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SubjectRE: [PATCH 4/4] x86/efi: print size and base in binary units in efi_print_memmap
Date
> -----Original Message-----
> From: Matt Fleming [mailto:matt@codeblueprint.co.uk]
> Sent: Friday, January 8, 2016 6:19 AM
> To: Andy Shevchenko <andy.shevchenko@gmail.com>
> Cc: Elliott, Robert (Persistent Memory) <elliott@hpe.com>; Thomas Gleixner
> <tglx@linutronix.de>; Ingo Molnar <mingo@redhat.com>; H. Peter Anvin
> <hpa@zytor.com>; x86@kernel.org; linux-efi@vger.kernel.org; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH 4/4] x86/efi: print size and base in binary units in
> efi_print_memmap
>
> On Sun, 27 Dec, at 04:35:12PM, Andy Shevchenko wrote:
> > On Mon, Dec 21, 2015 at 6:16 PM, Matt Fleming <matt@codeblueprint.co.uk>
> wrote:
> > >> diff --git a/arch/x86/platform/efi/efi.c
> b/arch/x86/platform/efi/efi.c
> > >> index 635a955..030ba91 100644
> > >> --- a/arch/x86/platform/efi/efi.c
> > >> +++ b/arch/x86/platform/efi/efi.c
> > >> @@ -222,6 +222,25 @@ int __init efi_memblock_x86_reserve_range(void)
> > >> return 0;
> > >> }
> > >>
> > >> +char * __init efi_size_format(char *buf, size_t size, u64 bytes)
> > >> +{
> > >> + if (!bytes || (bytes & 0x3ff))
> > >> + snprintf(buf, size, "%llu B", bytes);
> > >> + else if (bytes & 0xfffff)
> > >> + snprintf(buf, size, "%llu KiB", bytes >> 10);
> > >> + else if (bytes & 0x3fffffff)
> > >> + snprintf(buf, size, "%llu MiB", bytes >> 20);
> > >> + else if (bytes & 0xffffffffff)
> > >> + snprintf(buf, size, "%llu GiB", bytes >> 30);
> > >> + else if (bytes & 0x3ffffffffffff)
> > >> + snprintf(buf, size, "%llu TiB", bytes >> 40);
> > >> + else if (bytes & 0xfffffffffffffff)
> > >> + snprintf(buf, size, "%llu PiB", bytes >> 50);
> > >> + else
> > >> + snprintf(buf, size, "%llu EiB", bytes >> 60);
> > >> + return buf;
> >
> > For me it looks like ffs with name in the table can be used.
>
> Could you provide a patch?

I think this is functionally equivalent:
#include <string.h>

char * efi_size_format_ffsl(char *buf, size_t size, u64 bytes)
{
if (!bytes || ffsl(bytes) < 10)
snprintf(buf, size, "%llu B", bytes);
else if (ffsl(bytes) < 20)
snprintf(buf, size, "%llu KiB", bytes >> 10);
else if (ffsl(bytes) < 30)
snprintf(buf, size, "%llu MiB", bytes >> 20);
else if (ffsl(bytes) < 40)
snprintf(buf, size, "%llu GiB", bytes >> 30);
else if (ffsl(bytes) < 50)
snprintf(buf, size, "%llu TiB", bytes >> 40);
else if (ffsl(bytes) < 60)
snprintf(buf, size, "%llu PiB", bytes >> 50);
else
snprintf(buf, size, "%llu EiB", bytes >> 60);
return buf;
}

Compiled as a user program with gcc -O2, the original results
in mov and testq instructions:
movq %rdi, %rbx
je .L2
testl $1023, %edx
jne .L2
testl $1048575, %edx
jne .L15
testl $1073741823, %edx
jne .L16
movabsq $1099511627775, %rax
testq %rax, %rdx
jne .L17
movabsq $1125899906842623, %rax
testq %rax, %rdx
jne .L18
movabsq $1152921504606846975, %rax
movq %rdx, %rcx
testq %rax, %rdx
jne .L19

while the ffs version uses bit scan forward (bsfq)
and only needs cmpl instructions since the values
are smaller:
movq %rdi, %rbx
je .L21
bsfq %rdx, %rcx
addq $1, %rcx
cmpl $9, %ecx
jle .L21
cmpl $19, %ecx
jle .L33
cmpl $29, %ecx
jle .L34
cmpl $39, %ecx
.p2align 4,,2
jle .L35
cmpl $49, %ecx
.p2align 4,,2
jle .L36
cmpl $59, %ecx
.p2align 4,,2
jle .L37

The kernel offers ffs(int x) but not ffsl(), and it
uses inline assembly for one of these:
bsfl
bsfl, cmovzl
bsfl, jnz, movl

I don't know which code is the most efficient.
---
Robert Elliott, HPE Persistent Memory

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