x86_64: align loop headers to 64 bytes

Author: pepyakin

cranelift/codegen/src/context.rs

@@ -300,7 +300,7 @@ impl Context {
     /// Compute the loop analysis.
     pub fn compute_loop_analysis(&mut self) {
         self.loop_analysis
-            .compute(&self.func, &self.cfg, &self.domtree)
+            .compute(&mut self.func, &self.cfg, &self.domtree)
     }
 
     /// Compute the control flow graph and dominator tree.

cranelift/codegen/src/ir/layout.rs

@@ -482,6 +482,16 @@ impl Layout {
     pub fn is_cold(&self, block: Block) -> bool {
         self.blocks[block].cold
     }
+
+    /// This block is the start of a loop.
+    pub fn set_loop_header(&mut self, block: Block) {
+        self.blocks[block].loop_header = true;
+    }
+
+    /// Is the given block known to be a loop header?
+    pub fn is_loop_header(&self, block: Block) -> bool {
+        self.blocks[block].loop_header
+    }
 }
 
 /// A single node in the linked-list of blocks.
@@ -494,6 +504,7 @@ struct BlockNode {
     last_inst: PackedOption<Inst>,
     seq: SequenceNumber,
     cold: bool,
+    loop_header: bool,
 }
 
 /// Iterate over blocks in layout order. See [crate::ir::layout::Layout::blocks].

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -2211,6 +2211,17 @@ impl MachInst for Inst {
         Inst::nop(std::cmp::min(preferred_size, 15) as u8)
     }
 
+    fn align_basic_block(offset: CodeOffset, loop_header: bool) -> CodeOffset {
+        if loop_header {
+            // Unaligned loop headers can cause severe performance problems.
+            // See https://github.com/bytecodealliance/wasmtime/issues/4883.
+            // Here we use conservative 64 bytes alignment.
+            align_to(offset, 64)
+        } else {
+            offset
+        }
+    }
+
     fn rc_for_type(ty: Type) -> CodegenResult<(&'static [RegClass], &'static [Type])> {
         match ty {
             types::I8 => Ok((&[RegClass::Int], &[types::I8])),

cranelift/codegen/src/loop_analysis.rs

@@ -100,12 +100,17 @@ impl LoopAnalysis {
 
 impl LoopAnalysis {
     /// Detects the loops in a function. Needs the control flow graph and the dominator tree.
-    pub fn compute(&mut self, func: &Function, cfg: &ControlFlowGraph, domtree: &DominatorTree) {
+    pub fn compute(
+        &mut self,
+        func: &mut Function,
+        cfg: &ControlFlowGraph,
+        domtree: &DominatorTree,
+    ) {
         let _tt = timing::loop_analysis();
         self.loops.clear();
         self.block_loop_map.clear();
         self.block_loop_map.resize(func.dfg.num_blocks());
-        self.find_loop_headers(cfg, domtree, &func.layout);
+        self.find_loop_headers(cfg, domtree, &mut func.layout);
         self.discover_loop_blocks(cfg, domtree, &func.layout);
         self.valid = true;
     }
@@ -134,7 +139,7 @@ impl LoopAnalysis {
         &mut self,
         cfg: &ControlFlowGraph,
         domtree: &DominatorTree,
-        layout: &Layout,
+        layout: &mut Layout,
     ) {
         // We traverse the CFG in reverse postorder
         for &block in domtree.cfg_postorder().iter().rev() {
@@ -147,6 +152,8 @@ impl LoopAnalysis {
                     // This block is a loop header, so we create its associated loop
                     let lp = self.loops.push(LoopData::new(block, None));
                     self.block_loop_map[block] = lp.into();
+                    // We also need to mark this block as a loop header in the layout.
+                    layout.set_loop_header(block);
                     break;
                     // We break because we only need one back edge to identify a loop header.
                 }
@@ -270,7 +277,7 @@ mod tests {
         let mut domtree = DominatorTree::new();
         cfg.compute(&func);
         domtree.compute(&func, &cfg);
-        loop_analysis.compute(&func, &cfg, &domtree);
+        loop_analysis.compute(&mut func, &cfg, &domtree);
 
