[PATCH 2/2] riscv: Restrict DWARF5 when building with LLVM to known working versions

[Nathan Chancellor]

On Fri, Dec 01, 2023 at 08:31:12PM -0800, Fangrui Song wrote:
> Perhaps: Clang generates .uleb128 with label differences for DWARF v5,
> a feature that older binutils ports, utilizing RISC-V style linker
> relaxation, do not support.
> 
> (Signed LEB128 is used by DWARF as well, but it is encoded directly
> into DW_OP_fbreg/DW_LNS_advance_line/etc, and explicit .sleb128
> directives are not emitted)
> (In assemblers, "label difference" is a more common term.)
> (People working on DWARF tend to prefer DWARF v5 to DWARF5, when the
> full "DWARF version 5" feels too verbose.)

Thanks, does this diff seem reasonable to you? I'd like to keep the
issue link as a paper trail and I figured that we might as well make the
configuration name slightly more accurate with that additional
information. I will send it as a separate patch in v2 tomorrow if so in
order to give people a chance to further chime in on the series.

Cheers,
Nathan

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a0ebce05a368..76c2cc697573 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -231,9 +231,10 @@ config DEBUG_INFO
 	  in the "Debug information" choice below, indicating that debug
 	  information will be generated for build targets.
 
-# Clang is known to generate .{s,u}leb128 with symbol deltas with DWARF5, which
-# some targets may not support: https://sourceware.org/bugzilla/show_bug.cgi?id=27215
-config AS_HAS_NON_CONST_LEB128
+# Clang generates .uleb128 with label differences for DWARF v5, a feature that
+# older binutils ports do not support when utilizing RISC-V style linker
+# relaxation: https://sourceware.org/bugzilla/show_bug.cgi?id=27215
+config AS_HAS_NON_CONST_ULEB128
 	def_bool $(as-instr,.uleb128 .Lexpr_end4 - .Lexpr_start3\n.Lexpr_start3:\n.Lexpr_end4:)
 
 choice
@@ -258,7 +259,7 @@ config DEBUG_INFO_NONE
 config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
 	bool "Rely on the toolchain's implicit default DWARF version"
 	select DEBUG_INFO
-	depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
+	depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_ULEB128)
 	help
 	  The implicit default version of DWARF debug info produced by a
 	  toolchain changes over time.
@@ -283,7 +284,7 @@ config DEBUG_INFO_DWARF5
 	bool "Generate DWARF Version 5 debuginfo"
 	select DEBUG_INFO
 	depends on !ARCH_HAS_BROKEN_DWARF5
-	depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
+	depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_ULEB128)
 	help
 	  Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
 	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some