From e2efec89764f4ee68cc9f537eda722d2fb830bba Mon Sep 17 00:00:00 2001
From: Mark Rousskov <mark.simulacrum@gmail.com>
Date: Thu, 15 Oct 2020 09:28:27 -0400
Subject: Prevent miscompilation in trivial loop {}

Ideally, we would want to handle a broader set of cases to fully fix the
underlying bug here. That is currently relatively expensive at compile and
runtime, so we don't do that for now.
---
 compiler/rustc_codegen_ssa/src/mir/block.rs | 20 ++++++++++++++++++--
 compiler/rustc_codegen_ssa/src/mir/mod.rs   |  2 +-
 2 files changed, 19 insertions(+), 3 deletions(-)

(limited to 'compiler/rustc_codegen_ssa/src/mir')

diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 353189ae1f0..1e61f3ba2df 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -163,7 +163,7 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
                 target <= self.bb
                     && target.start_location().is_predecessor_of(self.bb.start_location(), mir)
             }) {
-                bx.sideeffect();
+                bx.sideeffect(false);
             }
         }
     }
@@ -964,7 +964,23 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             }
 
             mir::TerminatorKind::Goto { target } => {
-                helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
+                if bb == target {
+                    // This is an unconditional branch back to this same basic
+                    // block. That means we have something like a `loop {}`
+                    // statement. Currently LLVM miscompiles this because it
+                    // assumes forward progress. We want to prevent this in all
+                    // cases, but that has a fairly high cost to compile times
+                    // currently. Instead, try to handle this specific case
+                    // which comes up commonly in practice (e.g., in embedded
+                    // code).
+                    //
+                    // The `true` here means we insert side effects regardless
+                    // of -Zinsert-sideeffect being passed on unconditional
+                    // branching to the same basic block.
+                    bx.sideeffect(true);
+                } else {
+                    helper.maybe_sideeffect(self.mir, &mut bx, &[target]);
+                }
                 helper.funclet_br(self, &mut bx, target);
             }
 
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 64d456fb7aa..bff263567bf 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -153,7 +153,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         bx.set_personality_fn(cx.eh_personality());
     }
 
-    bx.sideeffect();
+    bx.sideeffect(false);
 
     let cleanup_kinds = analyze::cleanup_kinds(&mir);
     // Allocate a `Block` for every basic block, except
-- 
cgit 1.4.1-3-g733a5