libunwind: Do not use __attribute__((target("gcs"))) with non-clang compilers #138077
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@llvm/pr-subscribers-libunwind Author: Khem Raj (kraj) Changes…compilers This attribute is unsupported in GCC, so far it worked because before GCC15 did not define this macros in _CHKFEAT_GCS in arm_acle.h [1] With gcc15 compiler libunwind's check for this macros is succeeding and it ends up enabling 'gcs' by using function attribute, this works with clang but not with gcc. We can see this in rust compiler bootstrap for aarch64/musl when system uses gcc15, it ends up with these errors Building libunwind.a for aarch64-poky-linux-musl [1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af Full diff: https://github.com/llvm/llvm-project/pull/138077.diff 1 Files Affected:
diff --git a/libunwind/src/UnwindLevel1.c b/libunwind/src/UnwindLevel1.c
index a258a832a9c31..4771632c49c31 100644
--- a/libunwind/src/UnwindLevel1.c
+++ b/libunwind/src/UnwindLevel1.c
@@ -186,7 +186,7 @@ unwind_phase1(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
}
extern int __unw_step_stage2(unw_cursor_t *);
-#if defined(_LIBUNWIND_USE_GCS)
+#if defined(_LIBUNWIND_USE_GCS) && defined(__clang__)
// Enable the GCS target feature to permit gcspop instructions to be used.
__attribute__((target("gcs")))
#endif
@@ -330,7 +330,7 @@ unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *except
return _URC_FATAL_PHASE2_ERROR;
}
-#if defined(_LIBUNWIND_USE_GCS)
+#if defined(_LIBUNWIND_USE_GCS) && defined(__clang__)
// Enable the GCS target feature to permit gcspop instructions to be used.
__attribute__((target("gcs")))
#endif
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Actually GCC 15 does support GCS, the syntax is just |
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✅ With the latest revision this PR passed the C/C++ code formatter. |
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__attribute__((target("gcs"))) does not work with gcc
GCC-15 has added gcs intrinsics [1] but the syntax for enabling it is
slightly different. This syntax works with clang too.
With gcc15 compiler libunwind's check for this macros is succeeding and it
ends up enabling 'gcs' by using function attribute, this works with clang
but not with gcc but '+gcs' works with both
We can see this in rust compiler bootstrap for aarch64/musl when system
uses gcc15, it ends up with these errors
Building libunwind.a for aarch64-poky-linux-musl
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning= 191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
cargo:warning= | ^~~~~~~~~~~~~
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning= 337 | _Unwind_Stop_Fn stop, void *stop_parameter) {
cargo:warning= | ^~~~~~~~~~~~~~~
Fix clang-tidy warnings along the way
[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af
Signed-off-by: Khem Raj <[email protected]>
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MaskRay
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Jun 12, 2025
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LGTM.
I vaguely recall that clang's target attribute parsing is loose and may accept some forms that is not supported by gcc. @john-brawn-arm
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__attribute__((target("gcs"))) does not work with gcc
GCC-15 has added gcs intrinsics [1] but the syntax for enabling it is
slightly different. This syntax works with clang too.
With gcc15 compiler libunwind's check for this macros is succeeding and it
ends up enabling 'gcs' by using function attribute, this works with clang
but not with gcc but '+gcs' works with both
We can see this in rust compiler bootstrap for aarch64/musl when system
uses gcc15, it ends up with these errors
Building libunwind.a for aarch64-poky-linux-musl
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning= 191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
cargo:warning= | ^~~~~~~~~~~~~
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning= 337 | _Unwind_Stop_Fn stop, void *stop_parameter) {
cargo:warning= | ^~~~~~~~~~~~~~~
[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af
Upstream-Status: Submitted [llvm#138077]
Signed-off-by: Khem Raj <[email protected]>
tomtor
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…ompilers (llvm#138077) This attribute is unsupported in GCC, so far it worked because before GCC15 did not define this macros in _CHKFEAT_GCS in arm_acle.h [1] With gcc15 compiler libunwind's check for this macros is succeeding and it ends up enabling 'gcs' by using function attribute, this works with clang but not with gcc. We can see this in rust compiler bootstrap for aarch64/musl when system uses gcc15, it ends up with these errors Building libunwind.a for aarch64-poky-linux-musl ``` cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+' cargo:warning= 191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) { cargo:warning= | ^~~~~~~~~~~~~ cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+' cargo:warning= 337 | _Unwind_Stop_Fn stop, void *stop_parameter) { cargo:warning= | ^~~~~~~~~~~~~~~ ``` [1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af Signed-off-by: Khem Raj <[email protected]>
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…ompilers (llvm#138077) This attribute is unsupported in GCC, so far it worked because before GCC15 did not define this macros in _CHKFEAT_GCS in arm_acle.h [1] With gcc15 compiler libunwind's check for this macros is succeeding and it ends up enabling 'gcs' by using function attribute, this works with clang but not with gcc. We can see this in rust compiler bootstrap for aarch64/musl when system uses gcc15, it ends up with these errors Building libunwind.a for aarch64-poky-linux-musl ``` cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+' cargo:warning= 191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) { cargo:warning= | ^~~~~~~~~~~~~ cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+' cargo:warning= 337 | _Unwind_Stop_Fn stop, void *stop_parameter) { cargo:warning= | ^~~~~~~~~~~~~~~ ``` [1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af Signed-off-by: Khem Raj <[email protected]>
kraj
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__attribute__((target("gcs"))) does not work with gcc
GCC-15 has added gcs intrinsics [1] but the syntax for enabling it is
slightly different. This syntax works with clang too.
With gcc15 compiler libunwind's check for this macros is succeeding and it
ends up enabling 'gcs' by using function attribute, this works with clang
but not with gcc but '+gcs' works with both
We can see this in rust compiler bootstrap for aarch64/musl when system
uses gcc15, it ends up with these errors
Building libunwind.a for aarch64-poky-linux-musl
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:191:1: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning= 191 | unwind_phase2(unw_context_t *uc, unw_cursor_t *cursor, _Unwind_Exception *exception_object) {
cargo:warning= | ^~~~~~~~~~~~~
cargo:warning=/mnt/b/yoe/master/sources/poky/build/tmp/work/cortexa57-poky-linux-musl/rust/1.85.1/rustc-1.85.1-src/src/llvm-project/libunwind/src/UnwindLevel1.c:337:22: error: arch extension 'gcs' should be prefixed by '+'
cargo:warning= 337 | _Unwind_Stop_Fn stop, void *stop_parameter) {
cargo:warning= | ^~~~~~~~~~~~~~~
[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af
Upstream-Status: Submitted [llvm#138077]
Signed-off-by: Khem Raj <[email protected]>
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This attribute is unsupported in GCC, so far it worked because before GCC15 did not define this macros in _CHKFEAT_GCS in arm_acle.h [1]
With gcc15 compiler libunwind's check for this macros is succeeding and it ends up enabling 'gcs' by using function attribute, this works with clang but not with gcc.
We can see this in rust compiler bootstrap for aarch64/musl when system uses gcc15, it ends up with these errors
Building libunwind.a for aarch64-poky-linux-musl
[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=5a6af707f0af