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
@@ -812,8 +812,32 @@
   return unknown();
 }
 
-SizeOffsetType ObjectSizeOffsetVisitor::visitPHINode(PHINode&) {
+SizeOffsetType ObjectSizeOffsetVisitor::visitPHINode(PHINode &PN) {
   // too complex to analyze statically.
+  SmallVector<std::pair<SizeOffsetType, APInt>> Results;
+  for (Value *IV : PN.incoming_values()) {
+    SizeOffsetType SOT = compute(IV);
+    if (!bothKnown(SOT))
+      return unknown();
+    APInt Size = getSizeWithOverflow(SOT);
+    Results.push_back(std::make_pair(SOT, Size));
+  }
+
+  if (Options.EvalMode == ObjectSizeOpts::Mode::Min) {
+    return std::min_element(Results.begin(), Results.end(),
+                            [](auto const &LHS, auto const &RHS) {
+                              return LHS.second.slt(RHS.second);
+                            })
+        ->first;
+  }
+  if (Options.EvalMode == ObjectSizeOpts::Mode::Max) {
+    return std::max_element(Results.begin(), Results.end(),
+                            [](auto const &LHS, auto const &RHS) {
+                              return LHS.second.slt(RHS.second);
+                            })
+        ->first;
+  }
+
   return unknown();
 }
 
diff --git a/llvm/test/Transforms/InstCombine/builtin-object-size-phi.ll b/llvm/test/Transforms/InstCombine/builtin-object-size-phi.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/builtin-object-size-phi.ll
@@ -0,0 +1,46 @@
+; RUN: opt -O1 -S < %s | FileCheck %s
+
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare dso_local noalias noundef i8* @malloc(i64 noundef) local_unnamed_addr
+declare i64 @llvm.objectsize.i64.p0i8(i8*, i1 immarg, i1 immarg, i1 immarg)
+
+@buffer = dso_local global [4 x i8] zeroinitializer, align 1
+
+define dso_local i64 @pick_max(i32 noundef %n) local_unnamed_addr {
+; CHECK-LABEL: @pick_max(
+; CHECK-NEXT:    entry:
+; CHECK-NEXT:      ret i64 8
+entry:
+  %cond = icmp eq i32 %n, 0
+  br i1 %cond, label %if.else, label %if.end
+
+if.else:
+  %malloced = call noalias dereferenceable_or_null(8) i8* @malloc(i64 noundef 8)
+  br label %if.end
+
+if.end:
+  %p = phi i8* [ %malloced, %if.else ], [ getelementptr inbounds ([4 x i8], [4 x i8]* @buffer, i64 0, i64 0), %entry ]
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %p, i1 false, i1 true, i1 false)
+  ret i64 %size
+}
+
+define dso_local i64 @pick_min(i32 noundef %n) local_unnamed_addr {
+; CHECK-LABEL: @pick_min(
+; CHECK-NEXT:    entry:
+; CHECK-NEXT:      ret i64 4
+entry:
+  %cond = icmp eq i32 %n, 0
+  br i1 %cond, label %if.else, label %if.end
+
+if.else:
+  %malloced = call noalias dereferenceable_or_null(8) i8* @malloc(i64 noundef 8)
+  br label %if.end
+
+if.end:
+  %p = phi i8* [ %malloced, %if.else ], [ getelementptr inbounds ([4 x i8], [4 x i8]* @buffer, i64 0, i64 0), %entry ]
+  %size = call i64 @llvm.objectsize.i64.p0i8(i8* %p, i1 true, i1 true, i1 false)
+  ret i64 %size
+}