On 3/6/25 9:19 PM, Andrew Cooper wrote:
On 05/03/2025 7:34 am, Jan Beulich wrote:
On 28.02.2025 17:24, Andrew Cooper wrote:
On 27/02/2025 8:11 am, Jan Beulich wrote:
On 26.02.2025 18:20, Andrew Cooper wrote:
--- a/xen/arch/riscv/include/asm/bitops.h
+++ b/xen/arch/riscv/include/asm/bitops.h
@@ -125,6 +125,13 @@ static inline void clear_bit(int nr, volatile void *p)
#undef NOT
#undef __AMO
+#define arch_ffs(x) ((x) ? 1 + __builtin_ctz(x) : 0)
+#define arch_ffsl(x) ((x) ? 1 + __builtin_ctzl(x) : 0)
+#define arch_fls(x) ((x) ? 32 - __builtin_clz(x) : 0)
I fear you won't like me to say this, but can't we avoid baking in yet
another assumption on sizeof(int) == 4, by using at least sizeof(int) * 8
here (yet better might be sizeof(int) * BITS_PER_BYTE)?
Yes and no.
No, because 32 here is consistent with ARM and PPC when it comes to
arch_fls(). Given the effort it took to get these consistent, I'm not
interested in letting them diverge.
But, if someone wants to introduce BITS_PER_INT to mirror BITS_PER_LONG
and use it consistently, then that would be ok too.
Oleksii: I see your patch is committed, but when I said "use it
consistently", I meant "patch ARM and PPC too".
I planned to do that in a separate patch next week.
I was actually hoping to eliminate BITS_PER_LONG at some point, in favor
of using sizeof(long) * BITS_PER_BYTE. (Surely in common code we could
retain a shorthand of that name, if so desired, but I see no reason why
each arch would need to provide all three BITS_PER_{BYTE,INT,LONG}.)
The concern is legibility and clarity.
This:
((x) ? 32 - __builtin_clz(x) : 0)
is a clear expression in a way that this:
((x) ? (sizeof(int) * BITS_PER_BYTE) - __builtin_clz(x) : 0)
is not. The problem is the extra binary expression, and this:
((x) ? BITS_PER_INT - __builtin_clz(x) : 0)
is still clear, because the reader doesn't have to perform a multiply to
just to figure out what's going on.
It is definitely stupid to have each architecture provide their own
BITS_PER_*. The compiler is in a superior position to provide those
details, and it should be in a common location.
I don't particularly mind how those constants are derived, but one key
thing that BITS_PER_* can do which sizeof() can't is be used in #ifdef/etc.
What about moving them to xen/config.h? (if it isn't the best one place, any
suggestion which is better?)
#define BYTES_PER_INT (1 << INT_BYTEORDER)
#define BITS_PER_INT (BYTES_PER_INT << 3)
#define BYTES_PER_LONG (1 << LONG_BYTEORDER)
#define BITS_PER_LONG (BYTES_PER_LONG << 3)
#define BITS_PER_BYTE 8
Also, it seems like the follwoing could be moved there too:
#define POINTER_ALIGN BYTES_PER_LONG
#define BITS_PER_LLONG 64
#define BITS_PER_BYTE 8
Only, BITS_PER_XEN_ULONG is looking different for x86, so should be still in
arch specific config.h.
~ Oleksii
The following files use BITS_PER_LONG preprocessing:
arch/arm/include/asm/div64.h
arch/x86/cpu/mcheck/mce.c
arch/x86/smpboot.c
common/bitops.c
common/coverage/gcov.h
common/coverage/llvm.c
common/cpu.c
common/event_channel.c
common/time.c
common/ubsan/ubsan.c
include/asm-generic/div64.h
include/xen/cpumask.h
include/xen/inttypes.h
include/xen/nodemask.h
include/xen/sched.h
include/xen/xxhash.h
lib/find-next-bit.c
lib/generic-ffsl.c
lib/generic-flsl.c
lib/generic-hweightl.c
And I really don't think they can be replaced with a sizeof().
~Andrew
