Re: [Qemu-devel] [RFC PATCH v4 1/6] coccinelle: add a script to optimize tcg op using tcg_gen_extract()

[Paolo Bonzini]

On 15/05/2017 16:04, Eric Blake wrote:
> On 05/12/2017 06:38 PM, Philippe Mathieu-Daudé wrote:
>> If you have coccinelle installed you can apply this script using:
>>
>>     $ spatch \
>>         --macro-file scripts/cocci-macro-file.h \
>>         --dir target --in-place
>>
>> You can also use directly Peter Senna Tschudin docker image (easier):
>>
>>     $ docker run -v `pwd`:`pwd` -w `pwd` petersenna/coccinelle \
>>         --sp-file scripts/coccinelle/tcg_gen_extract.cocci \
>>         --macro-file scripts/cocci-macro-file.h \
>>         --dir target --in-place
>>
>> Then verified that no manual touchups are required.
>>
>> The following thread was helpful while writing this script:
>>
>>     https://github.com/coccinelle/coccinelle/issues/86
>>
>> Signed-off-by: Philippe Mathieu-Daudé <address@hidden>
>> ---
>>  scripts/coccinelle/tcg_gen_extract.cocci | 103 
>> +++++++++++++++++++++++++++++++
>>  1 file changed, 103 insertions(+)
>>  create mode 100644 scripts/coccinelle/tcg_gen_extract.cocci
> 
> It's still not obvious to me whether we want this script in the tree (as
> something we plan to rerun regularly to check for regressions), or just
> in the commit message (useful for the one-time location of spots to
> optimize, but something we don't anticipate repeating).

I think it's useful.  New backends can have this issue, plus it shows
some advanced Coccinelle techniques.

Paolo

> 
> 
>> +@@
>> +import sys
>> +fd = sys.stderr
>> +def debug(msg="", trailer="\n"):
>> +    fd.write("[DBG] " + msg + trailer)
>> +def low_bits_count(value):
>> +    bits_count = 0
>> +    while (value & (1 << bits_count)):
>> +        bits_count += 1
> 
> Surely python has a faster method than this (after all, we have ctz and
> friends in C code)?  But my python is limited enough that I don't know
> of one off-hand.
> 
>> +    return bits_count
>> +def Mn(order): # Mersenne number
>> +    return (1 << order) - 1
> 
> Correct name...
> 
> 
>> +else:
>> +    debug("candidate at %s:%s" % (shr_p[0].file, shr_p[0].line))
>> +    try: # only eval integer, no #define like 'SR_M' (cpp did this, else 
>> some headers are missing).
>> +        msk_v = long(msk_s.strip("UL"), 0)
>> +        msk_b = low_bits_count(msk_v)
>> +        if msk_b == 0:
>> +            debug("  value: 0x%x low_bits: %d" % (msk_v, msk_b))
>> +        else:
>> +            debug("  value: 0x%x low_bits: %d [Mersenne prime: 0x%x]" % 
>> (msk_v, msk_b, Mn(msk_b)))
> 
> ...but this name is still wrong.
>