Move all cold code to the end of the function

Fixes #836

Benchmark #1: simple-raytracer/raytracer_cg_clif
  Time (mean ± σ):      9.250 s ±  0.056 s    [User: 9.213 s, System: 0.015 s]
  Range (min … max):    9.151 s …  9.348 s    20 runs

Benchmark #2: simple-raytracer/raytracer_cg_clif_cold_separated
  Time (mean ± σ):      9.179 s ±  0.101 s    [User: 9.141 s, System: 0.016 s]
  Range (min … max):    9.070 s …  9.473 s    20 runs

Summary
  'simple-raytracer/raytracer_cg_clif_cold_separated' ran
    1.01 ± 0.01 times faster than 'simple-raytracer/raytracer_cg_clif'

src/abi/mod.rs

@@ -506,6 +506,7 @@ fn codegen_call_inner<'tcx>(
     args: Vec<CValue<'tcx>>,
     ret_place: Option<CPlace<'tcx>>,
 ) {
+    // FIXME mark the current ebb as cold when calling a `#[cold]` function.
     let fn_sig = fx
         .tcx
         .normalize_erasing_late_bound_regions(ParamEnv::reveal_all(), &fn_ty.fn_sig(fx.tcx));

src/base.rs

@@ -32,6 +32,12 @@ pub fn trans_fn<'clif, 'tcx, B: Backend + 'static>(
     // Predefine ebb's
     let start_ebb = bcx.create_ebb();
     let ebb_map: IndexVec<BasicBlock, Ebb> = (0..mir.basic_blocks().len()).map(|_| bcx.create_ebb()).collect();
+    let mut cold_ebbs = EntitySet::new();
+    for (bb, &ebb) in ebb_map.iter_enumerated() {
+        if mir.basic_blocks()[bb].is_cleanup {
+            cold_ebbs.insert(ebb);
+        }
+    }
 
     // Make FunctionCx
     let pointer_type = cx.module.target_config().pointer_type();
@@ -49,6 +55,7 @@ pub fn trans_fn<'clif, 'tcx, B: Backend + 'static>(
         ebb_map,
         local_map: HashMap::new(),
         caller_location: None, // set by `codegen_fn_prelude`
+        cold_ebbs,
 
         clif_comments,
         constants_cx: &mut cx.constants_cx,
@@ -73,6 +80,7 @@ pub fn trans_fn<'clif, 'tcx, B: Backend + 'static>(
     let mut clif_comments = fx.clif_comments;
     let source_info_set = fx.source_info_set;
     let local_map = fx.local_map;
+    let cold_ebbs = fx.cold_ebbs;
 
     #[cfg(debug_assertions)]
     crate::pretty_clif::write_clif_file(cx.tcx, "unopt", instance, &context.func, &clif_comments, None);
@@ -82,7 +90,7 @@ pub fn trans_fn<'clif, 'tcx, B: Backend + 'static>(
 
     // Perform rust specific optimizations
     tcx.sess.time("optimize clif ir", || {
-        crate::optimize::optimize_function(tcx, instance, context, &mut clif_comments);
+        crate::optimize::optimize_function(tcx, instance, context, &cold_ebbs, &mut clif_comments);
     });
 
     // Define function
@@ -191,8 +199,11 @@ fn codegen_fn_content(fx: &mut FunctionCx<'_, '_, impl Backend>) {
                     }
                 }
                 let cond = trans_operand(fx, cond).load_scalar(fx);
+
                 let target = fx.get_ebb(*target);
                 let failure = fx.bcx.create_ebb();
+                fx.cold_ebbs.insert(failure);
+
                 if *expected {
                     fx.bcx.ins().brz(cond, failure, &[]);
                 } else {
@@ -200,8 +211,6 @@ fn codegen_fn_content(fx: &mut FunctionCx<'_, '_, impl Backend>) {
                 };
                 fx.bcx.ins().jump(target, &[]);
 
