[v2] meson: add support for clang LTO
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Commit Message
Currently, building DPDK with clang + LTO is unsupported, which is a
showstopper for cross-language interprocedural optimizations. The root
cause is that pmdinfogen expects to scan through the section headers of
generated ELF object files, but clang's "-flto" flag generates LLVM
bitcode instead. This patch enables LTO builds with clang, provided that
the linker is set to lld.
The mechanism of action is clang's "-fembed-bitcode" flag, which acts
similarly to gcc's "-ffat-lto-objects" - the compiler outputs a normal
ELF object file that keeps pmdinfogen happy, but also embeds IR bitcode
in a special section of the ELF that lld can then use to construct the
final binary.
Signed-off-by: Mahdi Rakhshandehroo <mahdi.rakhshandehroo@gmail.com>
---
Forgot to remove get_linker_id() check from the previous patch - that
function is only available in Meson >= 0.54.
config/meson.build | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)
Comments
On Thu, 31 Mar 2022 21:54:35 -0700
Mahdi Rakhshandehroo <mahdi.rakhshandehroo@gmail.com> wrote:
> Currently, building DPDK with clang + LTO is unsupported, which is a
> showstopper for cross-language interprocedural optimizations. The root
> cause is that pmdinfogen expects to scan through the section headers of
> generated ELF object files, but clang's "-flto" flag generates LLVM
> bitcode instead. This patch enables LTO builds with clang, provided that
> the linker is set to lld.
>
> The mechanism of action is clang's "-fembed-bitcode" flag, which acts
> similarly to gcc's "-ffat-lto-objects" - the compiler outputs a normal
> ELF object file that keeps pmdinfogen happy, but also embeds IR bitcode
> in a special section of the ELF that lld can then use to construct the
> final binary.
>
> Signed-off-by: Mahdi Rakhshandehroo <mahdi.rakhshandehroo@gmail.com>
> ---
This patch still has several build failures such as Intel compiler.
Please resolve and resubmit
@@ -406,16 +406,21 @@ if is_windows
endif
if get_option('b_lto')
- if cc.has_argument('-ffat-lto-objects')
+ if cc.get_id() == 'gcc' and cc.has_argument('-ffat-lto-objects')
add_project_arguments('-ffat-lto-objects', language: 'c')
+
+ # workaround for gcc bug 81440
+ if cc.version().version_compare('<8.0')
+ add_project_arguments('-Wno-lto-type-mismatch', language: 'c')
+ add_project_link_arguments('-Wno-lto-type-mismatch', language: 'c')
+ endif
+ elif cc.get_id() == 'clang' and cc.has_argument('-fembed-bitcode')
+ # TODO: enforce use of lld using cc.get_linker_id() with Meson >= 0.54
+ add_project_arguments('-fno-lto', '-fembed-bitcode', language: 'c')
+ endif
else
error('compiler does not support fat LTO objects - please turn LTO off')
endif
- # workaround for gcc bug 81440
- if cc.get_id() == 'gcc' and cc.version().version_compare('<8.0')
- add_project_arguments('-Wno-lto-type-mismatch', language: 'c')
- add_project_link_arguments('-Wno-lto-type-mismatch', language: 'c')
- endif
endif
if get_option('b_sanitize') == 'address' or get_option('b_sanitize') == 'address,undefined'