1 files changed, 66 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/simplify_branches.rs b/compiler/rustc_mir_transform/src/simplify_branches.rs
new file mode 100644
index 00000000000..df90cfa318d
--- /dev/null
+++ b/compiler/rustc_mir_transform/src/simplify_branches.rs
@@ -0,0 +1,66 @@
+//! A pass that simplifies branches when their condition is known.
+
+use crate::MirPass;
+use rustc_middle::mir::*;
+use rustc_middle::ty::TyCtxt;
+
+use std::borrow::Cow;
+
+pub struct SimplifyBranches {
+    label: String,
+}
+
+impl SimplifyBranches {
+    pub fn new(label: &str) -> Self {
+        SimplifyBranches { label: format!("SimplifyBranches-{}", label) }
+    }
+}
+
+impl<'tcx> MirPass<'tcx> for SimplifyBranches {
+    fn name(&self) -> Cow<'_, str> {
+        Cow::Borrowed(&self.label)
+    }
+
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        let param_env = tcx.param_env(body.source.def_id());
+        for block in body.basic_blocks_mut() {
+            let terminator = block.terminator_mut();
+            terminator.kind = match terminator.kind {
+                TerminatorKind::SwitchInt {
+                    discr: Operand::Constant(ref c),
+                    switch_ty,
+                    ref targets,
+                    ..
+                } => {
+                    let constant = c.literal.try_eval_bits(tcx, param_env, switch_ty);
+                    if let Some(constant) = constant {
+                        let otherwise = targets.otherwise();
+                        let mut ret = TerminatorKind::Goto { target: otherwise };
+                        for (v, t) in targets.iter() {
+                            if v == constant {
+                                ret = TerminatorKind::Goto { target: t };
+                                break;
+                            }
+                        }
+                        ret
+                    } else {
+                        continue;
+                    }
+                }
+                TerminatorKind::Assert {
+                    target, cond: Operand::Constant(ref c), expected, ..
+                } => match c.literal.try_eval_bool(tcx, param_env) {
+                    Some(v) if v == expected => TerminatorKind::Goto { target },
+                    _ => continue,
+                },
+                TerminatorKind::FalseEdge { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+                TerminatorKind::FalseUnwind { real_target, .. } => {
+                    TerminatorKind::Goto { target: real_target }
+                }
+                _ => continue,
+            };
+        }
+    }
+}