cranelift: Remove redundant branch and select instructions (#5097)

As discussed in the 2022/10/19 meeting, this PR removes many of the branch and select instructions that used iflags, in favor if using brz/brnz and select in their place. Additionally, it reworks selectif_spectre_guard to take an i8 input instead of an iflags input.

For reference, the removed instructions are: br_icmp, brif, brff, trueif, trueff, and selectif.

cranelift/codegen/src/legalizer/heap.rs

@@ -250,11 +250,11 @@ fn compute_addr(
     if let Some((cc, a, b)) = spectre_oob_comparison {
         let final_addr = pos.ins().iadd(base, offset);
         let zero = pos.ins().iconst(addr_ty, 0);
-        let flags = pos.ins().ifcmp(a, b);
+        let cmp = pos.ins().icmp(cc, a, b);
         pos.func
             .dfg
             .replace(inst)
-            .selectif_spectre_guard(addr_ty, cc, flags, zero, final_addr);
+            .select_spectre_guard(cmp, zero, final_addr);
     } else {
         pos.func.dfg.replace(inst).iadd(base, offset);
     }

cranelift/codegen/src/legalizer/mod.rs

@@ -54,27 +54,6 @@ pub fn simple_legalize(func: &mut ir::Function, cfg: &mut ControlFlowGraph, isa:
         while let Some(inst) = pos.next_inst() {
             match pos.func.dfg[inst] {
                 // control flow
-                InstructionData::BranchIcmp {
-                    opcode: ir::Opcode::BrIcmp,
-                    cond,
-                    destination,
-                    ref args,
-                } => {
-                    let a = args.get(0, &pos.func.dfg.value_lists).unwrap();
-                    let b = args.get(1, &pos.func.dfg.value_lists).unwrap();
-                    let block_args = args.as_slice(&pos.func.dfg.value_lists)[2..].to_vec();
-
-                    let old_block = pos.func.layout.pp_block(inst);
-                    pos.func.dfg.clear_results(inst);
-
-                    let icmp_res = pos.func.dfg.replace(inst).icmp(cond, a, b);
-                    let mut pos = FuncCursor::new(pos.func).after_inst(inst);
-                    pos.use_srcloc(inst);
-                    pos.ins().brnz(icmp_res, destination, &block_args);
-
-                    cfg.recompute_block(pos.func, destination);
-                    cfg.recompute_block(pos.func, old_block);
-                }
                 InstructionData::CondTrap {
                     opcode:
                         opcode @ (ir::Opcode::Trapnz | ir::Opcode::Trapz | ir::Opcode::ResumableTrapnz),

cranelift/codegen/src/legalizer/table.rs

@@ -99,15 +99,11 @@ fn compute_addr(
     };
 
     let element_addr = if let Some((index, bound)) = spectre_oob_cmp {
-        let flags = pos.ins().ifcmp(index, bound);
+        let cond = pos
+            .ins()
+            .icmp(IntCC::UnsignedGreaterThanOrEqual, index, bound);
         // If out-of-bounds, choose the table base on the misspeculation path.
-        pos.ins().selectif_spectre_guard(
-            addr_ty,
-            IntCC::UnsignedGreaterThanOrEqual,
-            flags,
-            base,
-            element_addr,
-        )
+        pos.ins().select_spectre_guard(cond, base, element_addr)
     } else {
         element_addr
     };