Auto merge of #59546 - sfanxiang:interminable-ub, r=nagisa

Add llvm.sideeffect to potential infinite loops and recursions

LLVM assumes that a thread will eventually cause side effect. This is
not true in Rust if a loop or recursion does nothing in its body,
causing undefined behavior even in common cases like `loop {}`.
Inserting llvm.sideeffect fixes the undefined behavior.

As a micro-optimization, only insert llvm.sideeffect when jumping back
in blocks or calling a function.

A patch for LLVM is expected to allow empty non-terminate code by
default and fix this issue from LLVM side.

https://github.com/rust-lang/rust/issues/28728

**UPDATE:** [Mentoring instructions here](https://github.com/rust-lang/rust/pull/59546#issuecomment-515072429) to unstall this PR

3 files changed, 50 insertions, 0 deletions

diff --git a/src/test/codegen/non-terminate/infinite-loop-1.rs b/src/test/codegen/non-terminate/infinite-loop-1.rs
new file mode 100644
index 00000000000..56b360e0a7f
--- /dev/null
+++ b/src/test/codegen/non-terminate/infinite-loop-1.rs
@@ -0,0 +1,17 @@
+// compile-flags: -C opt-level=3 -Z insert-sideeffect
+
+#![crate_type = "lib"]
+
+fn infinite_loop() -> u8 {
+    loop {}
+}
+
+// CHECK-LABEL: @test
+#[no_mangle]
+fn test() -> u8 {
+    // CHECK-NOT: unreachable
+    // CHECK: br label %{{.+}}
+    // CHECK-NOT: unreachable
+    let x = infinite_loop();
+    x
+}
diff --git a/src/test/codegen/non-terminate/infinite-loop-2.rs b/src/test/codegen/non-terminate/infinite-loop-2.rs
new file mode 100644
index 00000000000..2921ab6dc04
--- /dev/null
+++ b/src/test/codegen/non-terminate/infinite-loop-2.rs
@@ -0,0 +1,19 @@
+// compile-flags: -C opt-level=3 -Z insert-sideeffect
+
+#![crate_type = "lib"]
+
+fn infinite_loop() -> u8 {
+    let i = 2;
+    while i > 1 {}
+    1
+}
+
+// CHECK-LABEL: @test
+#[no_mangle]
+fn test() -> u8 {
+    // CHECK-NOT: unreachable
+    // CHECK: br label %{{.+}}
+    // CHECK-NOT: unreachable
+    let x = infinite_loop();
+    x
+}
diff --git a/src/test/codegen/non-terminate/infinite-recursion.rs b/src/test/codegen/non-terminate/infinite-recursion.rs
new file mode 100644
index 00000000000..1f292ce379f
--- /dev/null
+++ b/src/test/codegen/non-terminate/infinite-recursion.rs
@@ -0,0 +1,14 @@
+// compile-flags: -C opt-level=3 -Z insert-sideeffect
+
+#![crate_type = "lib"]
+
+#![allow(unconditional_recursion)]
+
+// CHECK-LABEL: @infinite_recursion
+#[no_mangle]
+fn infinite_recursion() -> u8 {
+    // CHECK-NOT: ret i8 undef
+    // CHECK: br label %{{.+}}
+    // CHECK-NOT: ret i8 undef
+    infinite_recursion()
+}