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/cranelift/src/lib.rs

@@ -229,19 +229,28 @@ impl StackMapSink {
 fn get_function_address_map<'data>(
     context: &Context,
     data: &FunctionBodyData<'data>,
-    body_len: usize,
+    body_len: u32,
     isa: &dyn isa::TargetIsa,
 ) -> FunctionAddressMap {
+    // Generate artificial srcloc for function start/end to identify boundary
+    // within module.
+    let data = data.body.get_binary_reader();
+    let offset = data.original_position();
+    let len = data.bytes_remaining();
+    assert!((offset + len) <= u32::max_value() as usize);
+    let start_srcloc = ir::SourceLoc::new(offset as u32);
+    let end_srcloc = ir::SourceLoc::new((offset + len) as u32);
+
     let instructions = if let Some(ref mcr) = &context.mach_compile_result {
         // New-style backend: we have a `MachCompileResult` that will give us `MachSrcLoc` mapping
         // tuples.
-        collect_address_maps(mcr.buffer.get_srclocs_sorted().into_iter().map(
-            |&MachSrcLoc { start, end, loc }| InstructionAddressMap {
-                srcloc: loc,
-                code_offset: start as usize,
-                code_len: (end - start) as usize,
-            },
-        ))
+        collect_address_maps(
+            body_len,
+            mcr.buffer
+                .get_srclocs_sorted()
+                .into_iter()
+                .map(|&MachSrcLoc { start, end, loc }| (loc, start, (end - start))),
+        )
     } else {
         // Old-style backend: we need to traverse the instruction/encoding info in the function.
         let func = &context.func;
@@ -250,28 +259,16 @@ fn get_function_address_map<'data>(
 
         let encinfo = isa.encoding_info();
         collect_address_maps(
+            body_len,
             blocks
                 .into_iter()
                 .flat_map(|block| func.inst_offsets(block, &encinfo))
-                .map(|(offset, inst, size)| InstructionAddressMap {
-                    srcloc: func.srclocs[inst],
-                    code_offset: offset as usize,
-                    code_len: size as usize,
-                }),
+                .map(|(offset, inst, size)| (func.srclocs[inst], offset, size)),
         )
     };
 
-    // Generate artificial srcloc for function start/end to identify boundary
-    // within module. Similar to FuncTranslator::cur_srcloc(): it will wrap around
-    // if byte code is larger than 4 GB.
-    let data = data.body.get_binary_reader();
-    let offset = data.original_position();
-    let len = data.bytes_remaining();
-    let start_srcloc = ir::SourceLoc::new(offset as u32);
-    let end_srcloc = ir::SourceLoc::new((offset + len) as u32);
-
     FunctionAddressMap {
-        instructions,
+        instructions: instructions.into(),
         start_srcloc,
         end_srcloc,
         body_offset: 0,
@@ -283,23 +280,54 @@ fn get_function_address_map<'data>(
 // into a `FunctionAddressMap`. This will automatically coalesce adjacent
 // instructions which map to the same original source position.
 fn collect_address_maps(
-    iter: impl IntoIterator<Item = InstructionAddressMap>,
+    code_size: u32,
+    iter: impl IntoIterator<Item = (ir::SourceLoc, u32, u32)>,
 ) -> Vec<InstructionAddressMap> {
     let mut iter = iter.into_iter();
-    let mut cur = match iter.next() {
+    let (mut cur_loc, mut cur_offset, mut cur_len) = match iter.next() {
         Some(i) => i,
         None => return Vec::new(),
     };
     let mut ret = Vec::new();
-    for item in iter {
-        if cur.code_offset + cur.code_len == item.code_offset && item.srcloc == cur.srcloc {
-            cur.code_len += item.code_len;
-        } else {
-            ret.push(cur);
-            cur = item;
+    for (loc, offset, len) in iter {
+        // If this instruction is adjacent to the previous and has the same
+        // source location then we can "coalesce" it with the current
+        // instruction.
+        if cur_offset + cur_len == offset && loc == cur_loc {
+            cur_len += len;
+            continue;
         }
+
+        // Push an entry for the previous source item.
+        ret.push(InstructionAddressMap {
+            srcloc: cur_loc,
+            code_offset: cur_offset,
+        });
+        // And push a "dummy" entry if necessary to cover the span of ranges,
+        // if any, between the previous source offset and this one.
+        if cur_offset + cur_len != offset {
+            ret.push(InstructionAddressMap {
+                srcloc: ir::SourceLoc::default(),
+                code_offset: cur_offset + cur_len,
+            });
+        }
+        // Update our current location to get extended later or pushed on at
+        // the end.
+        cur_loc = loc;
+        cur_offset = offset;
+        cur_len = len;
     }
-    ret.push(cur);
+    ret.push(InstructionAddressMap {
+        srcloc: cur_loc,
+        code_offset: cur_offset,
+    });
+    if cur_offset + cur_len != code_size {
+        ret.push(InstructionAddressMap {
+            srcloc: ir::SourceLoc::default(),
+            code_offset: cur_offset + cur_len,
+        });
+    }
+
     return ret;
 }
 