-                // FIXME insert bb after all other bb's to reduce the amount of jumps in the common
-                // case and improve code locality.
                 fx.bcx.switch_to_block(failure);
                 trap_panic(
                     fx,

src/common.rs

@@ -270,6 +270,9 @@ pub struct FunctionCx<'clif, 'tcx, B: Backend + 'static> {
     /// When `#[track_caller]` is used, the implicit caller location is stored in this variable.
     pub caller_location: Option<CValue<'tcx>>,
 
+    /// See [crate::optimize::code_layout] for more information.
+    pub cold_ebbs: EntitySet<Ebb>,
+
     pub clif_comments: crate::pretty_clif::CommentWriter,
     pub constants_cx: &'clif mut crate::constant::ConstantCx,
     pub vtables: &'clif mut HashMap<(Ty<'tcx>, Option<ty::PolyExistentialTraitRef<'tcx>>), DataId>,

src/lib.rs

@@ -95,6 +95,7 @@ mod prelude {
     pub use rustc_codegen_ssa::{CodegenResults, CompiledModule, ModuleKind};
 
     pub use cranelift_codegen::Context;
+    pub use cranelift_codegen::entity::EntitySet;
     pub use cranelift_codegen::ir::{AbiParam, Ebb, ExternalName, FuncRef, Inst, InstBuilder, MemFlags, Signature, SourceLoc, StackSlot, StackSlotData, StackSlotKind, TrapCode, Type, Value};
     pub use cranelift_codegen::ir::condcodes::{FloatCC, IntCC};
     pub use cranelift_codegen::ir::function::Function;

src/optimize/code_layout.rs

@@ -0,0 +1,34 @@
+//! This optimization moves cold code to the end of the function.
+//!
+//! Some code is executed much less often than other code. For example panicking or the
+//! landingpads for unwinding. By moving this cold code to the end of the function the average
+//! amount of jumps is reduced and the code locality is improved.
+//!
+//! # Undefined behaviour
+//!
+//! This optimization doesn't assume anything that isn't already assumed by Cranelift itself.
+
+use crate::prelude::*;
+
+pub fn optimize_function(ctx: &mut Context, cold_ebbs: &EntitySet<Ebb>) {
+    // FIXME Move the ebb in place instead of remove and append once
+    // bytecodealliance/cranelift#1339 is implemented.
+
+    let mut ebb_insts = HashMap::new();
+    for ebb in cold_ebbs.keys().filter(|&ebb| cold_ebbs.contains(ebb)) {
+        let insts = ctx.func.layout.ebb_insts(ebb).collect::<Vec<_>>();
+        for &inst in &insts {
+            ctx.func.layout.remove_inst(inst);
+        }
+        ebb_insts.insert(ebb, insts);
+        ctx.func.layout.remove_ebb(ebb);
+    }
+
+    // And then append them at the back again.
+    for ebb in cold_ebbs.keys().filter(|&ebb| cold_ebbs.contains(ebb)) {
+        ctx.func.layout.append_ebb(ebb);
+        for inst in ebb_insts.remove(&ebb).unwrap() {
+            ctx.func.layout.append_inst(inst, ebb);
+        }
+    }
+}

src/optimize/mod.rs

@@ -1,13 +1,18 @@
 use crate::prelude::*;
 
+mod code_layout;
 mod stack2reg;
 
 pub fn optimize_function<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
     ctx: &mut Context,
+    cold_ebbs: &EntitySet<Ebb>,
     clif_comments: &mut crate::pretty_clif::CommentWriter,
 ) {
+    // The code_layout optimization is very cheap.
+    self::code_layout::optimize_function(ctx, cold_ebbs);
+
     if tcx.sess.opts.optimize == rustc_session::config::OptLevel::No {
         return; // FIXME classify optimizations over opt levels
     }

src/optimize/stack2reg.rs

@@ -13,7 +13,6 @@ use std::collections::{BTreeMap, HashSet};
 use std::ops::Not;
 
 use cranelift_codegen::cursor::{Cursor, FuncCursor};
-use cranelift_codegen::entity::EntitySet;
 use cranelift_codegen::ir::{InstructionData, Opcode, ValueDef};
 use cranelift_codegen::ir::immediates::Offset32;