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SubjectRe: [PATCH v4 1/3] powerpc/bitops: Use immediate operand when possible
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Le 20/09/2021 à 23:23, Segher Boessenkool a écrit :
> Hi!
>
> On Mon, Sep 20, 2021 at 10:31:17AM +0200, Christophe Leroy wrote:
>> Today we get the following code generation for bitops like
>> set or clear bit:
>>
>> c0009fe0: 39 40 08 00 li r10,2048
>> c0009fe4: 7c e0 40 28 lwarx r7,0,r8
>> c0009fe8: 7c e7 53 78 or r7,r7,r10
>> c0009fec: 7c e0 41 2d stwcx. r7,0,r8
>>
>> c000d568: 39 00 18 00 li r8,6144
>> c000d56c: 7c c0 38 28 lwarx r6,0,r7
>> c000d570: 7c c6 40 78 andc r6,r6,r8
>> c000d574: 7c c0 39 2d stwcx. r6,0,r7
>>
>> Most set bits are constant on lower 16 bits, so it can easily
>> be replaced by the "immediate" version of the operation. Allow
>> GCC to choose between the normal or immediate form.
>
> You can also handle the second sixteen bits (the "shifted" half), by
> using oris etc. The "%eN" output modifier prints an "s" for this:
> /* If the low 16 bits are 0, but some other bit is set, write 's'. */
> But this doesn't handle non-constant arguments, so you're likely better
> off using what you have noe.
>
>> For clear bits, on 32 bits 'rlwinm' can be used instead of 'andc' for
>> when all bits to be cleared are consecutive.
>
> Or when all you want to keep are consecutive (you do handle that now :-) )
>
>> On 64 bits we don't have any equivalent single operation for clearing,
>> single bits or a few bits, we'd need two 'rldicl' so it is not
>> worth it, the li/andc sequence is doing the same.
>
> You can use rlwinm whenever you want to clear all top 32 bits.
>
> A sometimes nice idiom is ori x,x,N ; xori x,x,N to clear the bits N
> (or oris/xoris). But it's two insns no matter what (but no spare
> register is needed).

Could be a candidate for a follow-up change if someone want to focus on
PPC64.

>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>
>> +static inline unsigned long test_and_clear_bits(unsigned long mask, volatile unsigned long *_p)
>> +{
>> + unsigned long old, t;
>> + unsigned long *p = (unsigned long *)_p;
>> +
>> + if (IS_ENABLED(CONFIG_PPC32) &&
>> + __builtin_constant_p(mask) && is_rlwinm_mask_valid(mask)) {
>
> is_rlwinm_mask_valid(~mask)? So that test_and_clear_bits(0, ...) will
> work with rlwinm, and test_and_clear_bits(0xffffffff, ...) will not make
> gas scream bloody murder ("illegal bitmask"). Tha mask you pass to the
> instruction is ~mask after all.

Ok, fixed in v5.

>
> Looks great except that one nit. Thanks :-)
>
> Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>

Thanks

Christophe

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