From: George Spelvin <lkml@SDF.ORG>
To: Stephen Hemminger <stephen@networkplumber.org>
Cc: linux-kernel@vger.kernel.org,
Hannes Frederic Sowa <hannes@stressinduktion.org>,
lkml@sdf.org
Subject: Re: [RFC PATCH v1 09/50] <linux/random.h> prandom_u32_max() for power-of-2 ranges
Date: Sat, 28 Mar 2020 18:07:11 +0000 [thread overview]
Message-ID: <20200328180711.GC5859@SDF.ORG> (raw)
In-Reply-To: <20200328103229.132a047f@hermes.lan>
On Sat, Mar 28, 2020 at 10:32:29AM -0700, Stephen Hemminger wrote:
> On Sat, 16 Mar 2019 02:32:04 -0400
> George Spelvin <lkml@sdf.org> wrote:
>
>> +static inline u32 prandom_u32_max(u32 range)
>> {
>> - return (u32)(((u64) prandom_u32() * ep_ro) >> 32);
>> + /*
>> + * If the range is a compile-time constant power of 2, then use
>> + * a simple shift. This is mathematically equivalent to the
>> + * multiplication, but GCC 8.3 doesn't optimize that perfectly.
>> + *
>> + * We could do an AND with a mask, but
>> + * 1) The shift is the same speed on a decent CPU,
>> + * 2) It's generally smaller code (smaller immediate), and
>> + * 3) Many PRNGs have trouble with their low-order bits;
>> + * using the msbits is generaly preferred.
>> + */
>> + if (__builtin_constant_p(range) && (range & (range - 1)) == 0)
>> + return prandom_u32() / (u32)(0x100000000 / range);
>> + else
>> + return reciprocal_scale(prandom_u32(), range);
> The optimization is good, but I don't think that the compiler
> is able to propogate the constant property into the function.
> Did you actually check the generated code?
Yes, I checked repeatedly during development. I just rechecked the
exact code (it's been a while), and verified that
unsigned foo(void)
{
return prandom_u32_max(256);
}
compiles to
foo:
.LFB1:
.cfi_startproc
subq $8, %rsp
.cfi_def_cfa_offset 16
call prandom_u32@PLT
shrl $24, %eax
addq $8, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
.LFE1:
.size foo, .-foo
But you prompted me to check a few other architectures, and
it's true for them too. E.g. m68k:
foo:
jsr prandom_u32
moveq #24,%d1
lsr.l %d1,%d0
rts
(68k is one architecture where the mask is faster than the shift,
so I could handle it separately, but it makes the code even uglier.
Basically, use masks for small ranges, and shifts for large ranges,
and an arch-dependent threshold that depends on the available
immediate constant range.)
ARM, PowerPC, and MIPS all have some hideously large function preamble
code, but the core is a right shift. E.g.
foo:
.LFB1:
.cfi_startproc
stwu 1,-16(1)
.cfi_def_cfa_offset 16
mflr 0
.cfi_register 65, 0
bcl 20,31,.L2
.L2:
stw 30,8(1)
.cfi_offset 30, -8
mflr 30
addis 30,30,.LCTOC1-.L2@ha
stw 0,20(1)
addi 30,30,.LCTOC1-.L2@l
.cfi_offset 65, 4
bl prandom_u32+32768@plt
lwz 0,20(1)
lwz 30,8(1)
addi 1,1,16
.cfi_restore 30
.cfi_def_cfa_offset 0
srwi 3,3,24
mtlr 0
.cfi_restore 65
blr
prev parent reply other threads:[~2020-03-28 18:07 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-16 6:32 [RFC PATCH v1 09/50] <linux/random.h> prandom_u32_max() for power-of-2 ranges George Spelvin
2020-03-28 17:32 ` Stephen Hemminger
2020-03-28 18:07 ` George Spelvin [this message]
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