Implement jump tables (#453)

* Add 'jump_table_entry' and 'indirect_jump' instructions.

* Update CodeSink to keep track of code size. Pretty up clif-util's disassembly output.

* Only disassemble the machine portion of output. Pretty print the read-only data after it.

* Update switch frontend code to use new br_table instruction w/ default.

lib/codegen/src/binemit/memorysink.rs

@@ -32,6 +32,8 @@ use std::ptr::write_unaligned;
 pub struct MemoryCodeSink<'a> {
     data: *mut u8,
     offset: isize,
+    /// Size of the machine code portion of output
+    pub code_size: isize,
     relocs: &'a mut RelocSink,
     traps: &'a mut TrapSink,
 }
@@ -49,6 +51,7 @@ impl<'a> MemoryCodeSink<'a> {
         MemoryCodeSink {
             data,
             offset: 0,
+            code_size: 0,
             relocs,
             traps,
         }
@@ -131,6 +134,10 @@ impl<'a> CodeSink for MemoryCodeSink<'a> {
         let ofs = self.offset();
         self.traps.trap(ofs, srcloc, code);
     }
+
+    fn begin_rodata(&mut self) {
+        self.code_size = self.offset;
+    }
 }
 
 /// A `TrapSink` implementation that does nothing, which is convenient when

lib/codegen/src/binemit/mod.rs

@@ -94,6 +94,9 @@ pub trait CodeSink {
 
     /// Add trap information for the current offset.
     fn trap(&mut self, TrapCode, SourceLoc);
+
+    /// Code output is complete, read-only data may follow.
+    fn begin_rodata(&mut self);
 }
 
 /// Report a bad encoding error.
@@ -123,4 +126,20 @@ where
             emit_inst(func, inst, &mut divert, sink);
         }
     }
+
+    sink.begin_rodata();
+
+    // output jump tables
+    for (jt, jt_data) in func.jump_tables.iter() {
+        let jt_offset = func.jt_offsets[jt];
+        for idx in 0..jt_data.len() {
+            match jt_data.get_entry(idx) {
+                Some(ebb) => {
+                    let rel_offset: i32 = func.offsets[ebb] as i32 - jt_offset as i32;
+                    sink.put4(rel_offset as u32)
+                }
+                None => sink.put4(0),
+            }
+        }
+    }
 }

lib/codegen/src/binemit/relaxation.rs

@@ -95,6 +95,13 @@ pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> CodegenResult<Cod
         }
     }
 
+    for (jt, jt_data) in func.jump_tables.iter() {
+        func.jt_offsets[jt] = offset;
+        // TODO: this should be computed based on the min size needed to hold
+        //        the furthest branch.
+        offset += jt_data.len() as u32 * 4;
+    }
+
     Ok(offset)
 }