Migrate cranelift-wasm to brif (#5638)

Incrementally working towards removing brz and brnz completely.

cranelift/frontend/src/frontend.rs

@@ -680,14 +680,14 @@ impl<'a> FunctionBuilder<'a> {
     ///
     /// **Note:** You are responsible for maintaining the coherence with the arguments of
     /// other jump instructions.
-    pub fn change_jump_destination(&mut self, inst: Inst, new_dest: Block) {
+    pub fn change_jump_destination(&mut self, inst: Inst, old_block: Block, new_block: Block) {
         let dfg = &mut self.func.dfg;
-        for old_dest in dfg.insts[inst].branch_destination_mut() {
-            self.func_ctx
-                .ssa
-                .remove_block_predecessor(old_dest.block(&dfg.value_lists), inst);
-            old_dest.set_block(new_dest, &mut dfg.value_lists);
-            self.func_ctx.ssa.declare_block_predecessor(new_dest, inst);
+        for block in dfg.insts[inst].branch_destination_mut() {
+            if block.block(&dfg.value_lists) == old_block {
+                self.func_ctx.ssa.remove_block_predecessor(old_block, inst);
+                block.set_block(new_block, &mut dfg.value_lists);
+                self.func_ctx.ssa.declare_block_predecessor(new_block, inst);
+            }
         }
     }
 

cranelift/wasm/src/code_translator.rs

@@ -292,6 +292,7 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
         Operator::If { blockty } => {
             let val = state.pop1();
 
+            let next_block = builder.create_block();
             let (params, results) = blocktype_params_results(validator, *blockty)?;
             let (destination, else_data) = if params.clone().eq(results.clone()) {
                 // It is possible there is no `else` block, so we will only
@@ -301,21 +302,38 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                 // up discovering an `else`, then we will allocate a block for it
                 // and go back and patch the jump.
                 let destination = block_with_params(builder, results.clone(), environ)?;
-                let branch_inst =
-                    canonicalise_then_brz(builder, val, destination, state.peekn(params.len()));
-                (destination, ElseData::NoElse { branch_inst })
+                let branch_inst = canonicalise_brif(
+                    builder,
+                    val,
+                    next_block,
+                    &[],
+                    destination,
+                    state.peekn(params.len()),
+                );
+                (
+                    destination,
+                    ElseData::NoElse {
+                        branch_inst,
+                        placeholder: destination,
+                    },
+                )
             } else {
                 // The `if` type signature is not valid without an `else` block,
                 // so we eagerly allocate the `else` block here.
                 let destination = block_with_params(builder, results.clone(), environ)?;
                 let else_block = block_with_params(builder, params.clone(), environ)?;
-                canonicalise_then_brz(builder, val, else_block, state.peekn(params.len()));
+                canonicalise_brif(
+                    builder,
+                    val,
+                    next_block,
+                    &[],
+                    else_block,
+                    state.peekn(params.len()),
+                );
                 builder.seal_block(else_block);
                 (destination, ElseData::WithElse { else_block })
             };
 
-            let next_block = builder.create_block();
-            canonicalise_then_jump(builder, next_block, &[]);
             builder.seal_block(next_block); // Only predecessor is the current block.
             builder.switch_to_block(next_block);
 
@@ -357,7 +375,10 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                         // Ensure we have a block for the `else` block (it may have
                         // already been pre-allocated, see `ElseData` for details).
                         let else_block = match *else_data {
-                            ElseData::NoElse { branch_inst } => {
+                            ElseData::NoElse {
+                                branch_inst,
+                                placeholder,
+                            } => {
                                 let (params, _results) =
                                     blocktype_params_results(validator, blocktype)?;
                                 debug_assert_eq!(params.len(), num_return_values);
@@ -370,7 +391,11 @@ pub fn translate_operator<FE: FuncEnvironment + ?Sized>(
                                 );
                                 state.popn(params.len());
 
-                                builder.change_jump_destination(branch_inst, else_block);
+                                builder.change_jump_destination(
+                                    branch_inst,
+                                    placeholder,
+                                    else_block,
+                                );
                                 builder.seal_block(else_block);
                                 else_block
                             }
@@ -2171,6 +2196,7 @@ fn translate_unreachable_operator<FE: FuncEnvironment + ?Sized>(
                 ir::Block::reserved_value(),
                 ElseData::NoElse {
                     branch_inst: ir::Inst::reserved_value(),
+                    placeholder: ir::Block::reserved_value(),
                 },
                 0,
                 0,
@@ -2198,7 +2224,10 @@ fn translate_unreachable_operator<FE: FuncEnvironment + ?Sized>(
                         state.reachable = true;
 
