[dpdk-dev] [RFC PATCH] build/pkg-config: remove machine arch flag

[Bruce Richardson <bruce.richardson@intel.com>]

Traditionally any apps built using DPDK had to support the same
instruction sets as supported when the DPDK SDK itself was built, since
that was "leaked" through to the end-app and DPDK headers via
RTE_MACHINE_CPUFLAG_* values. Therefore, when converting over to meson
for app build correctness the "-march=" flag was included in the
pkg-config cflags output.

However, since the DPDK-specific CPU flags are now obsolete, and we
instead check directly for compiler-defined flags, we should no longer
need to force the exact same architecture match in all cases. To
faciliate such flexibility, a new pkg_config_machine_args array - which
defaults to the existing machine_args array has been defined. The
individual architectures - x86, arm and ppc - can choose if and how to
override this value themselves.

For x86, since SSE4.2 is the minimum instruction-set level needed to run
DPDK, and since some header files assume that minimum level of
instruction set support, we override the "-march=" value with "-msse4"
for the pkg-config file. This allows end applications to set their own
"march" value while still ensuring valid DPDK compilation.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
---

CC: Jerin Jacob <jerinj@marvell.com>
CC: Ruifeng Wang <ruifeng.wang@arm.com>
CC: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
CC: David Christensen <drc@linux.vnet.ibm.com>
Feedback requested from ARM and PPC maintainers as to this change
and what flags, if any, need to be in the .pc file for DPDK on such
platforms. For example - is setting 'pkg_config_machine_args' to
'machine_args' by default necessary behaviour?

---
 buildtools/pkg-config/meson.build | 2 +-
 config/meson.build                | 1 +
 config/x86/meson.build            | 3 +++
 3 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/buildtools/pkg-config/meson.build b/buildtools/pkg-config/meson.build
index 5f1930428..8e2e24681 100644
--- a/buildtools/pkg-config/meson.build
+++ b/buildtools/pkg-config/meson.build
@@ -2,7 +2,7 @@
 # Copyright(c) 2020 Intel Corporation

 pkg = import('pkgconfig')
-pkg_extra_cflags = ['-include', 'rte_config.h'] + machine_args
+pkg_extra_cflags = ['-include', 'rte_config.h'] + pkg_config_machine_args
 if is_freebsd
 	pkg_extra_cflags += ['-D__BSD_VISIBLE']
 endif
diff --git a/config/meson.build b/config/meson.build
index 3ddcc3539..d58ff781d 100644
--- a/config/meson.build
+++ b/config/meson.build
@@ -100,6 +100,7 @@ if host_machine.cpu_family().startswith('ppc')
 else
 	machine_args += '-march=' + machine
 endif
+pkg_config_machine_args = machine_args # may be overriden late per arch

 toolchain = cc.get_id()
 dpdk_conf.set_quoted('RTE_TOOLCHAIN', toolchain)
diff --git a/config/x86/meson.build b/config/x86/meson.build
index 31bfa63b1..fcd75502e 100644
--- a/config/x86/meson.build
+++ b/config/x86/meson.build
@@ -16,6 +16,9 @@ if cc.get_define('__SSE4_2__', args: machine_args) == ''
 	machine_args += '-msse4'
 endif

+# require SSE4.2 in pkg-config file - don't use -march
+pkg_config_machine_args = ['-msse4']
+
 base_flags = ['SSE', 'SSE2', 'SSE3','SSSE3', 'SSE4_1', 'SSE4_2']
 foreach f:base_flags
 	compile_time_cpuflags += ['RTE_CPUFLAG_' + f]
--
2.27.0