Record information about sections of emitted code+data.

The result of the emitter is a vector of bytes holding machine code,
jump tables, and (in the future) other read-only data.  Some clients,
notably Firefox's Wasm compiler, needs to separate the machine code
from the data in order to insert more code directly after the code
generated by Cranelift.

To make such separation possible, we record more information about the
emitted bytes: the sizes of each of the sections of code, jump tables,
and read-only data, as well as the locations within the code that
reference (PC-relatively) the jump tables and read-only data.

cranelift/codegen/src/binemit/relaxation.rs

@@ -27,7 +27,7 @@
 //! ebb23:
 //! ```
 
-use crate::binemit::CodeOffset;
+use crate::binemit::{CodeInfo, CodeOffset};
 use crate::cursor::{Cursor, FuncCursor};
 use crate::ir::{Function, InstructionData, Opcode};
 use crate::isa::{EncInfo, TargetIsa};
@@ -40,7 +40,7 @@ use log::debug;
 /// Relax branches and compute the final layout of EBB headers in `func`.
 ///
 /// Fill in the `func.offsets` table so the function is ready for binary emission.
-pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> CodegenResult<CodeOffset> {
+pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> CodegenResult<CodeInfo> {
     let _tt = timing::relax_branches();
 
     let encinfo = isa.encoding_info();
@@ -109,6 +109,9 @@ pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> CodegenResult<Cod
         }
     }
 
+    let code_size = offset;
+    let jumptables = offset;
+
     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
@@ -116,7 +119,19 @@ pub fn relax_branches(func: &mut Function, isa: &TargetIsa) -> CodegenResult<Cod
         offset += jt_data.len() as u32 * 4;
     }
 
-    Ok(offset)
+    let jumptables_size = offset - jumptables;
+    let rodata = offset;
+
+    // TODO: Once we have constant pools we'll do some processing here to update offset.
+
+    let rodata_size = offset - rodata;
+
+    Ok(CodeInfo {
+        code_size,
+        jumptables_size,
+        rodata_size,
+        total_size: offset,
+    })
 }
 
 /// Convert `jump` instructions to `fallthrough` instructions where possible and verify that any