[pulseaudio-discuss] [PATCH webrtc-audio-processing] configure.ac: fix architecture detection

[Thomas Petazzoni]

The current architecture detection, based on the "host_cpu" part of the
tuple does not work properly for a number of reason:

 - The code assumes that if host_cpu starts with "arm" then ARM
   instructions are available, which is incorrect. Indeed, Cortex-M
   platforms can run Linux, they are ARM platforms (so host_cpu = arm),
   but they don't support ARM instructions: they support only the
   Thumb-2 instruction set.

 - The armv7 case is also not very useful, as it is not standard at all
   to pass armv7 as host_cpu even if the host system is actually ARMv7
   based.

 - For the same reason, the armv8 case is not very useful: ARMv8 is
   AArch64, and there is already a separate case to handle this
   architecture.

So, this commit moves away from a host_cpu based logic, and instead
tests using AC_CHECK_DECLS() the built-in definitions of the compiler:

 - If we have __ARM_ARCH_ISA_ARM defined, then it's an ARM processor
   that supports the ARM instruction set (this allows to exclude Thumb-2
   only processors).

 - If we have __ARM_ARCH_7A__, then we have an ARMv7-A processor, and
   we can enable the corresponding optimizations

 - Same for __aarch64__, __i386__ and __x86_64__.

In addition, we remove the AC_MSG_ERROR() that makes the build fail for
all architectures but the ones that are explicitly supported. Indeed,
webrtc-audio-processing builds just fine for other architectures (tested
on MIPS), it's just that none of the architecture-specific optimizations
will be used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni at free-electrons.com>
---
 configure.ac | 35 +++++++++++------------------------
 1 file changed, 11 insertions(+), 24 deletions(-)

diff --git a/configure.ac b/configure.ac
index b4b9ddf..acbb3e2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -83,30 +83,17 @@ AC_SUBST(PLATFORM_CFLAGS)
 AM_CONDITIONAL(HAVE_POSIX, [test "x${HAVE_POSIX}" = "x1"])
 AM_CONDITIONAL(HAVE_WIN, [test "x${HAVE_WIN}" = "x1"])
 
-AS_CASE(["${host_cpu}"],
-    [i?86|x86_64],
-        [
-         HAVE_X86=1
-        ],
-    [armv7*|armv8*],
-        [
-         HAVE_ARM=1
-         HAVE_ARMV7=1
-         ARCH_CFLAGS="-DWEBRTC_ARCH_ARM -DWEBRTC_ARCH_ARM_V7"
-        ],
-    [arm*],
-        [
-         HAVE_ARM=1
-         ARCH_CFLAGS="-DWEBRTC_ARCH_ARM"
-        ],
-    [aarch64*],
-        [
-         HAVE_NEON=1
-         ARCH_CFLAGS="-DWEBRTC_HAS_NEON -DWEBRTC_ARCH_ARM64"
-        ],
-    # FIXME: Add MIPS support, see webrtc/BUILD.gn for defines
-    [AC_MSG_ERROR([Unsupported CPU type $host_cpu])]
-)
+# Testing __ARM_ARCH_ISA_ARM since the code contains ARM instructions,
+# which don't work on Thumb-2 only platforms (ARMv7-M).
+AC_CHECK_DECLS([__ARM_ARCH_ISA_ARM],
+	[HAVE_ARM=1; ARCH_CFLAGS="${ARCH_CFLAGS} -DWEBRTC_ARCH_ARM"])
+AC_CHECK_DECLS([__ARM_ARCH_7A__],
+	[HAVE_ARMV7=1; ARCH_CFLAGS="${ARCH_CFLAGS} -DWEBRTC_ARCH_ARM_V7"])
+AC_CHECK_DECLS([__aarch64__],
+	[HAVE_NEON=1; ARCH_CFLAGS="${ARCH_CFLAGS} -DWEBRTC_HAS_NEON -DWEBRTC_ARCH_ARM64"])
+AC_CHECK_DECLS([__i386__], [HAVE_X86=1])
+AC_CHECK_DECLS([__x86_64__], [HAVE_X86=1])
+
 AM_CONDITIONAL(HAVE_X86, [test "x${HAVE_X86}" = "x1"])
 AM_CONDITIONAL(HAVE_ARM, [test "x${HAVE_ARM}" = "x1"])
 AM_CONDITIONAL(HAVE_ARMV7, [test "x${HAVE_ARMV7}" = "x1"])
-- 
2.7.4