Add side effect post (#534)

Author: Mark-Simulacrum

posts/inside-rust/2020-03-19-terminating-rust.md

@@ -0,0 +1,55 @@
+---
+layout: post
+title: "Resolving Rust's forward progress guarantees"
+author: Mark Rousskov
+description: "Should side-effect be the fix?"
+team: the compiler team <https://www.rust-lang.org/governance/teams/compiler>
+---
+
+There has been a longstanding miscompilation in Rust: programs that do not make
+[forward progress]. Note that the previous link is to the C++ definition; Rust
+is not C++, but currently LLVM optimizes all LLVM IR with the assumption that a
+lack of forward progress is undefined behavior.
+
+Note also that Rust does not define a lack of forward progress as [undefined
+behavior], while C++ does. It is particularly common to encounter the
+miscompilation "intentionally" when writing panic handlers and other such code
+with a body of `loop {}`. Some users also report that they've unintentionally
+hit this bug in recursive code which accidentally lacks a base case.
+
+Somewhat recently, LLVM added an intrinsic which tells the optimizer that
+forward progress has been made. On nightly Rust, you can enable this with
+`-Zinsert-sideeffect`, which will use some heuristics to insert it where it's
+possibly needed (currently, massively overshooting the minimal set).
+
+However, recent attempts to enable this intrinsic by default hit a snag: it's
+very expensive on compile times to do so ([3-30% regressions][compile-time
+regressions]). There is some runtime effect as well; check builds (which do not
+generate LLVM IR or run LLVM passes) regressed by up to 3-7%.
+
+The current implementation in rustc emits calls to the side effect intrinsic
+very aggressively; certainly in way more cases than is strictly necessary.
+However, there's not really any good ideas on how to improve the analysis rustc
+does without missing edge cases: we'd have to be "as good" as LLVM to emit only
+when necessary.
+
+Upstream, in LLVM, discussion has been ongoing for some time around whether, and
+how to, adjust LLVM's model to permit frontends for languages like Rust to
+opt-out of the forward progress guarantees. It seems unlikely that a solution
+will materialize in upstream LLVM that allows us to opt-out in the short term.
+
+However, having said that, side effect itself is likely improvable to at least
+avoid the excessive consecutive calls, as demonstrated by this [IR][IR-test]
+that occurs after LLVM optimizations. It seems plausible that those
+improvements may also reduce the compile time hit that we see when enabling
+side effect on the rustc side. Having said that, how simple these improvements
+are is unclear.
+
+We would love to hear feedback and suggestions on how to resolve this problem!
+Please leave feedback on this internals thread (link to be filled in right
+before merging).
+
+[IR-test]: https://gist.github.com/nikic/7e521def71d106c345a255e464b18d3f
+[compile-time regressions]: https://perf.rust-lang.org/compare.html?start=66b0c97070f422cb82baaaafc79ee94cab4396c5&end=548b5e75afd6bad696920dfdb69c9812ce0488f1
+[forward progress]: https://en.cppreference.com/w/cpp/language/memory_model#Forward_progress
+[undefined behavior]: https://rust-lang.github.io/unsafe-code-guidelines/glossary.html#undefined-behavior