proper fix for stapsdt note and gc-sections interaction

Author: conradludgate

tokio/src/util/usdt/stapsdt.rs

@@ -35,16 +35,13 @@ macro_rules! call_probe {
     ) => {
         // <https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation>
         // <https://github.com/cdkey/systemtap/blob/cbb34b7244ba60cb0904d61dc9167290855106aa/includes/sys/sdt.h#L240-L254>
-        //
-        // The `R` here means "retained". It prevents the linker from GC the note.
-        // This additionally means that the function containing the `nop` is also not GCd.
-        //
-        // This is not great, but without this we get notes that are not GCd but the functions
-        // they reference are GCd. This causes notes to have incorrect addresses.
         std::arch::asm!(
-            "990:   nop
-                    .pushsection .note.stapsdt, \"R\", \"note\"
-                    .balign 4
+            "990:   nop",
+            // We deviate from what systemtap source code does here.
+            // This is to fix an issue with how gc-sections is implemented while linking.
+            // This problem seems to manifest only when monomorphising certain functions with many CGUs.
+            concat!(".pushsection .note.stapsdt, \"G\", \"note\", \".note.stapsdt.tokio.", $name, "\", comdat"),
+                    ".balign 4
                     .4byte 992f-991f, 994f-993f, 3
             991:
                     .asciz \"stapsdt\"
@@ -76,16 +73,13 @@ macro_rules! call_probe {
     ) => {
         // <https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation>
         // <https://github.com/cdkey/systemtap/blob/cbb34b7244ba60cb0904d61dc9167290855106aa/includes/sys/sdt.h#L240-L254>
-        //
-        // The `R` here means "retained". It prevents the linker from GC the note.
-        // This additionally means that the function containing the `nop` is also not GCd.
-        //
-        // This is not great, but without this we get notes that are not GCd but the functions
-        // they reference are GCd. This causes notes to have incorrect addresses.
         std::arch::asm!(
-            "990:   nop
-                    .pushsection .note.stapsdt, \"R\", \"note\"
-                    .balign 4
+            "990:   nop",
+            // We deviate from what systemtap source code does here.
+            // This is to fix an issue with how gc-sections is implemented while linking.
+            // This problem seems to manifest only when monomorphising certain functions with many CGUs.
+            concat!(".pushsection .note.stapsdt, \"G\", \"note\", \".note.stapsdt.tokio.", $name, "\", comdat"),
+                    ".balign 4
                     .4byte 992f-991f, 994f-993f, 3
             991:
                     .asciz \"stapsdt\"