@@ -406,7 +434,8 @@ impl Compiler for Cranelift {
             CompileError::Codegen(pretty_error(&context.func, Some(isa), error))
         })?;
 
-        let address_transform = get_function_address_map(&context, &input, code_buf.len(), isa);
+        let address_transform =
+            get_function_address_map(&context, &input, code_buf.len() as u32, isa);
 
         let ranges = if tunables.debug_info {
             let ranges = context.build_value_labels_ranges(isa).map_err(|error| {

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,

crates/environ/src/address_map.rs

@@ -7,22 +7,24 @@ use serde::{Deserialize, Serialize};
 /// Single source location to generated address mapping.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq)]
 pub struct InstructionAddressMap {
-    /// Original source location.
+    /// Where in the source this instruction comes from.
     pub srcloc: ir::SourceLoc,
 
-    /// Generated instructions offset.
-    pub code_offset: usize,
-
-    /// Generated instructions length.
-    pub code_len: usize,
+    /// Offset from the start of the function's compiled code to where this
+    /// instruction is located, or the region where it starts.
+    pub code_offset: u32,
 }
 
 /// Function and its instructions addresses mappings.
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, Default)]
 pub struct FunctionAddressMap {
-    /// Instructions maps.
-    /// The array is sorted by the InstructionAddressMap::code_offset field.
-    pub instructions: Vec<InstructionAddressMap>,
+    /// An array of data for the instructions in this function, indicating where
+    /// each instruction maps back to in the original function.
+    ///
+    /// This array is sorted least-to-greatest by the `code_offset` field.
+    /// Additionally the span of each `InstructionAddressMap` is implicitly the
+    /// gap between it and the next item in the array.
+    pub instructions: Box<[InstructionAddressMap]>,
 
     /// Function start source location (normally declaration).
     pub start_srcloc: ir::SourceLoc,
@@ -34,7 +36,7 @@ pub struct FunctionAddressMap {
     pub body_offset: usize,
 
     /// Generated function body length.
-    pub body_len: usize,
+    pub body_len: u32,
 }
 
 /// Memory definition offset in the VMContext structure.

crates/wasmtime/src/frame_info.rs

@@ -64,7 +64,7 @@ impl GlobalFrameInfo {
         // Use our relative position from the start of the function to find the
         // machine instruction that corresponds to `pc`, which then allows us to
         // map that to a wasm original source location.
-        let rel_pos = pc - func.start;
+        let rel_pos = (pc - func.start) as u32;
         let pos = match func
             .instr_map
             .instructions
@@ -77,19 +77,8 @@ impl GlobalFrameInfo {
             // instructions cover `pc`.
             Err(0) => None,
 
-            // This would be at the `nth` slot, so check `n-1` to see if we're
-            // part of that instruction. This happens due to the minus one when
-            // this function is called form trap symbolication, where we don't
-            // always get called with a `pc` that's an exact instruction
-            // boundary.
-            Err(n) => {
-                let instr = &func.instr_map.instructions[n - 1];
-                if instr.code_offset <= rel_pos && rel_pos < instr.code_offset + instr.code_len {
-                    Some(n - 1)
-                } else {
-                    None
-                }
-            }
+            // This would be at the `nth` slot, so we're at the `n-1`th slot.
+            Err(n) => Some(n - 1),
         };
 
         // In debug mode for now assert that we found a mapping for `pc` within