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linux/arch/arc/include/asm
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Kees Cook 1ba13dde6a arc: Fix __fls() const-foldability via __builtin_clzl() 
[ Upstream commit a3fecb9160 ]

While tracking down a problem where constant expressions used by
BUILD_BUG_ON() suddenly stopped working[1], we found that an added static
initializer was convincing the compiler that it couldn't track the state
of the prior statically initialized value. Tracing this down found that
ffs() was used in the initializer macro, but since it wasn't marked with
__attribute__const__, the compiler had to assume the function might
change variable states as a side-effect (which is not true for ffs(),
which provides deterministic math results).

For arc architecture with CONFIG_ISA_ARCV2=y, the __fls() function
uses __builtin_arc_fls() which lacks GCC's const attribute, preventing
compile-time constant folding, and KUnit testing of ffs/fls fails on
arc[3]. A patch[2] to GCC to solve this has been sent.

Add a fix for this by handling compile-time constants with the standard
__builtin_clzl() builtin (which has const attribute) while preserving
the optimized arc-specific builtin for runtime cases. This has the added
benefit of skipping runtime calculation of compile-time constant values.
Even with the GCC bug fixed (which is about "attribute const") this is a
good change to avoid needless runtime costs, and should be done
regardless of the state of GCC's bug.

Build tested ARCH=arc allyesconfig with GCC arc-linux 15.2.0.