                         let else_block = match *else_data {
-                            ElseData::NoElse { branch_inst } => {
+                            ElseData::NoElse {
+                                branch_inst,
+                                placeholder,
+                            } => {
                                 let (params, _results) =
                                     blocktype_params_results(validator, blocktype)?;
                                 let else_block = block_with_params(builder, params, environ)?;
@@ -2206,7 +2235,11 @@ fn translate_unreachable_operator<FE: FuncEnvironment + ?Sized>(
                                 frame.truncate_value_stack_to_else_params(&mut state.stack);
 
                                 // We change the target of the branch instruction.
-                                builder.change_jump_destination(branch_inst, else_block);
+                                builder.change_jump_destination(
+                                    branch_inst,
+                                    placeholder,
+                                    else_block,
+                                );
                                 builder.seal_block(else_block);
                                 else_block
                             }
@@ -2816,10 +2849,9 @@ fn translate_br_if(
 ) {
     let val = state.pop1();
     let (br_destination, inputs) = translate_br_if_args(relative_depth, state);
-    canonicalise_then_brnz(builder, val, br_destination, inputs);
-
     let next_block = builder.create_block();
-    canonicalise_then_jump(builder, next_block, &[]);
+    canonicalise_brif(builder, val, br_destination, inputs, next_block, &[]);
+
     builder.seal_block(next_block); // The only predecessor is the current block.
     builder.switch_to_block(next_block);
 }
@@ -3120,28 +3152,28 @@ fn canonicalise_then_jump(
     builder.ins().jump(destination, canonicalised)
 }
 
-/// The same but for a `brz` instruction.
-fn canonicalise_then_brz(
-    builder: &mut FunctionBuilder,
-    cond: ir::Value,
-    destination: ir::Block,
-    params: &[Value],
-) -> ir::Inst {
-    let mut tmp_canonicalised = SmallVec::<[ir::Value; 16]>::new();
-    let canonicalised = canonicalise_v128_values(&mut tmp_canonicalised, builder, params);
-    builder.ins().brz(cond, destination, canonicalised)
-}
-
-/// The same but for a `brnz` instruction.
-fn canonicalise_then_brnz(
+/// The same but for a `brif` instruction.
+fn canonicalise_brif(
     builder: &mut FunctionBuilder,
     cond: ir::Value,
-    destination: ir::Block,
-    params: &[Value],
+    block_then: ir::Block,
+    params_then: &[ir::Value],
+    block_else: ir::Block,
+    params_else: &[ir::Value],
 ) -> ir::Inst {
-    let mut tmp_canonicalised = SmallVec::<[ir::Value; 16]>::new();
-    let canonicalised = canonicalise_v128_values(&mut tmp_canonicalised, builder, params);
-    builder.ins().brnz(cond, destination, canonicalised)
+    let mut tmp_canonicalised_then = SmallVec::<[ir::Value; 16]>::new();
+    let canonicalised_then =
+        canonicalise_v128_values(&mut tmp_canonicalised_then, builder, params_then);
+    let mut tmp_canonicalised_else = SmallVec::<[ir::Value; 16]>::new();
+    let canonicalised_else =
+        canonicalise_v128_values(&mut tmp_canonicalised_else, builder, params_else);
+    builder.ins().brif(
+        cond,
+        block_then,
+        canonicalised_then,
+        block_else,
+        canonicalised_else,
+    )
 }
 
 /// A helper for popping and bitcasting a single value; since SIMD values can lose their type by

cranelift/wasm/src/state.rs

@@ -22,6 +22,9 @@ pub enum ElseData {
         /// instruction that needs to be fixed up to point to the new `else`
         /// block rather than the destination block after the `if...end`.
         branch_inst: Inst,
+
+        /// The placeholder block we're replacing.
+        placeholder: Block,
     },
 
     /// We have already allocated an `else` block.
@@ -43,9 +46,8 @@ pub enum ElseData {
 /// - `num_return_values`: number of values returned by the control block;
 /// - `original_stack_size`: size of the value stack at the beginning of the control block.
 ///
-/// Moreover, the `if` frame has the `branch_inst` field that points to the `brz` instruction
-/// separating the `true` and `false` branch. The `loop` frame has a `header` field that references
-/// the `Block` that contains the beginning of the body of the loop.
+/// The `loop` frame has a `header` field that references the `Block` that contains the beginning
+/// of the body of the loop.
 #[derive(Debug)]
 pub enum ControlStackFrame {
     If {