From 12bfbdfaca3ecd3da95b30670df1b71fdba01be0 Mon Sep 17 00:00:00 2001
From: Adam Bratschi-Kaye <ark.email@gmail.com>
Date: Mon, 8 Nov 2021 16:31:04 +0100
Subject: [PATCH] Skip generating DWARF info for dead code (#3498)

When encountering a subprogram that is dead code (as indicated by the
dead code proposal
https://dwarfstd.org/ShowIssue.php?issue=200609.1), don't generate debug
output for the subprogram or any of its children.
---
 crates/cranelift/src/debug/transform/unit.rs |  20 ++++++++++++++++
 tests/all/debug/testsuite/dead_code.c        |  19 +++++++++++++++
 tests/all/debug/testsuite/dead_code.wasm     | Bin 0 -> 1174 bytes
 tests/all/debug/translate.rs                 |  24 +++++++++++++++++++
 4 files changed, 63 insertions(+)
 create mode 100644 tests/all/debug/testsuite/dead_code.c
 create mode 100755 tests/all/debug/testsuite/dead_code.wasm

diff --git a/crates/cranelift/src/debug/transform/unit.rs b/crates/cranelift/src/debug/transform/unit.rs
index e4164942cb..4ef08b66f2 100644
--- a/crates/cranelift/src/debug/transform/unit.rs
+++ b/crates/cranelift/src/debug/transform/unit.rs
@@ -244,6 +244,15 @@ where
     Ok(wrapper_die_id)
 }
 
