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, 13 insertions, 12 deletions

diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 465dd2737b..146020887b 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -83,7 +83,7 @@ module DynFlags (
         unsafeFlags, unsafeFlagsForInfer,
 
         -- ** LLVM Targets
-        LlvmTarget(..), LlvmTargets, LlvmPasses, LlvmConfig,
+        LlvmTarget(..), LlvmConfig(..),
 
         -- ** System tool settings and locations
         Settings(..),
@@ -970,8 +970,9 @@ data DynFlags = DynFlags {
   integerLibrary        :: IntegerLibrary,
     -- ^ IntegerGMP or IntegerSimple. Set at configure time, but may be overriden
     --   by GHC-API users. See Note [The integer library] in PrelNames
-  llvmTargets           :: LlvmTargets,
-  llvmPasses            :: LlvmPasses,
+  llvmConfig            :: LlvmConfig,
+    -- ^ N.B. It's important that this field is lazy since we load the LLVM
+    -- configuration lazily. See Note [LLVM Configuration] in SysTools.
   verbosity             :: Int,         -- ^ Verbosity level: see Note [Verbosity levels]
   optLevel              :: Int,         -- ^ Optimisation level
   debugLevel            :: Int,         -- ^ How much debug information to produce
@@ -1391,9 +1392,10 @@ data LlvmTarget = LlvmTarget
   , lAttributes :: [String]
   }
 
-type LlvmTargets = [(String, LlvmTarget)]
-type LlvmPasses = [(Int, String)]
-type LlvmConfig = (LlvmTargets, LlvmPasses)
+-- | See Note [LLVM Configuration] in SysTools.
+data LlvmConfig = LlvmConfig { llvmTargets :: [(String, LlvmTarget)]
+                             , llvmPasses  :: [(Int, String)]
+                             }
 
 -----------------------------------------------------------------------------
 -- Accessessors from 'DynFlags'
@@ -1924,7 +1926,7 @@ initDynFlags dflags = do
 -- | The normal 'DynFlags'. Note that they are not suitable for use in this form
 -- and must be fully initialized by 'GHC.runGhc' first.
 defaultDynFlags :: Settings -> LlvmConfig -> DynFlags
-defaultDynFlags mySettings (myLlvmTargets, myLlvmPasses) =
+defaultDynFlags mySettings llvmConfig =
 -- See Note [Updating flag description in the User's Guide]
      DynFlags {
         ghcMode                 = CompManager,
@@ -2035,8 +2037,8 @@ defaultDynFlags mySettings (myLlvmTargets, myLlvmPasses) =
         platformConstants = sPlatformConstants mySettings,
         rawSettings = sRawSettings mySettings,
 
-        llvmTargets             = myLlvmTargets,
-        llvmPasses              = myLlvmPasses,
+        -- See Note [LLVM configuration].
+        llvmConfig              = llvmConfig,
 
         -- ghc -M values
         depMakefile       = "Makefile",
@@ -5731,11 +5733,10 @@ makeDynFlagsConsistent dflags
 -- initialized.
 defaultGlobalDynFlags :: DynFlags
 defaultGlobalDynFlags =
-    (defaultDynFlags settings (llvmTargets, llvmPasses)) { verbosity = 2 }
+    (defaultDynFlags settings llvmConfig) { verbosity = 2 }
   where
     settings = panic "v_unsafeGlobalDynFlags: settings not initialised"
-    llvmTargets = panic "v_unsafeGlobalDynFlags: llvmTargets not initialised"
-    llvmPasses = panic "v_unsafeGlobalDynFlags: llvmPasses not initialised"
+    llvmConfig = panic "v_unsafeGlobalDynFlags: llvmConfig not initialised"
 
 #if STAGE < 2
 GLOBAL_VAR(v_unsafeGlobalDynFlags, defaultGlobalDynFlags, DynFlags)