From f71e0daa29b232d8f689f77fecb84dcb87fce6da Mon Sep 17 00:00:00 2001 From: Xiang Fan Date: Thu, 6 Jun 2019 08:39:20 +0800 Subject: 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 --- src/librustc_codegen_llvm/context.rs | 1 + src/librustc_codegen_llvm/intrinsic.rs | 6 ++++++ 2 files changed, 7 insertions(+) (limited to 'src/librustc_codegen_llvm') 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) -- cgit 1.4.1-3-g733a5