         let loops = loop_analysis.loops().collect::<Vec<Loop>>();
         assert_eq!(loops.len(), 2);
@@ -329,7 +336,7 @@ mod tests {
         let mut domtree = DominatorTree::new();
         cfg.compute(&func);
         domtree.compute(&func, &cfg);
-        loop_analysis.compute(&func, &cfg, &domtree);
+        loop_analysis.compute(&mut func, &cfg, &domtree);
 
         let loops = loop_analysis.loops().collect::<Vec<Loop>>();
         assert_eq!(loops.len(), 3);

cranelift/codegen/src/machinst/blockorder.rs

@@ -106,6 +106,8 @@ pub struct BlockLoweringOrder {
     /// which is used by VCode emission to sink the blocks at the last
     /// moment (when we actually emit bytes into the MachBuffer).
     cold_blocks: FxHashSet<BlockIndex>,
+    /// These are loop headers. Used for alignment.
+    loop_headers: FxHashSet<BlockIndex>,
     /// Lowered blocks that are indirect branch targets.
     indirect_branch_targets: FxHashSet<BlockIndex>,
 }
@@ -438,6 +440,7 @@ impl BlockLoweringOrder {
         // Step 3: now that we have RPO, build the BlockIndex/BB fwd/rev maps.
         let mut lowered_order = vec![];
         let mut cold_blocks = FxHashSet::default();
+        let mut loop_headers = FxHashSet::default();
         let mut lowered_succ_ranges = vec![];
         let mut lb_to_bindex = FxHashMap::default();
         let mut indirect_branch_targets = FxHashSet::default();
@@ -455,6 +458,10 @@ impl BlockLoweringOrder {
                         cold_blocks.insert(index);
                     }
 
+                    if f.layout.is_loop_header(block) {
+                        loop_headers.insert(index);
+                    }
+
                     if indirect_branch_target_clif_blocks.contains(&block) {
                         indirect_branch_targets.insert(index);
                     }
@@ -464,6 +471,10 @@ impl BlockLoweringOrder {
                         cold_blocks.insert(index);
                     }
 
+                    if f.layout.is_loop_header(pred) || f.layout.is_loop_header(succ) {
+                        loop_headers.insert(index);
+                    }
+
                     if indirect_branch_target_clif_blocks.contains(&succ) {
                         indirect_branch_targets.insert(index);
                     }
@@ -491,6 +502,7 @@ impl BlockLoweringOrder {
             lowered_succ_ranges,
             orig_map,
             cold_blocks,
+            loop_headers,
             indirect_branch_targets,
         };
         trace!("BlockLoweringOrder: {:?}", result);
@@ -513,6 +525,11 @@ impl BlockLoweringOrder {
         self.cold_blocks.contains(&block)
     }
 
+    /// Determine whether the given lowered-block index is a loop header.
+    pub fn is_loop_header(&self, block: BlockIndex) -> bool {
+        self.loop_headers.contains(&block)
+    }
+
     /// Determine whether the given lowered block index is an indirect branch
     /// target.
     pub fn is_indirect_branch_target(&self, block: BlockIndex) -> bool {

cranelift/codegen/src/machinst/mod.rs

@@ -157,7 +157,7 @@ pub trait MachInst: Clone + Debug {
 
     /// Align a basic block offset (from start of function).  By default, no
     /// alignment occurs.
-    fn align_basic_block(offset: CodeOffset) -> CodeOffset {
+    fn align_basic_block(offset: CodeOffset, _loop_header: bool) -> CodeOffset {
         offset
     }
 

cranelift/codegen/src/machinst/vcode.rs

@@ -849,7 +849,8 @@ impl<I: VCodeInst> VCode<I> {
 
         for (block_order_idx, &block) in final_order.iter().enumerate() {
             trace!("emitting block {:?}", block);
-            let new_offset = I::align_basic_block(buffer.cur_offset());
+            let new_offset =
+                I::align_basic_block(buffer.cur_offset(), self.block_order.is_loop_header(block));
             while new_offset > buffer.cur_offset() {
                 // Pad with NOPs up to the aligned block offset.
                 let nop = I::gen_nop((new_offset - buffer.cur_offset()) as usize);