Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -277,10 +277,22 @@
   switch (II->getIntrinsicID()) {
   default: break;
   case Intrinsic::objectsize: {
+    bool MinTrue = CI.getArgOperand(1) == ConstantInt::getTrue(CI.getContext());
+    Constant *SizeC = nullptr;
     uint64_t Size;
+    // Try to lower objectsize.  If the size is unknown, lower it to 0 or -1,
+    // depending on the min argument.
+    // One could argue this is a bit early to do the lowering, since the size
+    // might be made apparent by later optimizations.  In practice however,
+    // the overwhelming majority of cases has a never-known objectsize, and in
+    // a lot of the remaining few, the size is immediately apparent (say from a
+    // global value, or an alloca).
+    // Keeping the intrinsic call intact mostly prevents optimizations.
     if (getObjectSize(II->getArgOperand(0), Size, DL, TLI))
-      return ReplaceInstUsesWith(CI, ConstantInt::get(CI.getType(), Size));
-    return nullptr;
+      SizeC = ConstantInt::get(CI.getType(), Size);
+    else
+      SizeC = ConstantInt::get(CI.getType(), MinTrue ? 0 : -1);
+    return ReplaceInstUsesWith(CI, SizeC);
   }
   case Intrinsic::bswap: {
     Value *IIOperand = II->getArgOperand(0);
Index: test/Transforms/InstCombine/objsize.ll
===================================================================
--- test/Transforms/InstCombine/objsize.ll
+++ test/Transforms/InstCombine/objsize.ll
@@ -41,7 +41,7 @@
 
 define i1 @baz() nounwind {
 ; CHECK-LABEL: @baz(
-; CHECK-NEXT: objectsize
+; CHECK-NEXT: ret i1 true
   %1 = tail call i32 @llvm.objectsize.i32.p0i8(i8* getelementptr inbounds ([0 x i8]* @window, i32 0, i32 0), i1 false)
   %2 = icmp eq i32 %1, -1
   ret i1 %2
@@ -49,7 +49,7 @@
 
 define void @test1(i8* %q, i32 %x) nounwind noinline {
 ; CHECK-LABEL: @test1(
-; CHECK: objectsize.i32.p0i8
+; CHECK: br i1 true, label %"47", label %"46"
 entry:
   %0 = call i32 @llvm.objectsize.i32.p0i8(i8* getelementptr inbounds ([0 x i8]* @window, i32 0, i32 10), i1 false) ; <i64> [#uses=1]
   %1 = icmp eq i32 %0, -1                         ; <i1> [#uses=1]
@@ -231,7 +231,7 @@
   %0 = tail call i32 @llvm.objectsize.i32.p0i8(i8* %select, i1 true)
   %1 = tail call i32 @llvm.objectsize.i32.p0i8(i8* %select2, i1 true)
   %2 = add i32 %0, %1
-; CHECK: ret i32 undef
+; CHECK: ret i32 0
   ret i32 %2
 
 return:
@@ -249,7 +249,7 @@
   %gep2 = getelementptr i8* %gep, i32 1
   %gep = getelementptr i8* %gep2, i32 1
   %o = call i32 @llvm.objectsize.i32.p0i8(i8* %gep, i1 true)
-; CHECK: ret i32 undef
+; CHECK: ret i32 0
   ret i32 %o
 
 return:
@@ -269,7 +269,7 @@
 @globalalias2 = weak alias [60 x i8]* @a
 
 ; CHECK-LABEL: @test19(
-; CHECK: llvm.objectsize
+; CHECK: ret i32 -1
 define i32 @test19() {
   %bc = bitcast [60 x i8]* @globalalias2 to i8*
   %1 = call i32 @llvm.objectsize.i32.p0i8(i8* %bc, i1 false)
Index: test/Transforms/InstCombine/stpcpy_chk-1.ll
===================================================================
--- test/Transforms/InstCombine/stpcpy_chk-1.ll
+++ test/Transforms/InstCombine/stpcpy_chk-1.ll
@@ -60,7 +60,7 @@
   %dst = getelementptr inbounds [60 x i8]* @a, i32 0, i32 0
   %src = getelementptr inbounds [12 x i8]* @.str, i32 0, i32 0
 
-; CHECK: @__memcpy_chk
+; CHECK: call void @llvm.memcpy.p0i8.p0i8.i32
   %len = call i32 @llvm.objectsize.i32.p0i8(i8* %dst, i1 false)
   %ret = call i8* @__stpcpy_chk(i8* %dst, i8* %src, i32 %len)
 ; CHECK: ret i8* getelementptr inbounds ([60 x i8]* @a, i32 0, i32 11)
Index: test/Transforms/InstCombine/strcpy_chk-1.ll
===================================================================
--- test/Transforms/InstCombine/strcpy_chk-1.ll
+++ test/Transforms/InstCombine/strcpy_chk-1.ll
@@ -60,7 +60,7 @@
   %dst = getelementptr inbounds [60 x i8]* @a, i32 0, i32 0
   %src = getelementptr inbounds [12 x i8]* @.str, i32 0, i32 0
 
-; CHECK: @__memcpy_chk
+; CHECK: call void @llvm.memcpy.p0i8.p0i8.i32
   %len = call i32 @llvm.objectsize.i32.p0i8(i8* %dst, i1 false)
   call i8* @__strcpy_chk(i8* %dst, i8* %src, i32 %len)
   ret void