Remove the cold block optimization

It isn't effective with the new backend framework

src/abi/mod.rs

@@ -295,7 +295,6 @@ pub(crate) fn codegen_fn_prelude<'tcx>(fx: &mut FunctionCx<'_, '_, 'tcx>, start_
 pub(crate) fn codegen_terminator_call<'tcx>(
     fx: &mut FunctionCx<'_, '_, 'tcx>,
     span: Span,
-    current_block: Block,
     func: &Operand<'tcx>,
     args: &[Operand<'tcx>],
     destination: Option<(Place<'tcx>, BasicBlock)>,
@@ -357,7 +356,7 @@ pub(crate) fn codegen_terminator_call<'tcx>(
         .map(|inst| fx.tcx.codegen_fn_attrs(inst.def_id()).flags.contains(CodegenFnAttrFlags::COLD))
         .unwrap_or(false);
     if is_cold {
-        fx.cold_blocks.insert(current_block);
+        // FIXME Mark current_block block as cold once Cranelift supports it
     }
 
     // Unpack arguments tuple for closures

src/base.rs

@@ -55,7 +55,6 @@ pub(crate) fn codegen_fn<'tcx>(cx: &mut crate::CodegenCx<'_, 'tcx>, instance: In
         block_map,
         local_map: IndexVec::with_capacity(mir.local_decls.len()),
         caller_location: None, // set by `codegen_fn_prelude`
-        cold_blocks: EntitySet::new(),
 
         clif_comments,
         source_info_set: indexmap::IndexSet::new(),
@@ -90,7 +89,6 @@ pub(crate) fn codegen_fn<'tcx>(cx: &mut crate::CodegenCx<'_, 'tcx>, instance: In
     let mut clif_comments = fx.clif_comments;
     let source_info_set = fx.source_info_set;
     let local_map = fx.local_map;
-    let cold_blocks = fx.cold_blocks;
 
     // Store function in context
     let context = &mut cx.cached_context;
@@ -107,7 +105,6 @@ pub(crate) fn codegen_fn<'tcx>(cx: &mut crate::CodegenCx<'_, 'tcx>, instance: In
             tcx,
             instance,
             context,
-            &cold_blocks,
             &mut clif_comments,
         );
     });
@@ -205,9 +202,8 @@ fn codegen_fn_content(fx: &mut FunctionCx<'_, '_, '_>) {
             // Unwinding after panicking is not supported
             continue;
 
-            // FIXME once unwinding is supported uncomment next lines
+            // FIXME Once unwinding is supported and Cranelift supports marking blocks as cold, do
-            // // Unwinding is unlikely to happen, so mark cleanup block's as cold.
+            // so for cleanup blocks.
-            // fx.cold_blocks.insert(block);
         }
 
         fx.bcx.ins().nop();
@@ -262,7 +258,7 @@ fn codegen_fn_content(fx: &mut FunctionCx<'_, '_, '_>) {
 
                 let target = fx.get_block(*target);
                 let failure = fx.bcx.create_block();
-                fx.cold_blocks.insert(failure);
+                // FIXME Mark failure block as cold once Cranelift supports it
 
                 if *expected {
                     fx.bcx.ins().brz(cond, failure, &[]);
@@ -358,7 +354,6 @@ fn codegen_fn_content(fx: &mut FunctionCx<'_, '_, '_>) {
                     crate::abi::codegen_terminator_call(
                         fx,
                         *fn_span,
-                        block,
                         func,
                         args,
                         *destination,

src/common.rs

@@ -242,9 +242,6 @@ pub(crate) struct FunctionCx<'m, 'clif, 'tcx> {
     /// When `#[track_caller]` is used, the implicit caller location is stored in this variable.
     pub(crate) caller_location: Option<CValue<'tcx>>,
 
-    /// See [`crate::optimize::code_layout`] for more information.
-    pub(crate) cold_blocks: EntitySet<Block>,
-
     pub(crate) clif_comments: crate::pretty_clif::CommentWriter,
     pub(crate) source_info_set: indexmap::IndexSet<SourceInfo>,
 

src/lib.rs

@@ -87,7 +87,6 @@ mod prelude {
 
     pub(crate) use rustc_index::vec::Idx;
 
-    pub(crate) use cranelift_codegen::entity::EntitySet;
     pub(crate) use cranelift_codegen::ir::condcodes::{FloatCC, IntCC};
     pub(crate) use cranelift_codegen::ir::function::Function;
     pub(crate) use cranelift_codegen::ir::types;

src/optimize/code_layout.rs

@@ -1,34 +0,0 @@
-//! 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(super) fn optimize_function(ctx: &mut Context, cold_blocks: &EntitySet<Block>) {
-    // FIXME Move the block in place instead of remove and append once
-    // bytecodealliance/cranelift#1339 is implemented.
-
-    let mut block_insts = FxHashMap::default();
-    for block in cold_blocks.keys().filter(|&block| cold_blocks.contains(block)) {
-        let insts = ctx.func.layout.block_insts(block).collect::<Vec<_>>();
-        for &inst in &insts {
-            ctx.func.layout.remove_inst(inst);
-        }
-        block_insts.insert(block, insts);
-        ctx.func.layout.remove_block(block);
-    }
-
-    // And then append them at the back again.
-    for block in cold_blocks.keys().filter(|&block| cold_blocks.contains(block)) {
-        ctx.func.layout.append_block(block);
-        for inst in block_insts.remove(&block).unwrap() {
-            ctx.func.layout.append_inst(inst, block);
-        }
-    }
-}

src/optimize/mod.rs

@@ -2,21 +2,16 @@
 
 use crate::prelude::*;
 
-mod code_layout;
 pub(crate) mod peephole;
 
 pub(crate) fn optimize_function<'tcx>(
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
     ctx: &mut Context,
-    cold_blocks: &EntitySet<Block>,
     clif_comments: &mut crate::pretty_clif::CommentWriter,
 ) {
     // FIXME classify optimizations over opt levels once we have more
 
-    // The code_layout optimization is very cheap.
-    self::code_layout::optimize_function(ctx, cold_blocks);
-
     crate::pretty_clif::write_clif_file(tcx, "preopt", None, instance, &ctx, &*clif_comments);
     crate::base::verify_func(tcx, &*clif_comments, &ctx.func);
 }