From 33cc4959dca16ded32cf2d1f546ccbfd37991c8c Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Mon, 10 Jun 2024 17:07:39 -0500
Subject: [PATCH] Fix meaning of generated `.debug_loc` sections (#8753)

This commit is a fix to Wasmtime's DWARF processing transform to correct
the meaning of the `.debug_loc` section. This section's addresses are
relative to the `DW_AT_low_pc` entry located in the
`DW_TAG_compile_unit` container, but Wasmtime's construction of this
section didn't take this into account. Instead all addresses in
`.debug_loc` are relative to the start of the compiled object, not to
the start of the compile unit itself. This commit fixes this by
unconditionally describing `DW_TAG_compile_unit` locations with
`DW_AT_ranges` instead of `DW_AT_low_pc`. This ends up fixing debug
information for debug information using `.debug_loc` with multiple
codegen units.

Closes #8752
---
 .../src/debug/transform/range_info_builder.rs | 15 ++++++-
 .../src/bin/dwarf_multiple_codegen_units.rs   | 14 +++++++
 tests/all/debug/lldb.rs                       | 42 +++++++++++++++++++
 3 files changed, 70 insertions(+), 1 deletion(-)
 create mode 100644 crates/test-programs/src/bin/dwarf_multiple_codegen_units.rs

diff --git a/crates/cranelift/src/debug/transform/range_info_builder.rs b/crates/cranelift/src/debug/transform/range_info_builder.rs
index 568eeac940..980a495018 100644
--- a/crates/cranelift/src/debug/transform/range_info_builder.rs
+++ b/crates/cranelift/src/debug/transform/range_info_builder.rs
@@ -145,7 +145,20 @@ impl RangeInfoBuilder {
                 for (begin, end) in ranges {
                     result.extend(addr_tr.translate_ranges(*begin, *end));
                 }
-                if result.len() != 1 {
+
+                // If we're seeing the ranges for a `DW_TAG_compile_unit` DIE
+                // then don't use `DW_AT_low_pc` and `DW_AT_high_pc`. These
+                // attributes, if set, will configure the base address of all
+                // location lists that this unit refers to. Currently this
+                // debug transform does not take this base address into account
+                // when generate the `.debug_loc` section. Consequently when a
+                // compile unit is configured here the `DW_AT_ranges` attribute
+                // is unconditionally used instead of
+                // `DW_AT_low_pc`/`DW_AT_high_pc`.
+                let is_attr_for_compile_unit =
+                    out_unit.get(current_scope_id).tag() == gimli::DW_TAG_compile_unit;
+
+                if result.len() != 1 || is_attr_for_compile_unit {
                     let range_list = result
                         .iter()
                         .map(|tr| write::Range::StartLength {
diff --git a/crates/test-programs/src/bin/dwarf_multiple_codegen_units.rs b/crates/test-programs/src/bin/dwarf_multiple_codegen_units.rs
new file mode 100644
index 0000000000..afbfa94ffc
--- /dev/null
+++ b/crates/test-programs/src/bin/dwarf_multiple_codegen_units.rs
@@ -0,0 +1,14 @@
+fn main() {
+    let a = 3;
+    foo::bar(a);
+}
+
+mod foo {
+    pub fn bar(x: u32) -> u32 {
+        let mut sum = 0;
+        for i in 0..x {
+            sum += i;
+        }
+        sum
+    }
+}
diff --git a/tests/all/debug/lldb.rs b/tests/all/debug/lldb.rs
index ee347a162c..232ab540f7 100644
--- a/tests/all/debug/lldb.rs
+++ b/tests/all/debug/lldb.rs
@@ -376,4 +376,46 @@ check: exited with status = 0
     fn dwarf_shared_memory() -> Result<()> {
         test_dwarf_simple(DWARF_SHARED_MEMORY, &[])
     }
+
+    #[test]
+    #[ignore]
+    fn dwarf_multiple_codegen_units() -> Result<()> {
+        for wasm in [
+            DWARF_MULTIPLE_CODEGEN_UNITS,
+            DWARF_MULTIPLE_CODEGEN_UNITS_COMPONENT,
+        ] {
+            println!("testing {wasm:?}");
+            let output = lldb_with_script(
+                &["-Ccache=n", "-Oopt-level=0", "-Ddebug-info", wasm],
+                r#"
+breakpoint set --file dwarf_multiple_codegen_units.rs --line 3
+breakpoint set --file dwarf_multiple_codegen_units.rs --line 10
+r
+fr v
+c
+fr v
+breakpoint delete 2
+finish
+c"#,
+            )?;
+
+            check_lldb_output(
+                &output,
+                r#"
+check: Breakpoint 1: no locations (pending)
+check: Breakpoint 2: no locations (pending)
+check: stop reason = breakpoint 1.1
+check: foo::bar(a)
+check: a = 3
+check: sum += i
+check: x = 3
+check: sum = 0
+check: 1 breakpoints deleted
+check: Return value: $(=.*) 3
+check: exited with status = 0
+"#,
+            )?;
+        }
+        Ok(())
+    }
 }