[commit: ghc] master: LlvmCodeGen: Fix generation of malformed LLVM blocks (92821ec)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/92821ec9a57817e1429ae94c756539259488b728/ghc

>---------------------------------------------------------------

commit 92821ec9a57817e1429ae94c756539259488b728
Author: Erik de Castro Lopo <erikd at mega-nerd.com>
Date:   Sat Mar 12 12:03:56 2016 +0100

    LlvmCodeGen: Fix generation of malformed LLVM blocks
    
    Commit 673efccb3b uncovered a bug in LLVM code generation that produced
    LLVM code that the LLVM compiler refused to compile:
    
        {
        clpH:
          br label %clpH
        }
    
    This may well be a bug in LLVM itself. The solution is to keep the
    existing entry label and rewrite the function as:
    
        {
        clpH:
          br label %nPV
        nPV:
          br label %nPV
        }
    
    Thanks to Ben Gamari for pointing me in the right direction on this
    one.
    
    Test Plan: Build GHC with BuildFlavour=quick-llvm
    
    Reviewers: hvr, austin, bgamari
    
    Reviewed By: bgamari
    
    Subscribers: thomie
    
    Differential Revision: https://phabricator.haskell.org/D1996
    
    GHC Trac Issues: #11649


>---------------------------------------------------------------

92821ec9a57817e1429ae94c756539259488b728
 compiler/llvmGen/LlvmCodeGen.hs               | 34 ++++++++++++++++++++++++++-
 testsuite/tests/llvm/should_compile/T11649.hs | 16 +++++++++++++
 testsuite/tests/llvm/should_compile/all.T     |  1 +
 3 files changed, 50 insertions(+), 1 deletion(-)

diff --git a/compiler/llvmGen/LlvmCodeGen.hs b/compiler/llvmGen/LlvmCodeGen.hs
index 3c63aa0..872ad8c 100644
--- a/compiler/llvmGen/LlvmCodeGen.hs
+++ b/compiler/llvmGen/LlvmCodeGen.hs
@@ -15,8 +15,10 @@ import LlvmCodeGen.Ppr
 import LlvmCodeGen.Regs
 import LlvmMangler
 
+import BlockId
 import CgUtils ( fixStgRegisters )
 import Cmm
+import CmmUtils
 import Hoopl
 import PprCmm
 
@@ -120,13 +122,43 @@ cmmDataLlvmGens statics
 
        renderLlvm $ pprLlvmData (concat gss', concat tss)
 
+-- | LLVM can't handle entry blocks which loop back to themselves (could be
+-- seen as an LLVM bug) so we rearrange the code to keep the original entry
+-- label which branches to a newly generated second label that branches back
+-- to itself. See: Trac #11649
+fixBottom :: RawCmmDecl -> LlvmM RawCmmDecl
+fixBottom cp@(CmmProc hdr entry_lbl live g) =
+    maybe (pure cp) fix_block $ mapLookup (g_entry g) blk_map
+  where
+    blk_map = toBlockMap g
+
+    fix_block :: CmmBlock -> LlvmM RawCmmDecl
+    fix_block blk
+        | (CmmEntry e_lbl tickscp, middle, CmmBranch b_lbl) <- blockSplit blk
+        , isEmptyBlock middle
+        , e_lbl == b_lbl = do
+            new_lbl <- mkBlockId <$> getUniqueM
+
+            let fst_blk =
+                    BlockCC (CmmEntry e_lbl tickscp) BNil (CmmBranch new_lbl)
+                snd_blk =
+                    BlockCC (CmmEntry new_lbl tickscp) BNil (CmmBranch new_lbl)
+
+            pure . CmmProc hdr entry_lbl live . ofBlockMap (g_entry g)
+                $ mapFromList [(e_lbl, fst_blk), (new_lbl, snd_blk)]
+
+    fix_block _ = pure cp
+
+fixBottom rcd = pure rcd
+
 -- | Complete LLVM code generation phase for a single top-level chunk of Cmm.
 cmmLlvmGen ::RawCmmDecl -> LlvmM ()
 cmmLlvmGen cmm at CmmProc{} = do
 
     -- rewrite assignments to global regs
     dflags <- getDynFlag id
-    let fixed_cmm = {-# SCC "llvm_fix_regs" #-}
+    fixed_cmm <- fixBottom $
+                    {-# SCC "llvm_fix_regs" #-}
                     fixStgRegisters dflags cmm
 
     dumpIfSetLlvm Opt_D_dump_opt_cmm "Optimised Cmm" (pprCmmGroup [fixed_cmm])
diff --git a/testsuite/tests/llvm/should_compile/T11649.hs b/testsuite/tests/llvm/should_compile/T11649.hs
new file mode 100644
index 0000000..9d09c3a
--- /dev/null
+++ b/testsuite/tests/llvm/should_compile/T11649.hs
@@ -0,0 +1,16 @@
+{-# LANGUAGE NoImplicitPrelude #-}
+module Test where
+import GHC.Base
+
+data U1 p = U1
+
+instance Functor U1 where
+    fmap f U1 = U1
+
+instance Applicative U1 where
+    pure _ = U1
+    U1 <*> U1 = U1
+
+instance Alternative U1 where
+    empty = U1
+    U1 <|> U1 = U1
diff --git a/testsuite/tests/llvm/should_compile/all.T b/testsuite/tests/llvm/should_compile/all.T
index 9da136d..6806c25 100644
--- a/testsuite/tests/llvm/should_compile/all.T
+++ b/testsuite/tests/llvm/should_compile/all.T
@@ -13,3 +13,4 @@ test('T6158', [reqlib('vector'), reqlib('primitive')], compile, ['-package vecto
 test('T7571', cmm_src, compile, [''])
 test('T7575', unless(wordsize(32), skip), compile, [''])
 test('T8131b', normal, compile, [''])
+test('T11649', normal, compile, [''])