diff --git a/src/test/codegen/internalize-closures.rs b/src/test/codegen/internalize-closures.rs
index 8d9192c6fa0..ab3dc3fba5e 100644
--- a/src/test/codegen/internalize-closures.rs
+++ b/src/test/codegen/internalize-closures.rs
@@ -1,4 +1,4 @@
-// compile-flags: -C no-prepopulate-passes
+// compile-flags: -C no-prepopulate-passes -Zmir-opt-level=0
 
 pub fn main() {
 
diff --git a/src/test/codegen/issue-37945.rs b/src/test/codegen/issue-37945.rs
index 0ca42b7ec83..a91e8e817e4 100644
--- a/src/test/codegen/issue-37945.rs
+++ b/src/test/codegen/issue-37945.rs
@@ -1,26 +1,34 @@
-// compile-flags: -O
+// compile-flags: -O -Zmerge-functions=disabled
 // ignore-x86
 // ignore-arm
 // ignore-emscripten
 // ignore-gnux32
 // ignore 32-bit platforms (LLVM has a bug with them)
 
-// See issue #37945.
+// Check that LLVM understands that `Iter` pointer is not null. Issue #37945.
 
 #![crate_type = "lib"]
 
 use std::slice::Iter;
 
-// CHECK-LABEL: @is_empty_1
 #[no_mangle]
 pub fn is_empty_1(xs: Iter<f32>) -> bool {
-// CHECK-NOT: icmp eq float* {{.*}}, null
+// CHECK-LABEL: @is_empty_1(
+// CHECK-NEXT:  start:
+// CHECK-NEXT:    [[A:%.*]] = icmp ne i32* %xs.1, null
+// CHECK-NEXT:    tail call void @llvm.assume(i1 [[A]])
+// CHECK-NEXT:    [[B:%.*]] = icmp eq i32* %xs.0, %xs.1
+// CHECK-NEXT:    ret i1 [[B:%.*]]
     {xs}.next().is_none()
 }
 
-// CHECK-LABEL: @is_empty_2
 #[no_mangle]
 pub fn is_empty_2(xs: Iter<f32>) -> bool {
-// CHECK-NOT: icmp eq float* {{.*}}, null
+// CHECK-LABEL: @is_empty_2
+// CHECK-NEXT:  start:
+// CHECK-NEXT:    [[C:%.*]] = icmp ne i32* %xs.1, null
+// CHECK-NEXT:    tail call void @llvm.assume(i1 [[C]])
+// CHECK-NEXT:    [[D:%.*]] = icmp eq i32* %xs.0, %xs.1
+// CHECK-NEXT:    ret i1 [[D:%.*]]
     xs.map(|&x| x).next().is_none()
 }
diff --git a/src/test/codegen/vec-shrink-panic.rs b/src/test/codegen/vec-shrink-panic.rs
index 690c7e6d6ce..cee4128c650 100644
--- a/src/test/codegen/vec-shrink-panic.rs
+++ b/src/test/codegen/vec-shrink-panic.rs
@@ -15,9 +15,9 @@ pub fn shrink_to_fit(vec: &mut Vec<u32>) {
 
 // CHECK-LABEL: @issue71861
 #[no_mangle]
-pub fn issue71861(n: usize) -> Box<[u32]> {
+pub fn issue71861(vec: Vec<u32>) -> Box<[u32]> {
     // CHECK-NOT: panic
-    vec![0; n].into_boxed_slice()
+    vec.into_boxed_slice()
 }
 
 // CHECK-LABEL: @issue75636