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, 10 insertions, 4 deletions

diff --git a/compiler/llvmGen/LlvmCodeGen.hs b/compiler/llvmGen/LlvmCodeGen.hs
index cc86c4254e..b566b99a1f 100644
--- a/compiler/llvmGen/LlvmCodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen.hs
@@ -94,12 +94,18 @@ llvmCodeGen' cmm_stream
     header :: SDoc
     header = sdocWithDynFlags $ \dflags ->
       let target = platformMisc_llvmTarget $ platformMisc dflags
-          layout = case lookup target (llvmTargets dflags) of
-            Just (LlvmTarget dl _ _) -> dl
-            Nothing -> error $ "Failed to lookup the datalayout for " ++ target ++ "; available targets: " ++ show (map fst $ llvmTargets dflags)
-      in     text ("target datalayout = \"" ++ layout ++ "\"")
+      in     text ("target datalayout = \"" ++ getDataLayout dflags target ++ "\"")
          $+$ text ("target triple = \"" ++ target ++ "\"")
 
+    getDataLayout :: DynFlags -> String -> String
+    getDataLayout dflags target =
+      case lookup target (llvmTargets $ llvmConfig dflags) of
+        Just (LlvmTarget {lDataLayout=dl}) -> dl
+        Nothing -> pprPanic "Failed to lookup LLVM data layout" $
+                   text "Target:" <+> text target $$
+                   hang (text "Available targets:") 4
+                        (vcat $ map (text . fst) $ llvmTargets $ llvmConfig dflags)
+
 llvmGroupLlvmGens :: RawCmmGroup -> LlvmM ()
 llvmGroupLlvmGens cmm = do