+fn is_dead_code<R: Reader>(entry: &DebuggingInformationEntry<R>) -> bool {
+    const TOMBSTONE: u64 = u32::MAX as u64;
+
+    match entry.attr_value(gimli::DW_AT_low_pc) {
+        Ok(Some(AttributeValue::Addr(addr))) => addr == TOMBSTONE,
+        _ => false,
+    }
+}
+
 pub(crate) fn clone_unit<'a, R>(
     dwarf: &gimli::Dwarf<R>,
     unit: Unit<R, R::Offset>,
@@ -346,12 +355,20 @@ where
     let mut current_value_range = InheritedAttr::new();
     let mut current_scope_ranges = InheritedAttr::new();
     while let Some((depth_delta, entry)) = entries.next_dfs()? {
+        // If `skip_at_depth` is `Some` then we previously decided to skip over
+        // a node and all it's children. Let A be the last node processed, B be
+        // the first node skipped, C be previous node, and D the current node.
+        // Then `cached` is the difference from A to B, `depth` is the diffence
+        // from B to C, and `depth_delta` is the differenc from C to D.
         let depth_delta = if let Some((depth, cached)) = skip_at_depth {
+            // `new_depth` = B to D
             let new_depth = depth + depth_delta;
+            // if D is below B continue to skip
             if new_depth > 0 {
                 skip_at_depth = Some((new_depth, cached));
                 continue;
             }
+            // otherwise process D with `depth_delta` being the difference from A to D
             skip_at_depth = None;
             new_depth + cached
         } else {
@@ -361,8 +378,11 @@ where
         if !context
             .reachable
             .contains(&entry.offset().to_unit_section_offset(&unit))
+            || is_dead_code(&entry)
         {
             // entry is not reachable: discarding all its info.
+            // Here B = C so `depth` is 0. A is the previous node so `cached` =
+            // `depth_delta`.
             skip_at_depth = Some((0, depth_delta));
             continue;
         }
diff --git a/tests/all/debug/testsuite/dead_code.c b/tests/all/debug/testsuite/dead_code.c
new file mode 100644
index 0000000000..61067a78da
--- /dev/null
+++ b/tests/all/debug/testsuite/dead_code.c
@@ -0,0 +1,19 @@
+int bar(int a)
+{
+  int b[50];
+  b[0] = a;
+  b[29] = a;
+  return a;
+}
+
+int baz(int a);
+
+__attribute__((export_name("foo"))) int foo()
+{
+  return baz(10);
+}
+
+__attribute__((noinline)) int baz(int a)
+{
+  return a + 5;
+}
diff --git a/tests/all/debug/testsuite/dead_code.wasm b/tests/all/debug/testsuite/dead_code.wasm
new file mode 100755
index 0000000000000000000000000000000000000000..b8c28a1f63d432907ef2838b3b7fe91cc7818bb7
GIT binary patch
literal 1174
zcmcIjOK;Oa5T5bcNouzyX%20Lgi@&p5Nh0prfG{>2&hQ8q4E&r;`*^kUF<lroj`kO
zRfGiMMh_ejXCycv!~r41fuF#Yf6!lm1k5^pAP`4Zns0Z$*_oZ0-32QS5CCw9tpTh7
zZm;1sZZkzDPEr_xl*tiO-+3%+*;6bN@F0vfCD2T-?IoV&ds`fkHNy}jwIq`v2l*Wu
zps7$-^AFQy7D`Z8HZ=H(TG{1xw?*#F@C%(x2MH8Oz#woj(RkXQQj~tu5Zki*Ob{<#
z5g@)MWb!`gACB}i*hitVcZ8tm1S-j)>swl*CR{Tk1R#wOWQ82Weot%6{}DUYA^V1G
zikuV1fzJdLP(RU>BmWEPL0Nw2h;)CDkSVQB2a)8Syw~Z^f=@?z=)ftm+ASUnA0Z;=
zQ4*0_iR5U!#6t(mwj<ud0-n~33e)o=C-nl}qKc$4k;9qGqtCC5F?~V1sGS*W6faRI
zrhqisry4~{YzCx}V^#UfukV$4vWrll%gIgv)0Z?&qk0iVgn@Br6l<ib!lY?Sv;=r|
zbiuyY=DWnmut<oA-}NCeiJK<DgvUv2SiWzh55h?Gu9wggaJl80PU!NoBeqi4(>Fk+
zsuL2UAVi2RF%1<}T~t`6DPf-$O=Edf2Gwz*@AT<!cz8W=5;;C~yVT=R?1k;pbfr91
zo+_PPPm)f&U>GegS?}6qCkzbV@AYA_6NMYxNlI>I&T7`2+3Jit=h{>D?3^=QtGag8
z;g#u%TbpUxRX%S!=ir~kQL)Fh8S7!d4a>CxgFoo-$P0LzdUBsWWkPGPtq8n!0^)bs
z))rV`gK5@RR_d$fjmF(&^XBUHWzz(>CbzAC<9QhhrYYP_hZ<`-Nf^cW`&X63Pi2gP
zWrq<?m}Z<<&ONge(lI=OM>s&Wx?P95BjrS+x>k!LyE;D)gJf}d9CLTl-b5vB<L&>`
FzF)0q4O;*J

literal 0
HcmV?d00001

diff --git a/tests/all/debug/translate.rs b/tests/all/debug/translate.rs
index db5e2af935..4e72dcd39f 100644
--- a/tests/all/debug/translate.rs
+++ b/tests/all/debug/translate.rs
@@ -43,6 +43,30 @@ fn check_line_program(wasm_path: &str, directives: &str) -> Result<()> {
     Ok(())
 }
 
+#[test]
+#[ignore]
+#[cfg(all(
+    any(target_os = "linux", target_os = "macos"),
+    target_pointer_width = "64"
+))]
+fn test_debug_dwarf_translate_dead_code() -> Result<()> {
+    check_wasm(
+        "tests/all/debug/testsuite/dead_code.wasm",
+        r##"
+check: DW_TAG_compile_unit
+# We don't have "bar" function because it is dead code
+not:      DW_AT_name	("bar")
+# We have "foo" function
+check: DW_TAG_subprogram
+check:      DW_AT_name	("foo")
+# We have "baz" function
+# it was marked `noinline` so isn't dead code
+check: DW_TAG_subprogram
+check:      DW_AT_name	("baz")
+    "##,
+    )
+}
+
 #[test]
 #[ignore]
 #[cfg(all(