Re: [build] Only support -gstabs on Mac OS X if assember supports it (PR target/67973)

[Iain Sandoe <iain at codesourcery dot com>]

Hi Rainer, 

Thanks for looking at this!

> On 14 Dec 2015, at 10:40, Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE> wrote:
> 
> As described in PR PR target/67973, newer assemblers on Mac OS X, which
> are based on LLVM instead of gas, don't support .stab* directives any
> longer.  The following patch detects this situation and tries to fall
> back to the older gas-based as if it is still accessible via as -Q.
> 
> Tested on x86_64-apple-darwin15.2.0 and as expected the -gstabs* tests
> now pass.
> 
> However, I'm not really comfortable with this solution.  Initially, I
> forgot to wrap the -Q option to as in %{gstabs*:...}, which lead to a
> bootstrap failure: the gas- and LLVM-based assemblers differ in a
> number of other ways, as can be seen when comparing gcc/auto-host.h:

FAOD, 
the changes below only occur if you omit the guard on â-Qâ ?
or they are present always?

> --- build.llvm-as/gcc/auto-host.h	2015-11-27 10:53:31.000000000 +0100
> +++ build.gas/gcc/auto-host.h	2015-12-04 20:25:30.000000000 +0100
> @@ -351 +357 @@
> -/* #undef HAVE_AS_GDWARF2_DEBUG_FLAG */
> +#define HAVE_AS_GDWARF2_DEBUG_FLAG 1
> @@ -369 +375 @@
> -/* #undef HAVE_AS_GSTABS_DEBUG_FLAG */
> +#define HAVE_AS_GSTABS_DEBUG_FLAG 1
> @@ -388 +394 @@
> -/* #undef HAVE_AS_IX86_FFREEP */
> +#define HAVE_AS_IX86_FFREEP 1
> @@ -412 +418 @@
> -#define HAVE_AS_IX86_INTERUNIT_MOVQ 1
> +#define HAVE_AS_IX86_INTERUNIT_MOVQ 0
> @@ -424 +430 @@
> -#define HAVE_AS_IX86_REP_LOCK_PREFIX 1
> +/* #undef HAVE_AS_IX86_REP_LOCK_PREFIX */
> @@ -1176 +1182 @@

> -#define HAVE_GAS_CFI_PERSONALITY_DIRECTIVE 1
> +#define HAVE_GAS_CFI_PERSONALITY_DIRECTIVE 0
> @@ -1179 +1185 @@
> -#define HAVE_GAS_CFI_SECTIONS_DIRECTIVE 1
> +#define HAVE_GAS_CFI_SECTIONS_DIRECTIVE 0

^^^ These two are definitely not safe without other changes (I have a local patch, but it might not be stage3 stuff).

> @@ -1183 +1189 @@
> -#define HAVE_GAS_DISCRIMINATOR 1
> +/* #undef HAVE_GAS_DISCRIMINATOR */
> @@ -1298 +1304 @@
> -#define HAVE_GNU_AS 0
> +#define HAVE_GNU_AS 1
> 
> So, we can be pretty certain to hit cases where some file compiles and
> assembles without -gstabs, but fails to assemble with -gstabs.  Not
> exactly the user experience I prefer.
> 
> Given this, I'd rather have us not support stabs at all than via this
> half-hearted approach.

I agree, we should just disable it if the âdefaultâ assembler doesnât support - so most of your patch would be unchanged.
The actual practical outcome is really not terribly severe, it ony affects folks trying to target x86-darwin8 ( the last OS to use stabs as the first choice), and even those systems support some debugging with DWARF.

> 
> What do you think?

My view is it should still be a configure test, rather than a blanket disable for d15+ (simply because Iâve got back-ports of the newer tools that should be useable across the Darwin range, and hopefully those back-ports will be published before Xmas).  The same issues would generally apply there.

Iain

> 
> 	Rainer
> 
> 
> 2015-12-11  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
> 
> 	PR target/67973
> 	* configure.ac (gcc_cv_as_stabs_directive): New test.
> 	(gcc_cv_as_darwin_stabs_Q): New test.
> 	* configure: Regenerate.
> 	* config.in: Regenerate.
> 	* config/darwin.h (DBX_DEBUGGING_INFO): Wrap in
> 	HAVE_AS_STABS_DIRECTIVE.
> 	(PREFERRED_DEBUGGING_TYPE): Likewise.
> 	* config/i386/darwin.h (PREFERRED_DEBUGGING_TYPE): Only include
> 	DBX_DEBUG if HAVE_AS_STABS_DIRECTIVE.
> 	(ASM_STABS_Q_SPEC): Define.
> 	(ASM_SPEC): Use it.
> 
> # HG changeset patch
> # Parent  7029fd86ac40d7ff34e1c43d729c6bf469416643
> Only support -gstabs on Mac OS X if assember supports it (PR target/67973)
> 
> diff --git a/gcc/config/darwin.h b/gcc/config/darwin.h
> --- a/gcc/config/darwin.h
> +++ b/gcc/config/darwin.h
> @@ -400,12 +400,13 @@ extern GTY(()) int darwin_ms_struct;
> 
> #define ASM_DEBUG_SPEC  "%{g*:%{!g0:%{!gdwarf*:--gstabs}}}"
> 
> -/* We still allow output of STABS.  */
> -
> +/* We still allow output of STABS if the assembler supports it.  */
> +#ifdef HAVE_AS_STABS_DIRECTIVE
> #define DBX_DEBUGGING_INFO 1
> +#define PREFERRED_DEBUGGING_TYPE DBX_DEBUG
> +#endif
> 
> #define DWARF2_DEBUGGING_INFO 1
> -#define PREFERRED_DEBUGGING_TYPE DBX_DEBUG
> 
> #define DEBUG_FRAME_SECTION	"__DWARF,__debug_frame,regular,debug"
> #define DEBUG_INFO_SECTION	"__DWARF,__debug_info,regular,debug"
> diff --git a/gcc/config/i386/darwin.h b/gcc/config/i386/darwin.h
> --- a/gcc/config/i386/darwin.h
> +++ b/gcc/config/i386/darwin.h
> @@ -111,9 +111,16 @@ extern int darwin_emit_branch_islands;
>   %{g: %{!fno-eliminate-unused-debug-symbols: -feliminate-unused-debug-symbols }} " \
>   DARWIN_CC1_SPEC
> 
> +/* Pass -Q to assembler if necessary for stabs support.  */
> +#ifdef HAVE_AS_DARWIN_STABS_Q
> +#define ASM_STABS_Q_SPEC " %{gstabs*:-Q}"
> +#else
> +#define ASM_STABS_Q_SPEC ""
> +#endif
> +
> #undef ASM_SPEC
> #define ASM_SPEC "-arch %(darwin_arch) -force_cpusubtype_ALL \
> -  %{static}"
> +  %{static}" ASM_STABS_Q_SPEC
> 
> #define DARWIN_ARCH_SPEC "%{m64:x86_64;:i386}"
> #define DARWIN_SUBARCH_SPEC DARWIN_ARCH_SPEC
> @@ -226,7 +233,11 @@ do {									\
>    compiles default to stabs+.  darwin9+ defaults to dwarf-2.  */
> #ifndef DARWIN_PREFER_DWARF
> #undef PREFERRED_DEBUGGING_TYPE
> +#ifdef HAVE_AS_STABS_DIRECTIVE
> #define PREFERRED_DEBUGGING_TYPE (TARGET_64BIT ? DWARF2_DEBUG : DBX_DEBUG)
> +#else
> +#define PREFERRED_DEBUGGING_TYPE DWARF2_DEBUG
> +#endif
> #endif
> 
> /* Darwin uses the standard DWARF register numbers but the default
> diff --git a/gcc/configure.ac b/gcc/configure.ac
> --- a/gcc/configure.ac
> +++ b/gcc/configure.ac
> @@ -2909,6 +2909,11 @@ AC_DEFINE_UNQUOTED(HAVE_GAS_SHF_MERGE,
>   [`if test $gcc_cv_as_shf_merge = yes; then echo 1; else echo 0; fi`],
> [Define 0/1 if your assembler supports marking sections with SHF_MERGE flag.])
> 
> +gcc_GAS_CHECK_FEATURE([stabs directive], gcc_cv_as_stabs_directive, ,,
> +[.stabs "gcc2_compiled.",60,0,0,0],,
> +[AC_DEFINE(HAVE_AS_STABS_DIRECTIVE, 1,
> +  [Define if your assembler supports .stabs.])])
> +
> gcc_GAS_CHECK_FEATURE([COMDAT group support (GNU as)],
>  gcc_cv_as_comdat_group,
>  [elf,2,16,0], [--fatal-warnings],
> @@ -3619,6 +3624,26 @@ AC_DEFINE_UNQUOTED(HAVE_LTO_PLUGIN, $gcc
>   [Define to the level of your linker's plugin support.])
> AC_MSG_RESULT($gcc_cv_lto_plugin)
> 
> +# Target OS-specific assembler checks.
> +
> +case "$target_os" in
> +  darwin*)
> +    # If the default assembler doesn't support .stabs, check if the
> +    # gas-based one does.
> +    if test x$gcc_cv_as_stabs_directive = xno; then
> +      gcc_GAS_CHECK_FEATURE([.stabs directive via -Q],
> +        gcc_cv_as_darwin_stabs_Q,, [-Q],
> +	[.stabs "gcc2_compiled.",60,0,0,0],,
> +      [AC_DEFINE(HAVE_AS_STABS_DIRECTIVE, 1,
> +        [Define if your assembler supports .stabs.])
> +       AC_DEFINE(HAVE_AS_DARWIN_STABS_Q, 1,
> +        [Define if your assembler needs -Q to support .stabs.])])
> +    fi
> +    ;;
> +esac
> +
> +# Target CPU-specific assembler checks.
> +
> case "$target" in
>   aarch64*-*-*)
>     gcc_GAS_CHECK_FEATURE([-mabi option], gcc_cv_as_aarch64_mabi,,
> 
> -- 
> -----------------------------------------------------------------------------
> Rainer Orth, Center for Biotechnology, Bielefeld University