Re: [PATCH Rust front-end v3 01/46] Use DW_ATE_UTF for the Rust 'char' type

[Mark Wielaard <mark@klomp.org>]

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Hi,

On Wed, Oct 26, 2022 at 10:39:09AM +0200, Jakub Jelinek wrote:
> I must say I don't understand nor like this DW_LANG_Rust_old stuff at all.
> Other languages don't do similar dances.
> Look for D, or Go.  Neither of them has any non-standard lang code as
> fallback, they use the DWARF assigned DW_LANG_* code, and DW_LANG_C as
> fallback.  On most arches, DWARF 5 is the default anyway, or non-strict
> DWARF at least.  Where neither is enabled because of prehistoric or buggy
> DWARF consumers, it is unlikely they'd handle Rust sanely anyway.
> Just follow what Go does in the same function.

DW_LANG_Rust_old was used by old rustc compilers <= 2016 before DWARF5
assigned an official number. It might be recognized by some
debuggers. But I agree that these days it doesn't really make sense to
emit it. When producing strict DWARF it is also slightly odd to emit a
non-standard language code. So I agree that it makes sense to do what
Go does, always emit DW_LANG_Rust unless we emit strict DWARF for
versions before 5 (and then just fall back to DW_LANG_C).

The attached patch (against "upstream gccrs") does that. I kept the
oldlang.rs testcase just to see that the -gstrict-dwarf -gdwarf-3 case
does something sane.

The only "issue" is that is_rust () depends on the comp_unit_die
DW_AT_language being DW_LANG_Rust. But the only usage of is_rust
already depends on strict DWARF.

https://code.wildebeest.org/git/user/mjw/gccrs/commit/?h=no-Rust-old
if someone wants to push that, to merge for a v4.

Thanks,

Mark

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From cdcfe27cfba23402f91200c64c1ef8e0bf3528a0 Mon Sep 17 00:00:00 2001
From: Mark Wielaard <mark@klomp.org>
Date: Sun, 30 Oct 2022 16:03:16 +0100
Subject: [PATCH] dwarf2out.c: Don't emit DW_LANG_Rust_old

DW_LANG_Rust_old is a non-standard DWARF language code used by old
rustc compilers before DWARF5 (released in 2017). Just always emit
DW_LANG_Rust unless producing strict DWARF for versions before 5.
And in that old strict DWARF case just emit DW_LANG_C instead of a
non-standard language code.
---
 gcc/dwarf2out.cc                    | 14 +++++---------
 gcc/testsuite/rust/debug/oldlang.rs |  4 ++--
 2 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/gcc/dwarf2out.cc b/gcc/dwarf2out.cc
index 7b9d5ae33fc..87c0d103a27 100644
--- a/gcc/dwarf2out.cc
+++ b/gcc/dwarf2out.cc
@@ -5600,14 +5600,15 @@ is_fortran (const_tree decl)
   return is_fortran ();
 }
 
-/* Return TRUE if the language is Rust.  */
+/* Return TRUE if the language is Rust.
+   Note, returns FALSE for dwarf_version < 5 && dwarf_strict. */
 
 static inline bool
 is_rust ()
 {
   unsigned int lang = get_AT_unsigned (comp_unit_die (), DW_AT_language);
 
-  return lang == DW_LANG_Rust || lang == DW_LANG_Rust_old;
+  return lang == DW_LANG_Rust;
 }
 
 /* Return TRUE if the language is Ada.  */
@@ -25216,13 +25217,6 @@ gen_compile_unit_die (const char *filename)
     }
   else if (strcmp (language_string, "GNU F77") == 0)
     language = DW_LANG_Fortran77;
-  else if (strcmp (language_string, "GNU Rust") == 0)
-    {
-      if (dwarf_version >= 5 || !dwarf_strict)
-	language = DW_LANG_Rust;
-      else
-	language = DW_LANG_Rust_old;
-    }
   else if (dwarf_version >= 3 || !dwarf_strict)
     {
       if (strcmp (language_string, "GNU Ada") == 0)
@@ -25248,6 +25242,8 @@ gen_compile_unit_die (const char *filename)
 	{
 	  if (strcmp (language_string, "GNU Go") == 0)
 	    language = DW_LANG_Go;
+	  else if (strcmp (language_string, "GNU Rust") == 0)
+	    language = DW_LANG_Rust;
 	}
     }
   /* Use a degraded Fortran setting in strict DWARF2 so is_fortran works.  */
diff --git a/gcc/testsuite/rust/debug/oldlang.rs b/gcc/testsuite/rust/debug/oldlang.rs
index ddacf0e4392..648d6b78f06 100644
--- a/gcc/testsuite/rust/debug/oldlang.rs
+++ b/gcc/testsuite/rust/debug/oldlang.rs
@@ -1,6 +1,6 @@
 fn main () {
 // { dg-do compile }
 // { dg-options "-gstrict-dwarf -gdwarf-3 -dA" }
-// DW_LANG_Rust_old is 0x9000
-// { dg-final { scan-assembler "0x9000\[ \t]\[^\n\r]* DW_AT_language" } } */
+// Strict DWARF < 5 uses DW_LANG_C = 0x0002
+// { dg-final { scan-assembler "0x2\[ \t]\[^\n\r]* DW_AT_language" } } */
 }
-- 
2.30.2