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

2 files changed, 7 insertions, 0 deletions

diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index 58ce9703909..bac37369a29 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -537,6 +537,7 @@ impl CodegenCx<'b, 'tcx> {
         ifn!("llvm.trap", fn() -> void);
         ifn!("llvm.debugtrap", fn() -> void);
         ifn!("llvm.frameaddress", fn(t_i32) -> i8p);
+        ifn!("llvm.sideeffect", fn() -> void);
 
         ifn!("llvm.powi.f32", fn(t_f32, t_i32) -> t_f32);
         ifn!("llvm.powi.v2f32", fn(t_v2f32, t_i32) -> t_v2f32);
diff --git a/src/librustc_codegen_llvm/intrinsic.rs b/src/librustc_codegen_llvm/intrinsic.rs
index b7a410c3760..a8734d338df 100644
--- a/src/librustc_codegen_llvm/intrinsic.rs
+++ b/src/librustc_codegen_llvm/intrinsic.rs
@@ -124,6 +124,7 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
                 self.call(expect, &[args[0].immediate(), self.const_bool(false)], None)
             }
             "try" => {
+                self.sideeffect();
                 try_intrinsic(self,
                               args[0].immediate(),
                               args[1].immediate(),
@@ -724,6 +725,11 @@ impl IntrinsicCallMethods<'tcx> for Builder<'a, 'll, 'tcx> {
         self.call(expect, &[cond, self.const_bool(expected)], None)
     }
 
+    fn sideeffect(&mut self) {
+        let fnname = self.get_intrinsic(&("llvm.sideeffect"));
+        self.call(fnname, &[], None);
+    }
+
     fn va_start(&mut self, va_list: &'ll Value) -> &'ll Value {
         let intrinsic = self.cx().get_intrinsic("llvm.va_start");
         self.call(intrinsic, &[va_list], None)