linux-kernel - Re: [PATCH v4 1/3] powerpc/bitops: Use immediate operand when possible

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Message-ID: <20210920212303.GZ1583@gate.crashing.org>
Date:   Mon, 20 Sep 2021 16:23:04 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/3] powerpc/bitops: Use immediate operand when possible

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@...roup.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@...nel.crashing.org>


Segher