linux-kernel - RE: [PATCH] slab.h: Avoid using & for logical and of booleans

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Date:   Tue, 6 Nov 2018 11:07:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Vlastimil Babka' <vbabka@...e.cz>,
        'Bart Van Assche' <bvanassche@....org>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Christoph Lameter <cl@...ux.com>,
        "Roman Gushchin" <guro@...com>
Subject: RE: [PATCH] slab.h: Avoid using & for logical and of booleans

From: Vlastimil Babka [mailto:vbabka@...e.cz]
> Sent: 06 November 2018 10:22
...
> >> -	return type_dma + (is_reclaimable & !is_dma) * KMALLOC_RECLAIM;
> >> +	return type_dma + is_reclaimable * !is_dma * KMALLOC_RECLAIM;
> >
> > ISTM that changing is_dma and is_reclaimable from int to bool will stop the bleating.
> >
> > It is also strange that this code is trying so hard here to avoid conditional instructions

I've done some experiments, compiled with gcc 4.7.3 and -O2
The constants match those from the kernel headers.

It is noticable that there isn't a cmov in sight.
The code would also be better if the KMALLOC constants matched the GFP ones.


#define __GFP_DMA 1u
#define __GFP_RECLAIM 0x10u

#define KMALLOC_DMA 2
#define KMALLOC_RECLAIM 1

unsigned int f(unsigned int flags)
{
        return flags & __GFP_DMA ? KMALLOC_DMA : flags & __GFP_RECLAIM ? KMALLOC_RECLAIM : 0;
}

unsigned int f1(unsigned int flags)
{
        return !__builtin_expect(flags & (__GFP_DMA | __GFP_RECLAIM), 0) ? 0 : flags & __GFP_DMA ? KMALLOC_DMA : KMALLOC_RECLAIM;
}

unsigned int f2(unsigned int flags)
{
        int is_dma, type_dma, is_rec;

        is_dma = !!(flags & __GFP_DMA);
        type_dma = is_dma * KMALLOC_DMA;
        is_rec = !!(flags & __GFP_RECLAIM);

        return type_dma + (is_rec & !is_dma) * KMALLOC_RECLAIM;
}

unsigned int f3(unsigned int flags)
{
        int is_dma, type_dma, is_rec;

        is_dma = !!(flags & __GFP_DMA);
        type_dma = is_dma * KMALLOC_DMA;
        is_rec = !!(flags & __GFP_RECLAIM);

        return type_dma + (is_rec * !is_dma) * KMALLOC_RECLAIM;
}

Disassembly of section .text:

0000000000000000 <f>:
   0:   40 f6 c7 01             test   $0x1,%dil
   4:   b8 02 00 00 00          mov    $0x2,%eax
   9:   74 05                   je     10 <f+0x10>
   b:   f3 c3                   repz retq
   d:   0f 1f 00                nopl   (%rax)
  10:   89 f8                   mov    %edi,%eax
  12:   c1 e8 04                shr    $0x4,%eax
  15:   83 e0 01                and    $0x1,%eax
  18:   c3                      retq

This one has a misprediced branch in the common path.

0000000000000020 <f1>:
  20:   40 f6 c7 11             test   $0x11,%dil
  24:   75 03                   jne    29 <f1+0x9>
  26:   31 c0                   xor    %eax,%eax
  28:   c3                      retq
  29:   83 e7 01                and    $0x1,%edi
  2c:   83 ff 01                cmp    $0x1,%edi
  2f:   19 c0                   sbb    %eax,%eax
  31:   83 c0 02                add    $0x2,%eax
  34:   c3                      retq

The jne will be predicted not taken and the retq predicted.
So this might only be 1 clock in the normal case.

0000000000000040 <f2>:
  40:   89 f8                   mov    %edi,%eax
  42:   c1 ef 04                shr    $0x4,%edi
  45:   83 e0 01                and    $0x1,%eax
  48:   89 c2                   mov    %eax,%edx
  4a:   83 f2 01                xor    $0x1,%edx
  4d:   21 d7                   and    %edx,%edi
  4f:   8d 04 47                lea    (%rdi,%rax,2),%eax
  52:   c3                      retq

No conditionals, but a 4 deep dependency chain.

0000000000000060 <f3>:
  60:   89 fa                   mov    %edi,%edx
  62:   c1 ef 04                shr    $0x4,%edi
  65:   83 e2 01                and    $0x1,%edx
  68:   83 e7 01                and    $0x1,%edi
  6b:   89 d0                   mov    %edx,%eax
  6d:   83 f0 01                xor    $0x1,%eax
  70:   0f af c7                imul   %edi,%eax
  73:   8d 04 50                lea    (%rax,%rdx,2),%eax
  76:   c3                      retq

You really don't want the multiply.

	David

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