optimise linker in linux standalone tarballs

Trick the linker into not doing unncessary work searching for optimised
libraries that are not present, by symlinking the directories where
optimised libs would be to the main lib dir.

This reduces the ENOENT of git-annex init by about 1/2. The linker always
finds the files where it looks first time now. I have not looked at what
the wall clock speedup might be, it's probably rather small.

If a x86-64-v5 comes to be, the list will need to be extended. And there
may be other directories used on some machines that I have missed. Not done
for arm64 yet, or any uncommon architectures.

Sponsored-by: Dartmouth College's Datalad project

Build/LinuxMkLibs.hs

@@ -14,6 +14,7 @@ import Data.List
 import System.Posix.Files
 import Control.Monad.IfElse
 import Control.Applicative
+import qualified System.Info
 import Prelude
 
 import Utility.LinuxMkLibs
@@ -82,9 +83,57 @@ consolidateUsrLib top libdirs = go [] libdirs
 				unless (dirCruft f) $
 					unlessM (doesDirectoryExist src) $
 						renameFile src dst
+			symlinkHwCapDirs top d
 			go c rest
 		_ -> go (x:c) rest
 
+{- The linker looks for optimised versions of libraries depending on the
+ - hardware capabilities. That causes a lot of extra work searching for
+ - libraries, so to avoid it, make symlinks from the hwcap directories
+ - to the libdir. This way, the linker will find a library the first place
+ - it happens to look for it.
+ -}
+symlinkHwCapDirs :: FilePath -> FilePath -> IO ()
+symlinkHwCapDirs top libdir = forM_ hwcapdirs $ \d ->
+	unlessM (doesDirectoryExist (top ++ libdir </> d)) $ do
+		createDirectoryIfMissing True (top ++ libdir </> takeDirectory d)
+		link <- relPathDirToFile
+			(toRawFilePath (top ++ takeDirectory (libdir </> d)))
+			(toRawFilePath (top ++ libdir))
+		let link' = case fromRawFilePath link of
+			"" -> "."
+			l -> l
+		createSymbolicLink link' (top ++ libdir </> d)
+  where
+	hwcapdirs = case System.Info.arch of
+		"x86_64" -> 
+			-- See glibc's sysdeps/x86_64/dl-hwcaps-subdirs.c 
+			-- for list of subarchitecture directories.
+			[ "glibc-hwcaps/x86-64-v2"
+			, "glibc-hwcaps/x86-64-v3"
+			, "glibc-hwcaps/x86-64-v4"
+			-- The linker later checks these, and will check
+			-- them when none of the above subarchitectures
+			-- are supported by the processor, so make them
+			-- just in case.
+			, "tls/x86_64"
+			, "x86_64"
+			]
+		"i386" ->
+			[ "tls/i686"
+			[ "tls/i586"
+			, "i686"
+			,
+			"i586"
+			]
+		"arm" ->
+			-- Probably not complete, only what I have
+			-- observed.
+			[ "tls/v7l"
+			, "v7l"
+			]
+		_ -> []
+
 {- Installs a linker shim script around a binary.
  -
  - Note that each binary is put into its own separate directory,