Add a fold_redundant_jumps() pass to the branch relaxation phase. (#887)

cranelift/codegen/src/ir/function.rs

@@ -8,7 +8,7 @@ use crate::entity::{PrimaryMap, SecondaryMap};
 use crate::ir;
 use crate::ir::{DataFlowGraph, ExternalName, Layout, Signature};
 use crate::ir::{
-    Ebb, ExtFuncData, FuncRef, GlobalValue, GlobalValueData, Heap, HeapData, JumpTable,
+    Ebb, ExtFuncData, FuncRef, GlobalValue, GlobalValueData, Heap, HeapData, Inst, JumpTable,
     JumpTableData, SigRef, StackSlot, StackSlotData, Table, TableData,
 };
 use crate::ir::{EbbOffsets, InstEncodings, SourceLocs, StackSlots, ValueLocations};
@@ -20,7 +20,7 @@ use crate::write::write_function;
 use core::fmt;
 
 #[cfg(feature = "basic-blocks")]
-use crate::ir::{Inst, Opcode};
+use crate::ir::Opcode;
 
 /// A function.
 ///
@@ -223,6 +223,15 @@ impl Function {
         self.dfg.collect_debug_info();
     }
 
+    /// Changes the destination of a jump or branch instruction.
+    /// Does nothing if called with a non-jump or non-branch instruction.
+    pub fn change_branch_destination(&mut self, inst: Inst, new_dest: Ebb) {
+        match self.dfg[inst].branch_destination_mut() {
+            None => (),
+            Some(inst_dest) => *inst_dest = new_dest,
+        }
+    }
+
     /// Checks that the specified EBB can be encoded as a basic block.
     ///
     /// On error, returns the first invalid instruction and an error message.