Messages in this thread | | | Subject | Re: [PATCH v4 1/3] powerpc/bitops: Use immediate operand when possible | From | Christophe Leroy <> | Date | Tue, 21 Sep 2021 17:15:29 +0200 |
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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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