Further compress the in-memory representation of address maps (#2324)

This commit reduces the size of `InstructionAddressMap` from 24 bytes to
8 bytes by dropping the `code_len` field and reducing `code_offset` to
`u32` instead of `usize`. The intention is to primarily make the
in-memory version take up less space, and the hunch is that the
`code_len` is largely not necessary since most entries in this map are
always adjacent to one another. The `code_len` field is now implied by
the `code_offset` field of the next entry in the map.

This isn't as big of an improvement to serialized module size as #2321
or #2322, primarily because of the switch to variable-length encoding.
Despite this though it shaves about 10MB off the encoded size of the
module from #2318

crates/debug/src/transform/address_transform.rs

@@ -102,7 +102,7 @@ fn build_function_lookup(
     let mut ranges_index = BTreeMap::new();
     let mut current_range = Vec::new();
     let mut last_gen_inst_empty = false;
-    for t in &ft.instructions {
+    for (i, t) in ft.instructions.iter().enumerate() {
         if t.srcloc.is_default() {
             continue;
         }
@@ -111,8 +111,11 @@ fn build_function_lookup(
         assert_le!(fn_start, offset);
         assert_le!(offset, fn_end);
 
-        let inst_gen_start = t.code_offset;
-        let inst_gen_end = t.code_offset + t.code_len;
+        let inst_gen_start = t.code_offset as usize;
+        let inst_gen_end = match ft.instructions.get(i + 1) {
+            Some(i) => i.code_offset as usize,
+            None => ft.body_len as usize,
+        };
 
         if last_wasm_pos > offset {
             // Start new range.
@@ -149,7 +152,7 @@ fn build_function_lookup(
         }
         last_wasm_pos = offset;
     }
-    let last_gen_addr = ft.body_offset + ft.body_len;
+    let last_gen_addr = ft.body_offset + ft.body_len as usize;
     ranges_index.insert(range_wasm_start, ranges.len());
     ranges.push(Range {
         wasm_start: range_wasm_start,
@@ -193,13 +196,13 @@ fn build_function_addr_map(
     for (_, f) in funcs {
         let ft = &f.address_map;
         let mut fn_map = Vec::new();
-        for t in &ft.instructions {
+        for t in ft.instructions.iter() {
             if t.srcloc.is_default() {
                 continue;
             }
             let offset = get_wasm_code_offset(t.srcloc, code_section_offset);
             fn_map.push(AddressMap {
-                generated: t.code_offset,
+                generated: t.code_offset as usize,
                 wasm: offset,
             });
         }
@@ -213,7 +216,7 @@ fn build_function_addr_map(
 
         map.push(FunctionMap {
             offset: ft.body_offset,
-            len: ft.body_len,
+            len: ft.body_len as usize,
             wasm_start: get_wasm_code_offset(ft.start_srcloc, code_section_offset),
             wasm_end: get_wasm_code_offset(ft.end_srcloc, code_section_offset),
             addresses: fn_map.into_boxed_slice(),
@@ -605,6 +608,7 @@ mod tests {
     use super::{build_function_lookup, get_wasm_code_offset, AddressTransform};
     use gimli::write::Address;
     use std::iter::FromIterator;
+    use std::mem;
     use wasmtime_environ::entity::PrimaryMap;
     use wasmtime_environ::ir::SourceLoc;
     use wasmtime_environ::{CompiledFunction, WasmFileInfo};
@@ -626,14 +630,21 @@ mod tests {
                 InstructionAddressMap {
                     srcloc: SourceLoc::new(wasm_offset + 2),
                     code_offset: 5,
-                    code_len: 3,
+                },
+                InstructionAddressMap {
+                    srcloc: SourceLoc::default(),
+                    code_offset: 8,
                 },
                 InstructionAddressMap {
                     srcloc: SourceLoc::new(wasm_offset + 7),
                     code_offset: 15,
-                    code_len: 8,
                 },
-            ],
+                InstructionAddressMap {
+                    srcloc: SourceLoc::default(),
+                    code_offset: 23,
+                },
+            ]
+            .into(),
             start_srcloc: SourceLoc::new(wasm_offset),
             end_srcloc: SourceLoc::new(wasm_offset + 10),
             body_offset: 0,
@@ -678,11 +689,16 @@ mod tests {
     fn test_build_function_lookup_two_ranges() {
         let mut input = create_simple_func(11);
         // append instruction with same srcloc as input.instructions[0]
-        input.instructions.push(InstructionAddressMap {
+        let mut list = Vec::from(mem::take(&mut input.instructions));
+        list.push(InstructionAddressMap {
             srcloc: SourceLoc::new(11 + 2),
             code_offset: 23,
-            code_len: 3,
         });
+        list.push(InstructionAddressMap {
+            srcloc: SourceLoc::default(),
+            code_offset: 26,
+        });
+        input.instructions = list.into();
         let (start, end, lookup) = build_function_lookup(&input, 1);
         assert_eq!(10, start);
         assert_eq!(20, end);

crates/debug/src/transform/expression.rs

@@ -1145,14 +1145,21 @@ mod tests {
                     InstructionAddressMap {
                         srcloc: SourceLoc::new(code_section_offset + 12),
                         code_offset: 5,
-                        code_len: 3,
+                    },
+                    InstructionAddressMap {
+                        srcloc: SourceLoc::default(),
+                        code_offset: 8,
                     },
                     InstructionAddressMap {
                         srcloc: SourceLoc::new(code_section_offset + 17),
                         code_offset: 15,
-                        code_len: 8,
                     },
-                ],
+                    InstructionAddressMap {
+                        srcloc: SourceLoc::default(),
+                        code_offset: 23,
+                    },
+                ]
+                .into(),
                 start_srcloc: SourceLoc::new(code_section_offset + 10),
                 end_srcloc: SourceLoc::new(code_section_offset + 20),
                 body_offset: 0,