Re: [Qemu-devel] [RFC PATCH v3 1/5] coccinelle: add a script to optimize tcg op using tcg_gen_extract()

[Eric Blake]

On 05/12/2017 12:36 AM, Philippe Mathieu-Daudé wrote:

> In this patch
> http://lists.nongnu.org/archive/html/qemu-devel/2017-05/msg01466.html
> Aurelien does:
> 
> -    tcg_gen_shri_i32(cpu_sr_q, src, SR_Q);
> -    tcg_gen_andi_i32(cpu_sr_q, cpu_sr_q, 1);
> +    tcg_gen_extract_i32(cpu_sr_q, src, SR_Q, 1);
> 
> having:
> 
> #define SR_Q  8
> 
> I wanted to write a Coccinelle script to check for this pattern.
> My first version was wrong, as Richard Henderson reminded me this
> pattern can be applied as long as the len argument (here "1") is a
> Mersenne prime (all least significant bits as "1").

Side note: while you are correct that a Mersenne prime is one less than
a power of 2, your use of the term here is incorrect. You are looking
for ALL instances of numbers that are one less than a power of two, and
not just the Mersenne primes.  (For instance, 0xf is NOT a Mersenne
prime, but IS a candidate for an optimization using a length of 4.)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

signature.asc
Description: OpenPGP digital signature