diff options
authorGravatar Thomas Petazzoni <thomas.petazzoni@free-electrons.com>2016-02-02 16:31:20 +0100
committerGravatar Thomas Petazzoni <thomas.petazzoni@free-electrons.com>2016-02-06 11:16:00 +0100
commit6856e417da4f3aa77e2a814db2a89429af072f7d (patch)
parent971fae9896572aec3fdeca4cfa818e6f3933f717 (diff)
toolchain: add BR2_TOOLCHAIN_HAS_{SYNC_x, ATOMIC} hidden booleans
Currently, Buildroot provides one BR2_ARCH_HAS_ATOMICS boolean option to indicate whether the architecture supports atomic operations or not. However, the reality of atomic operations support is much more complicated and requires more than one option to be expressed properly. There are in fact two types of atomic built-ins provided by gcc: (1) The __sync_*() family of functions, which have been in gcc for a long time (probably gcc 4.1). They are available in variants operating on 1-byte, 2-byte, 4-byte and 8-byte integers. Some architectures implement a number of variants, some do not implement any, some implement all of them. They are now considered "legacy" by the gcc developers but are nonetheless still being used by a significant number of userspace libraries and applications. https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html (2) The __atomic_*() family of functions, which have been introduced in gcc 4.7. They have been introduced in order to support C++11 atomic operations. In gcc 4.8, they are available on all architectures, either built-in or in the libatomic library part of the gcc runtime (in which case the application needs to be linked with -latomic). In gcc 4.7, the __atomic_*() intrinsics are only supported on certain architectures, since libatomic did not exist at the time. For (1), a single BR2_ARCH_HAS_ATOMICS is not sufficient, because depending on the architecture, some variants may or may not be available. Setting BR2_ARCH_HAS_ATOMICS to false as soon as one of the variant is missing would cause a large number of packages to become unavailable, even if they in fact use only more common variants available on a large number of architectures. For this reason, we've chosen to introduce four new Config.in options: - BR2_TOOLCHAIN_HAS_SYNC_1 - BR2_TOOLCHAIN_HAS_SYNC_2 - BR2_TOOLCHAIN_HAS_SYNC_3 - BR2_TOOLCHAIN_HAS_SYNC_4 Which indicate whether the toolchain support 1-byte, 2-byte, 4-byte and 8-byte __sync_*() built-ins respectively. For (2), we introduce a BR2_TOOLCHAIN_HAS_ATOMIC, which indicates if the __atomic_*() built-ins are available. Note that it is up to the package to link with -latomic when gcc is >= 4.8. Since __atomic_*() intrinsics for all sizes are supported starting We conducted a fairly large analysis about various architectures supported by Buildroot, as well as with a number of different toolchains, to check which combinations support which variant. To do, we linked the following program with various toolchains: int main(void) { uint8_t a; uint16_t b; uint32_t c; uint64_t d; __sync_fetch_and_add(&a, 3); __sync_fetch_and_add(&b, 3); __sync_fetch_and_add(&c, 3); __sync_fetch_and_add(&d, 3); __sync_val_compare_and_swap(&a, 1, 2); __sync_val_compare_and_swap(&b, 1, 2); __sync_val_compare_and_swap(&c, 1, 2); __sync_val_compare_and_swap(&d, 1, 2); __atomic_add_fetch(&a, 3, __ATOMIC_RELAXED); __atomic_add_fetch(&b, 3, __ATOMIC_RELAXED); __atomic_add_fetch(&c, 3, __ATOMIC_RELAXED); __atomic_add_fetch(&d, 3, __ATOMIC_RELAXED); __atomic_compare_exchange_n(&a, &a, 2, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED); __atomic_compare_exchange_n(&b, &b, 2, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED); __atomic_compare_exchange_n(&c, &c, 2, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED); __atomic_compare_exchange_n(&d, &d, 2, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED); return 0; } And looked at which symbols were unresolved. For the __atomic_*() ones, we tested with and without -latomic to see which variants are built-in, which variants require libatomic. This testing effort has led to the following results: __sync __atomic gcc 1 2 4 8 1 2 4 8 ARC Y Y Y - Y Y Y L 4.8 [with BR2_ARC_ATOMIC_EXT] ARC - - - - L L L L 4.8 [without BR2_ARC_ATOMIC_EXT] ARM Y Y Y X Y Y Y Y 4.8, 4.7 ARM Y Y Y - 4.5 AArch64 Y Y Y Y Y Y Y Y 4.9, 5.1 Bfin - - Y - 4.3 i386 (i386) - - - - L L L L 4.9 i386 (i486..) Y Y Y - L L L L 4.9 [i486, c3, winchip2, winchip-c6] i386 (> i586) Y Y Y Y L L L L 4.9 Microblaze - - Y - L L Y L 4.9 MIPS Y Y Y - Y Y Y L 4.9 MIPS64 Y Y Y Y Y Y Y Y 4.9 NIOS 2 Y Y Y - Y Y Y L 4.9, 5.2 PowerPC Y Y Y - Y Y Y L 4.9 SuperH Y Y Y - Y Y Y L 4.9 SPARC - - - - L L L L 4.9 SPARC64 Y Y Y Y Y Y Y Y 4.9 x86_64 Y Y Y Y Y Y Y Y 4.7, 4.9 Xtensa Y Y Y - Y Y Y Y 4.9 Notes: * __atomic built-ins appeared in gcc 4.7, so for toolchais older than that, the __atomic column is empty. * Y means 'supported built-in' * L means 'supported via linking to libatomic' (only for __atomic functions) * X indicates a very special case for 8 bytes __sync built-ins on ARM. On ARMv7, there is no problem, starting from gcc 4.7, the __sync built-in for 8 bytes integers is implemented, fully in userspace. For cores < ARMv7, doing a 8 bytes atomic operation requires help from the kernel. Unfortunately, the libgcc code implementing this uses the __write() function to display an error, and this function is internal to glibc. Therefore, if you're using glibc everything is fine, but if you're using uClibc or musl, you cannot link an application that uses 8 bytes __sync operations. This has been fixed as part of gcc PR68095, merged in the gcc 5 branch but not yet part of any gcc release. * - means not supported This commit only introduces the new options. Follow-up commits will progressively change the packages using BR2_ARCH_HAS_ATOMICS to use the appropriate BR2_TOOLCHAIN_HAS_SYNC_x or BR2_TOOLCHAIN_HAS_ATOMIC until the point where BR2_ARCH_HAS_ATOMICS can be removed. Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
1 files changed, 79 insertions, 0 deletions
diff --git a/toolchain/toolchain-common.in b/toolchain/toolchain-common.in
index 8408852fda..1b7b41668c 100644
--- a/toolchain/toolchain-common.in
+++ b/toolchain/toolchain-common.in
@@ -307,3 +307,82 @@ config BR2_TOOLCHAIN_GCC_AT_LEAST
default "4.5" if BR2_TOOLCHAIN_GCC_AT_LEAST_4_5
default "4.4" if BR2_TOOLCHAIN_GCC_AT_LEAST_4_4
default "4.3" if BR2_TOOLCHAIN_GCC_AT_LEAST_4_3
+ bool
+ default y
+ depends on !BR2_x86_i386
+ depends on !BR2_bfin
+ depends on !BR2_microblaze
+ depends on !BR2_sparc
+ depends on !(BR2_arc && !BR2_ARC_ATOMIC_EXT)
+ bool
+ default y if BR2_TOOLCHAIN_HAS_SYNC_1
+ bool
+ default y
+ depends on !BR2_sparc
+ depends on !BR2_x86_i386
+ depends on !(BR2_arc && !BR2_ARC_ATOMIC_EXT)
+# The availability of __sync for 8-byte types on ARM is somewhat
+# complicated:
+# - It appeared in gcc starting with gcc 4.7.
+# - On ARMv7, there is no problem, it can be directly implemented in
+# userspace.
+# - On < ARMv7, it requires help from the kernel. Unfortunately, the
+# libgcc code implementing 8-byte __sync with the help from the
+# kernel calls __write() when a failure occurs, which is a function
+# internal to glibc, not available in uClibc and musl. This means
+# that the 8-byte __sync operations are not available on < ARMv7
+# with uClibc and musl. This problem was fixed as part of gcc
+# PR68059, which was backported to the gcc 5 branch, but isn't yet
+# part of any gcc 5.x release.
+ bool
+ default y
+ depends on BR2_arm || BR2_armeb
+ depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_7
+# 8-byte intrinsics available on most x86 CPUs, except a few old ones
+ bool
+ default y
+ depends on BR2_i386
+ depends on !BR2_x86_i386
+ depends on !BR2_x86_i486
+ depends on !BR2_x86_c3
+ depends on !BR2_x86_winchip_c6
+ depends on !BR2_x86_winchip2
+# 8-byte intrinsics available:
+# - On all 64 bits architecture
+# - On a certain combinations of ARM platforms
+# - On certain x86 32 bits CPUs
+ bool
+ default y if BR2_ARCH_IS_64
+ default y if BR2_TOOLCHAIN_ARM_HAS_SYNC_8
+ default y if BR2_TOOLCHAIN_X86_HAS_SYNC_8
+# __atomic intrinsics are available:
+# - with gcc 4.8, either through built-ins or libatomic, on all
+# architectures
+# - with gcc 4.7, libatomic did not exist, so only built-ins are
+# available. This means that __atomic can only be used in a subset
+# of the architectures
+ bool
+ default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_8
+ default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_arm
+ default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_armeb
+ default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_xtensa
+ default y if BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 && BR2_ARCH_IS_64