Refactor, document, and optimize LLVM configuration loading

As described in the new Note [LLVM Configuration] in SysTools, we now
load llvm-targets and llvm-passes lazily to avoid the overhead of doing
so when -fllvm isn't used (also known as "the common case").

Noticed in #17003.

Metric Decrease:
    T12234
    T12150

1 files changed, 27 insertions, 5 deletions

diff --git a/compiler/main/SysTools.hs b/compiler/main/SysTools.hs
index fa339bbb87..b3312b0dae 100644
--- a/compiler/main/SysTools.hs
+++ b/compiler/main/SysTools.hs
@@ -13,7 +13,7 @@
 module SysTools (
         -- * Initialisation
         initSysTools,
-        initLlvmConfig,
+        lazyInitLlvmConfig,
 
         -- * Interface to system tools
         module SysTools.Tasks,
@@ -52,6 +52,7 @@ import DynFlags
 import Control.Monad.Trans.Except (runExceptT)
 import System.FilePath
 import System.IO
+import System.IO.Unsafe (unsafeInterleaveIO)
 import SysTools.ExtraObj
 import SysTools.Info
 import SysTools.Tasks
@@ -110,13 +111,34 @@ stuff.
 ************************************************************************
 -}
 
-initLlvmConfig :: String
+-- Note [LLVM configuration]
+-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+-- The `llvm-targets` and `llvm-passes` files are shipped with GHC and contain
+-- information needed by the LLVM backend to invoke `llc` and `opt`.
+-- Specifically:
+--
+--  * llvm-targets maps autoconf host triples to the corresponding LLVM
+--    `data-layout` declarations. This information is extracted from clang using
+--    the script in utils/llvm-targets/gen-data-layout.sh and should be updated
+--    whenever we target a new version of LLVM.
+--
+--  * llvm-passes maps GHC optimization levels to sets of LLVM optimization
+--    flags that GHC should pass to `opt`.
+--
+-- This information is contained in files rather the GHC source to allow users
+-- to add new targets to GHC without having to recompile the compiler.
+--
+-- Since this information is only needed by the LLVM backend we load it lazily
+-- with unsafeInterleaveIO. Consequently it is important that we lazily pattern
+-- match on LlvmConfig until we actually need its contents.
+
+lazyInitLlvmConfig :: String
                -> IO LlvmConfig
-initLlvmConfig top_dir
-  = do
+lazyInitLlvmConfig top_dir
+  = unsafeInterleaveIO $ do    -- see Note [LLVM configuration]
       targets <- readAndParse "llvm-targets" mkLlvmTarget
       passes <- readAndParse "llvm-passes" id
-      return (targets, passes)
+      return $ LlvmConfig { llvmTargets = targets, llvmPasses = passes }
   where
     readAndParse name builder =
       do let llvmConfigFile = top_dir </> name