the alignment of allocas, byval
   /// arguments, and global variables.
   bool RoundToAlign = false;
diff --git a/llvm/lib/Analysis/MemoryBuiltins.cpp b/llvm/lib/Analysis/MemoryBuiltins.cpp
--- a/llvm/lib/Analysis/MemoryBuiltins.cpp
+++ b/llvm/lib/Analysis/MemoryBuiltins.cpp
@@ -641,7 +641,7 @@
     EvalOptions.EvalMode =
         MaxVal ? ObjectSizeOpts::Mode::Max : ObjectSizeOpts::Mode::Min;
   else
-    EvalOptions.EvalMode = ObjectSizeOpts::Mode::Exact;
+    EvalOptions.EvalMode = ObjectSizeOpts::Mode::ExactSizeFromOffset;
 
   EvalOptions.NullIsUnknownSize =
       cast<ConstantInt>(ObjectSize->getArgOperand(2))->isOne();
@@ -999,9 +999,11 @@
     return (getSizeWithOverflow(LHS).slt(getSizeWithOverflow(RHS))) ? LHS : RHS;
   case ObjectSizeOpts::Mode::Max:
     return (getSizeWithOverflow(LHS).sgt(getSizeWithOverflow(RHS))) ? LHS : RHS;
-  case ObjectSizeOpts::Mode::Exact:
+  case ObjectSizeOpts::Mode::ExactSizeFromOffset:
     return (getSizeWithOverflow(LHS).eq(getSizeWithOverflow(RHS))) ? LHS
                                                                    : unknown();
+  case ObjectSizeOpts::Mode::ExactUnderlyingSizeAndOffset:
+    return LHS == RHS && LHS.second.eq(RHS.second) ? LHS : unknown();
   }
   llvm_unreachable("missing an eval mode");
 }
diff --git a/llvm/lib/Transforms/Instrumentation/BoundsChecking.cpp b/llvm/lib/Transforms/Instrumentation/BoundsChecking.cpp
--- a/llvm/lib/Transforms/Instrumentation/BoundsChecking.cpp
+++ b/llvm/lib/Transforms/Instrumentation/BoundsChecking.cpp
@@ -146,6 +146,7 @@
   const DataLayout &DL = F.getParent()->getDataLayout();
   ObjectSizeOpts EvalOpts;
   EvalOpts.RoundToAlign = true;
+  EvalOpts.EvalMode = ObjectSizeOpts::Mode::ExactUnderlyingSizeAndOffset;
   ObjectSizeOffsetEvaluator ObjSizeEval(DL, &TLI, F.getContext(), EvalOpts);
 
   // check HANDLE_MEMORY_INST in include/llvm/Instruction.def for memory
diff --git a/llvm/test/Instrumentation/BoundsChecking/simple.ll b/llvm/test/Instrumentation/BoundsChecking/simple.ll
--- a/llvm/test/Instrumentation/BoundsChecking/simple.ll
+++ b/llvm/test/Instrumentation/BoundsChecking/simple.ll
@@ -330,7 +330,6 @@
 }
 
 ; Check that merging sizes in a phi works.
-; FIXME: bounds-checking thinks that %alloc has an underlying size of 0.
 define i8 @f14(i1 %i) {
 ; CHECK-LABEL: @f14(
 ; CHECK-NEXT:  entry:
@@ -340,16 +339,18 @@
 ; CHECK-NEXT:    [[G:%.*]] = getelementptr i8, ptr [[A]], i32 32
 ; CHECK-NEXT:    br label [[BB2]]
 ; CHECK:       bb2:
-; CHECK-NEXT:    [[ALLOC:%.*]] = phi ptr [ null, [[ENTRY:%.*]] ], [ [[G]], [[BB1]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ 32, [[BB1]] ]
+; CHECK-NEXT:    [[TMP1:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ 32, [[BB1]] ]
+; CHECK-NEXT:    [[ALLOC:%.*]] = phi ptr [ null, [[ENTRY]] ], [ [[G]], [[BB1]] ]
 ; CHECK-NEXT:    [[IND:%.*]] = phi i64 [ 0, [[ENTRY]] ], [ -4, [[BB1]] ]
-; CHECK-NEXT:    [[TMP0:%.*]] = add i64 0, [[IND]]
+; CHECK-NEXT:    [[TMP2:%.*]] = add i64 [[TMP1]], [[IND]]
 ; CHECK-NEXT:    [[P:%.*]] = getelementptr i8, ptr [[ALLOC]], i64 [[IND]]
-; CHECK-NEXT:    [[TMP1:%.*]] = sub i64 0, [[TMP0]]
-; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i64 0, [[TMP0]]
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp ult i64 [[TMP1]], 1
-; CHECK-NEXT:    [[TMP4:%.*]] = or i1 [[TMP2]], [[TMP3]]
-; CHECK-NEXT:    br i1 [[TMP4]], label [[TRAP:%.*]], label [[TMP5:%.*]]
-; CHECK:       5:
+; CHECK-NEXT:    [[TMP3:%.*]] = sub i64 [[TMP0]], [[TMP2]]
+; CHECK-NEXT:    [[TMP4:%.*]] = icmp ult i64 [[TMP0]], [[TMP2]]
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp ult i64 [[TMP3]], 1
+; CHECK-NEXT:    [[TMP6:%.*]] = or i1 [[TMP4]], [[TMP5]]
+; CHECK-NEXT:    br i1 [[TMP6]], label [[TRAP:%.*]], label [[TMP7:%.*]]
+; CHECK:       7:
 ; CHECK-NEXT:    [[RET:%.*]] = load i8, ptr [[P]], align 1
 ; CHECK-NEXT:    ret i8 [[RET]]
 ; CHECK:       trap: