[Buildroot] [PATCH] toolchain: add variadic MI thunk support flag

[Buildroot] [PATCH] toolchain: add variadic MI thunk support flag

[Max Filippov]

GCC uses thunk functions to adjust the 'this' pointer when calling
C++ member functions in classes derived with multiple inheritance.
Generation of thunk functions requires support from the compiler
back end. In the absence of that support target-independent code
in the C++ front end is used to generate thunk functions, but it
does not support vararg functions.

Support for this feature is currently missing in or1k and xtensa
toolchains.

Add flag BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK that indicates
presence of this feature in the toolchain. Add dependency to packages
that require this feature to be built.

Fixes:
http://autobuild.buildroot.net/results/c9e660c764edbd7cf0ae54ab0f0f412464721446/
http://autobuild.buildroot.net/results/9a3bf4b411c418ea78d59e35d23ba865dd453890/

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
---
 package/libtorrent-rasterbar/Config.in |  6 ++++--
 toolchain/Config.in                    | 13 +++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/package/libtorrent-rasterbar/Config.in b/package/libtorrent-rasterbar/Config.in
index aa986f2388e0..e9a1a40fb9b1 100644
--- a/package/libtorrent-rasterbar/Config.in
+++ b/package/libtorrent-rasterbar/Config.in
@@ -1,6 +1,7 @@
-comment "libtorrent-rasterbar needs a toolchain w/ C++, threads, wchar, gcc >= 4.8"
+comment "libtorrent-rasterbar needs a toolchain w/ C++, threads, wchar, variadic MI thunks, gcc >= 4.8"
 	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS || !BR2_USE_WCHAR || \
-		!BR2_HOST_GCC_AT_LEAST_4_8 || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
+		!BR2_HOST_GCC_AT_LEAST_4_8 || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 || \
+		!BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK
 
 config BR2_PACKAGE_LIBTORRENT_RASTERBAR
 	bool "libtorrent-rasterbar"
@@ -8,6 +9,7 @@ config BR2_PACKAGE_LIBTORRENT_RASTERBAR
 	depends on BR2_HOST_GCC_AT_LEAST_4_8
 	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
 	depends on BR2_TOOLCHAIN_HAS_THREADS # boost
+	depends on BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK
 	depends on BR2_USE_WCHAR # boost
 	select BR2_PACKAGE_BOOST
 	select BR2_PACKAGE_BOOST_CHRONO
diff --git a/toolchain/Config.in b/toolchain/Config.in
index baf192c936d4..5109f66946ef 100644
--- a/toolchain/Config.in
+++ b/toolchain/Config.in
@@ -93,6 +93,19 @@ config BR2_TOOLCHAIN_SUPPORTS_ALWAYS_LOCKFREE_ATOMIC_INTS
 	depends on !BR2_sparc_v8
 	depends on !BR2_m68k_cf5208
 
+# GCC uses thunk functions to adjust the 'this' pointer when calling
+# C++ member functions in classes derived with multiple inheritance.
+# Generation of thunk functions requires support from the compiler
+# back end. In the absence of that support target-independent code
+# in the C++ front end is used to generate thunk functions, but it
+# does not support vararg functions.
+# or1k will support it in the gcc mainline starting with gcc-9.
+config BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK
+	bool
+	default y
+	depends on !BR2_or1k
+	depends on !BR2_xtensa
+
 # Prior to gcc 7.x, exception_ptr, nested_exception and future from
 # libstdc++ would only be provided on architectures that support
 # always lock-free atomic ints. See
-- 
2.11.0

[Buildroot] [PATCH] toolchain: add variadic MI thunk support flag

[Thomas Petazzoni]

On Fri, 25 Jan 2019 15:34:52 -0800
Max Filippov <jcmvbkbc@gmail.com> wrote:

> GCC uses thunk functions to adjust the 'this' pointer when calling
> C++ member functions in classes derived with multiple inheritance.
> Generation of thunk functions requires support from the compiler
> back end. In the absence of that support target-independent code
> in the C++ front end is used to generate thunk functions, but it
> does not support vararg functions.
> 
> Support for this feature is currently missing in or1k and xtensa
> toolchains.
> 
> Add flag BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK that indicates
> presence of this feature in the toolchain. Add dependency to packages
> that require this feature to be built.
> 
> Fixes:
> http://autobuild.buildroot.net/results/c9e660c764edbd7cf0ae54ab0f0f412464721446/
> http://autobuild.buildroot.net/results/9a3bf4b411c418ea78d59e35d23ba865dd453890/
> 
> Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

Thanks for providing this patch!

> diff --git a/package/libtorrent-rasterbar/Config.in b/package/libtorrent-rasterbar/Config.in
> index aa986f2388e0..e9a1a40fb9b1 100644
> --- a/package/libtorrent-rasterbar/Config.in
> +++ b/package/libtorrent-rasterbar/Config.in
> @@ -1,6 +1,7 @@
> -comment "libtorrent-rasterbar needs a toolchain w/ C++, threads, wchar, gcc >= 4.8"
> +comment "libtorrent-rasterbar needs a toolchain w/ C++, threads, wchar, variadic MI thunks, gcc >= 4.8"

Actually, because the user can't do anything about variadic MI thunk
support, there is not much point in mentioning it in the comment. So
I've removed that from the comment...

>  	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_HAS_THREADS || !BR2_USE_WCHAR || \
> -		!BR2_HOST_GCC_AT_LEAST_4_8 || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
> +		!BR2_HOST_GCC_AT_LEAST_4_8 || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_8 || \
> +		!BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK

... and changed this to:

	depends on BR2_TOOLCHAIN_SUPPORTS_VARIADIC_MI_THUNK

so that the Config.in comment does not show up (just like the package
doesn't show up) on architectures where the toolchain does not have
variadic MI thunk support.

Applied with this change. Thanks!

Thomas
-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com