Build json_spirit as a lib (as much as possible, anyway) #1749

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What are the advantages and disadvantages of this?

I'm ok with the changes although I don't like "include three cpps in one cpp", as it adds an almost-empty cpp in src/. I'd like it somewhat better if you moved the resulting jsonrpc.cpp to src/json instead.

Also, I might misunderstand what you're trying to do but you should probably remove the individual files (src/json/json_spirit_*.cpp) from the build systems otherwise you'll link them twice?

This eliminates duplicating the templates in each rpc.o file.

src/json/json_spirit_*.cpp are never built or linked.

OK I guess that will save some compilation time. I still don't like the almost-empty cpp though.

They are linked individually in bitcoin-qt.pro... probably, that is a the mistake then.

Sadly, bitcoinrpc.cpp's requirement of templates kills this optimization. We can see that it makes a positive impact on compiled code size in several cases:

[jgarzik@localhost src]$ ls -l obj.vanilla/*rpc*.o 
-rw-rw-r-- 1 jgarzik jgarzik 9235016 Aug 29 00:52 obj.vanilla/bitcoinrpc.o
-rw-rw-r-- 1 jgarzik jgarzik 2193688 Aug 29 00:53 obj.vanilla/rpcblockchain.o
-rw-rw-r-- 1 jgarzik jgarzik 2427808 Aug 29 00:52 obj.vanilla/rpcdump.o
-rw-rw-r-- 1 jgarzik jgarzik 3848484 Aug 29 00:53 obj.vanilla/rpcmining.o
-rw-rw-r-- 1 jgarzik jgarzik 1433740 Aug 29 00:52 obj.vanilla/rpcnet.o
-rw-rw-r-- 1 jgarzik jgarzik 4616688 Aug 29 00:54 obj.vanilla/rpcrawtransaction.o
-rw-rw-r-- 1 jgarzik jgarzik 5919696 Aug 29 00:53 obj.vanilla/rpcwallet.o
[jgarzik@localhost src]$ ls -l obj/*rpc*.o 
-rw-rw-r-- 1 jgarzik jgarzik 9235044 Aug 29 01:21 obj/bitcoinrpc.o
-rw-rw-r-- 1 jgarzik jgarzik 2157036 Aug 29 01:22 obj/rpcblockchain.o
-rw-rw-r-- 1 jgarzik jgarzik 2377376 Aug 29 01:21 obj/rpcdump.o
-rw-rw-r-- 1 jgarzik jgarzik 3809184 Aug 29 01:21 obj/rpcmining.o
-rw-rw-r-- 1 jgarzik jgarzik 1445256 Aug 29 01:21 obj/rpcnet.o
-rw-rw-r-- 1 jgarzik jgarzik 4627340 Aug 29 01:23 obj/rpcrawtransaction.o
-rw-rw-r-- 1 jgarzik jgarzik 5849288 Aug 29 01:22 obj/rpcwallet.o

However, due to bitcoinrpc.cpp's templates requirement, and other caveats (SEE BELOW), the final size is worse:

[jgarzik@localhost src]$ size obj.vanilla/bitcoind obj/bitcoind
   text    data     bss     dec     hex filename
2688992    5368    7464 2701824  293a00 obj.vanilla/bitcoind
3987711    5508    8232 4001451  3d0eab obj/bitcoind

[jgarzik@localhost src]$ ls -l obj.vanilla/bitcoind obj/bitcoind
-rwxrwxr-x 1 jgarzik jgarzik 58337880 Aug 29 01:27 obj/bitcoind
-rwxrwxr-x 1 jgarzik jgarzik 36560716 Aug 29 00:56 obj.vanilla/bitcoind

If we can eliminate the template usage from bitcoinrpc.cpp, then this solution becomes viable.

Caveats: a) My i386 laptop does not work with -flto, which should dramatically improve the situation (but still the code is duplicated, due to bitcoinrpc.cpp's use of templates) b) libjson.o would be better constructed as a library archive (libjson.a). However, as we use all the modules, this would probably not result in code savings.