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Date: Fri, 10 Nov 2023 11:30:52 +0000
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Subject: Re: [PATCH] AArch64: Cleanup memset expansion
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To: Wilco Dijkstra <Wilco.Dijkstra@arm.com>,
 Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>,
 GCC Patches <gcc-patches@gcc.gnu.org>
Cc: Richard Sandiford <Richard.Sandiford@arm.com>,
 Richard Earnshaw <Richard.Earnshaw@arm.com>
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On 10/11/2023 10:17, Wilco Dijkstra wrote:
> Hi Kyrill,
> 
>> +  /* Reduce the maximum size with -Os.  */
>> +  if (optimize_function_for_size_p (cfun))
>> +    max_set_size = 96;
>> +
> 
>> .... This is a new "magic" number in this code. It looks sensible, but how did you arrive at it?
> 
> We need 1 instruction to create the value to store (DUP or MOVI) and 1 STP
> for every 32 bytes, so the 96 means 4 instructions for typical sizes 
> (sizes not
> a multiple of 16 can add one extra instruction).
> 
> I checked codesize on SPECINT2017, and 96 had practically identical size.
> Using 128 would also be a reasonable Os value with a very slight size 
> increase,
> and 384 looks good for O2 - however I didn't want to tune these values 
> as this
> is a cleanup patch.
> 
> Cheers,
> Wilco

Shouldn't this be a param then?  Also, manifest constants in the middle 
of code are a potential nightmare, please move it to a #define (even if 
that's then used as the default value for the param).