LLVM: Use musttail instead of tailwip/llvm_musttail_experiment

This forces LLVM to handle tail calls as tail calls. Otherwise LLVM is
free to ignore the tail and do an ordinary call.

https://llvm.org/docs/LangRef.html#call-instruction

2 files changed, 3 insertions, 6 deletions

diff --git a/compiler/GHC/CmmToLlvm/CodeGen.hs b/compiler/GHC/CmmToLlvm/CodeGen.hs
index 3ad52b6f79..13e46ae9d5 100644
--- a/compiler/GHC/CmmToLlvm/CodeGen.hs
+++ b/compiler/GHC/CmmToLlvm/CodeGen.hs
@@ -915,8 +915,7 @@ genJump (CmmLit (CmmLabel lbl)) live = do
     (vf, stmts, top) <- getHsFunc live lbl
     (stgRegs, stgStmts) <- funEpilogue live
     let s1  = Expr $ Call TailCall vf stgRegs llvmStdFunAttrs
-    let s2  = Return Nothing
-    return (stmts `appOL` stgStmts `snocOL` s1 `snocOL` s2, top)
+    return (stmts `appOL` stgStmts `snocOL` s1, top)
 
 
 -- Call to unknown function / address
@@ -935,9 +934,7 @@ genJump expr live = do
     (v1, s1) <- doExpr (pLift fty) $ Cast cast vf (pLift fty)
     (stgRegs, stgStmts) <- funEpilogue live
     let s2 = Expr $ Call TailCall v1 stgRegs llvmStdFunAttrs
-    let s3 = Return Nothing
-    return (stmts `snocOL` s1 `appOL` stgStmts `snocOL` s2 `snocOL` s3,
-            top)
+    return (stmts `snocOL` s1 `appOL` stgStmts `snocOL` s2, top)
 
 
 -- | CmmAssign operation
diff --git a/compiler/GHC/Llvm/Ppr.hs b/compiler/GHC/Llvm/Ppr.hs
index 3cc4ab5394..6aa2a15e0b 100644
--- a/compiler/GHC/Llvm/Ppr.hs
+++ b/compiler/GHC/Llvm/Ppr.hs
@@ -283,7 +283,7 @@ ppCall opts ct fptr args attrs = case fptr of
 
     where
         ppCall' (LlvmFunctionDecl _ _ cc ret argTy params _) =
-            let tc = if ct == TailCall then text "tail " else empty
+            let tc = if ct == TailCall then text "ret musttail " else empty
                 ppValues = ppCallParams opts (map snd params) args
                 ppArgTy  = (ppCommaJoin $ map (ppr . fst) params) <>
                            (case argTy of