Link: https://github.com/KSPP/linux/issues/364 [1]
Link: https://gcc.gnu.org/pipermail/gcc-patches/2025-August/693273.html
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202508031025.doWxtzzc-lkp@intel.com/ [3]
Signed-off-by: Kees Cook <kees@kernel.org>
Acked-by: Vineet Gupta <vgupta@kernel.org>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-12-07 06:09:16 +09:00
..
arcregs.h ARC: allow userspace DSP applications to use AGU extensions 2020-03-16 10:30:49 -07:00
asm-offsets.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
asserts.h ARC: allow userspace DSP applications to use AGU extensions 2020-03-16 10:30:49 -07:00
atomic-llsc.h ARC: atomics: Add compiler barrier to atomic operations... 2023-09-19 12:23:00 +02:00
atomic-spinlock.h ARC: atomic: !LLSC: use int data type consistently 2021-08-24 14:25:46 -07:00
atomic.h ARC: atomic_cmpxchg/atomic_xchg: implement relaxed variants 2021-08-24 14:25:47 -07:00
atomic64-arcv2.h ARC: atomics: Add compiler barrier to atomic operations... 2023-09-19 12:23:00 +02:00
barrier.h ARC: [plat-eznps]: Drop support for EZChip NPS platform 2020-10-05 21:02:29 -07:00
bitops.h arc: Fix __fls() const-foldability via __builtin_clzl() 2025-12-07 06:09:16 +09:00
bug.h arc: add show_stack_loglvl() 2020-06-09 09:39:10 -07:00
cache.h ARC: mm: move mmu/cache externs out to setup.h 2021-08-24 14:25:48 -07:00
cacheflush.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
checksum.h arc: Prefer unsigned int to bare use of unsigned 2021-08-03 18:58:33 -07:00
cmpxchg.h ARC: atomic_cmpxchg/atomic_xchg: implement relaxed variants 2021-08-24 14:25:47 -07:00
current.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
delay.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
disasm.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
dma.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
dsp-impl.h ARC: guard dsp early init against non ARCv2 2020-04-29 11:53:51 -07:00
dsp.h ARC: allow userspace DSP applications to use AGU extensions 2020-03-16 10:30:49 -07:00
dwarf.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
elf.h ARC: elf: use right ELF_ARCH 2020-06-16 12:56:37 -07:00
entry-arcv2.h ARC: entry: comment 2020-04-12 22:45:35 -07:00
entry-compact.h ARC: mm: use SCRATCH_DATA0 register for caching pgdir in ARCv2 only 2021-08-24 14:25:48 -07:00
entry.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
exec.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
fb.h License cleanup: add SPDX GPL-2.0 license identifier to files with no license 2017-11-02 11:10:55 +01:00
fpu.h ARC: fpu: fix randconfig build error reported by 0-day test service 2020-02-09 22:17:22 -08:00
futex.h futex: arch_futex_atomic_op_inuser() calling conventions change 2020-03-27 23:58:51 -04:00
highmem.h arc/mm/highmem: Use generic kmap atomic implementation 2020-11-06 23:14:55 +01:00
hugepage.h ARC: mm: switch pgtable_t back to struct page * 2021-08-26 13:42:42 -07:00
io.h arc: iounmap() arg is volatile 2022-11-03 23:59:15 +09:00
irq.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
irqflags-arcv2.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
irqflags-compact.h ARC: [arcompact] fix bitrot with 2 levels of interrupt 2020-06-16 12:56:37 -07:00
irqflags.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
jump_label.h ARC: ARCv2: jump label: implement jump label patching 2019-11-08 16:14:32 -08:00
Kbuild local64.h: make <asm/local64.h> mandatory 2020-12-29 15:36:49 -08:00
kdebug.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
kgdb.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
kprobes.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
linkage.h ARC: define ASM_NL and __ALIGN(_STR) outside #ifdef __ASSEMBLY__ guard 2023-07-23 13:47:15 +02:00
mach_desc.h treewide: Convert macro and uses of __section(foo) to __section("foo") 2020-10-25 14:51:49 -07:00
mmu-arcv2.h ARC: mm: disintegrate mmu.h (arcv2 bits out) 2021-08-25 15:53:19 -07:00
mmu.h ARC: mm: disintegrate mmu.h (arcv2 bits out) 2021-08-25 15:53:19 -07:00
mmu_context.h ARC: mm: move MMU specific bits out of ASID allocator 2021-08-24 14:25:48 -07:00
module.h arch: split MODULE_ARCH_VERMAGIC definitions out to <asm/vermagic.h> 2020-04-23 10:50:26 +09:00
page.h ARC: mm: support 4 levels of page tables 2021-08-26 13:43:19 -07:00
pci.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
perf_event.h arc: Prefer unsigned int to bare use of unsigned 2021-08-03 18:58:33 -07:00
pgalloc.h ARC: mm: support 4 levels of page tables 2021-08-26 13:43:19 -07:00
pgtable-bits-arcv2.h ARC: mm: disintegrate pgtable.h into levels and flags 2021-08-25 15:53:19 -07:00
pgtable-levels.h ARC: mm: fix leakage of memory allocated for PTE 2022-11-03 23:59:13 +09:00
pgtable.h ARC: fix potential build snafu 2021-10-15 18:06:32 -07:00
processor.h sched: Add wrapper for get_wchan() to keep task blocked 2025-08-28 16:24:03 +02:00
ptrace.h ARC: [plat-eznps]: Drop support for EZChip NPS platform 2020-10-05 21:02:29 -07:00
sections.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
segment.h uaccess: remove segment_eq 2020-08-12 10:57:58 -07:00
serial.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
setup.h ARC: mm: move mmu/cache externs out to setup.h 2021-08-24 14:25:48 -07:00
shmparam.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
smp.h ARC: switch to generic bitops 2021-08-24 14:25:47 -07:00
spinlock.h ARC: [plat-eznps]: Drop support for EZChip NPS platform 2020-10-05 21:02:29 -07:00
spinlock_types.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
stacktrace.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
string.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
switch_to.h ARC: [plat-eznps]: Drop support for EZChip NPS platform 2020-10-05 21:02:29 -07:00
syscall.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
syscalls.h ARC: wireup clone3 syscall 2020-01-15 16:08:12 -08:00
thread_info.h arc: add support for TIF_NOTIFY_SIGNAL 2020-11-09 08:16:55 -07:00
timex.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
tlb.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
tlbflush.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
uaccess.h asm-generic: remove extra strn{cpy_from,len}_user declarations 2021-07-27 23:01:13 +02:00
unaligned.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
unwind.h treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 500 2019-06-19 17:09:55 +02:00
vermagic.h arch: split MODULE_ARCH_VERMAGIC definitions out to <asm/vermagic.h> 2020-04-23 10:50:26 +09:00
vmalloc.h mm/vmalloc: Add empty <asm/vmalloc.h> headers and use them from <linux/vmalloc.h> 2019-12-10 10:12:55 +01:00