[commit: ghc] wip/T12357: FastString: Reduce allocations of concatFS (e3db922)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : wip/T12357
Link       : http://ghc.haskell.org/trac/ghc/changeset/e3db922012d73a29e6574e35b2e20f7285f4c550/ghc

>---------------------------------------------------------------

commit e3db922012d73a29e6574e35b2e20f7285f4c550
Author: Ben Gamari <ben at smart-cactus.org>
Date:   Mon Jul 4 20:33:57 2016 -0400

    FastString: Reduce allocations of concatFS
    
    Instead of unpacking and then repacking we simply concatenate all of the
    individual ByteStrings.


>---------------------------------------------------------------

e3db922012d73a29e6574e35b2e20f7285f4c550
 compiler/utils/FastString.hs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/utils/FastString.hs b/compiler/utils/FastString.hs
index 32330f2..f58a5b5 100644
--- a/compiler/utils/FastString.hs
+++ b/compiler/utils/FastString.hs
@@ -512,7 +512,7 @@ appendFS fs1 fs2 = mkFastStringByteString
                              (fastStringToByteString fs2)
 
 concatFS :: [FastString] -> FastString
-concatFS ls = mkFastString (Prelude.concat (map unpackFS ls)) -- ToDo: do better
+concatFS = mkFastStringByteString . BS.concat . map fs_bs
 
 headFS :: FastString -> Char
 headFS (FastString _ 0 _ _) = panic "headFS: Empty FastString"