Messages in this thread | | | Date | Mon, 20 Sep 2021 16:23:04 -0500 | From | Segher Boessenkool <> | Subject | Re: [PATCH v4 1/3] powerpc/bitops: Use immediate operand when possible |
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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).
> 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.
Looks great except that one nit. Thanks :-)
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Segher
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