Revert "Remove obsolete CallGuards MIR pass"

This reverts commit 8cee04a.

It turns out that DropElaboration may need the CallGuards pass,
even if trans doesn't.

Author: Mark-Simulacrum

src/librustc_driver/driver.rs

@@ -1050,6 +1050,7 @@ pub fn phase_4_translate_to_llvm<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         // From here on out, regions are gone.
         passes.push_pass(box mir::transform::erase_regions::EraseRegions);
 
+        passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
         passes.push_pass(box borrowck::ElaborateDrops);
         passes.push_pass(box mir::transform::no_landing_pads::NoLandingPads);
         passes.push_pass(box mir::transform::simplify::SimplifyCfg::new("elaborate-drops"));
@@ -1061,6 +1062,7 @@ pub fn phase_4_translate_to_llvm<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
         passes.push_pass(box mir::transform::copy_prop::CopyPropagation);
 
         passes.push_pass(box mir::transform::simplify::SimplifyLocals);
+        passes.push_pass(box mir::transform::add_call_guards::AddCallGuards);
         passes.push_pass(box mir::transform::dump_mir::Marker("PreTrans"));
 
         passes.run_passes(tcx);

src/librustc_mir/shim.rs

@@ -30,7 +30,7 @@ use std::fmt;
 use std::iter;
 use std::mem;
 
-use transform::{no_landing_pads, simplify};
+use transform::{add_call_guards, no_landing_pads, simplify};
 use util::elaborate_drops::{self, DropElaborator, DropStyle, DropFlagMode};
 use util::patch::MirPatch;
 
@@ -115,6 +115,7 @@ fn make_shim<'a, 'tcx>(tcx: ty::TyCtxt<'a, 'tcx, 'tcx>,
         debug!("make_shim({:?}) = untransformed {:?}", instance, result);
         no_landing_pads::no_landing_pads(tcx, &mut result);
         simplify::simplify_cfg(&mut result);
+        add_call_guards::add_call_guards(&mut result);
     debug!("make_shim({:?}) = {:?}", instance, result);
 
     let result = tcx.alloc_mir(result);

src/librustc_mir/transform/add_call_guards.rs

@@ -0,0 +1,86 @@
+// Copyright 2016 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use rustc::ty::TyCtxt;
+use rustc::mir::*;
+use rustc::mir::transform::{MirPass, MirSource, Pass};
+use rustc_data_structures::indexed_vec::{Idx, IndexVec};
+
+pub struct AddCallGuards;
+
+/**
+ * Breaks outgoing critical edges for call terminators in the MIR.
+ *
+ * Critical edges are edges that are neither the only edge leaving a
+ * block, nor the only edge entering one.
+ *
+ * When you want something to happen "along" an edge, you can either
+ * do at the end of the predecessor block, or at the start of the
+ * successor block. Critical edges have to be broken in order to prevent
+ * "edge actions" from affecting other edges. We need this for calls that are
+ * translated to LLVM invoke instructions, because invoke is a block terminator
+ * in LLVM so we can't insert any code to handle the call's result into the
+ * block that performs the call.
+ *
+ * This function will break those edges by inserting new blocks along them.
+ *
+ * NOTE: Simplify CFG will happily undo most of the work this pass does.
+ *
+ */
+
+impl<'tcx> MirPass<'tcx> for AddCallGuards {
+    fn run_pass<'a>(&mut self, _tcx: TyCtxt<'a, 'tcx, 'tcx>, _src: MirSource, mir: &mut Mir<'tcx>) {
+        add_call_guards(mir);
+    }
+}
+
+pub fn add_call_guards(mir: &mut Mir) {
+    let pred_count: IndexVec<_, _> =
+        mir.predecessors().iter().map(|ps| ps.len()).collect();
+
+    // We need a place to store the new blocks generated
+    let mut new_blocks = Vec::new();
+
+    let cur_len = mir.basic_blocks().len();
+
+    for block in mir.basic_blocks_mut() {
+        match block.terminator {
+            Some(Terminator {
+                kind: TerminatorKind::Call {
+                    destination: Some((_, ref mut destination)),
+                    cleanup: Some(_),
+                    ..
+                }, source_info
+            }) if pred_count[*destination] > 1 => {
+                // It's a critical edge, break it
+                let call_guard = BlockData {
+                    statements: vec![],
+                    is_cleanup: block.is_cleanup,
+                    terminator: Some(Terminator {
+                        source_info: source_info,
+                        kind: TerminatorKind::Goto { target: *destination }
+                    })
+                };
+
+                // Get the index it will be when inserted into the MIR
+                let idx = cur_len + new_blocks.len();
+                new_blocks.push(call_guard);
+                *destination = Block::new(idx);
+            }
+            _ => {}
+        }
+    }
+
+    debug!("Broke {} N edges", new_blocks.len());
+
+    mir.basic_blocks_mut().extend(new_blocks);
+}
+
+impl Pass for AddCallGuards {}

src/librustc_mir/transform/mod.rs

@@ -13,6 +13,7 @@ pub mod simplify;
 pub mod erase_regions;
 pub mod no_landing_pads;
 pub mod type_check;
+pub mod add_call_guards;
 pub mod promote_consts;
 pub mod qualify_consts;
 pub mod dump_mir;