[commit: ghc] wip/kavon-nosplit-llvm: adding a note about why we check for HscLlvm (02eb265)

[git at git.haskell.org]

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

On branch  : wip/kavon-nosplit-llvm
Link       : http://ghc.haskell.org/trac/ghc/changeset/02eb265a1998cfed1630936c60af884e899d63f4/ghc

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

commit 02eb265a1998cfed1630936c60af884e899d63f4
Author: Kavon Farvardin <kavon at farvard.in>
Date:   Fri May 12 17:36:07 2017 +0100

    adding a note about why we check for HscLlvm


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

02eb265a1998cfed1630936c60af884e899d63f4
 compiler/cmm/MkGraph.hs | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/compiler/cmm/MkGraph.hs b/compiler/cmm/MkGraph.hs
index 0c71210..eca5fbf 100644
--- a/compiler/cmm/MkGraph.hs
+++ b/compiler/cmm/MkGraph.hs
@@ -329,6 +329,12 @@ copyIn dflags conv area formals extra_stk
 -- Factoring out the common parts of the copyout functions yielded something
 -- more complicated:
 
+-- Note [llvm non-tail calls]
+-- We cannot explicily store a block address to the stack when using
+-- LLVM. Instead, we just leave space in the frame for the return 
+-- address, which LLVM's assembly code generator will fill in later 
+-- when it sees the CPSCALL pseudo-instruction.
+
 data Transfer = Call | JumpRet | Jump | Ret deriving Eq
 
 copyOutOflow :: DynFlags -> Convention -> Transfer -> Area -> [CmmExpr]
@@ -361,6 +367,7 @@ copyOutOflow dflags conv transfer area actuals updfr_off extra_stack_stuff
                          -- the return address if making a call
                   case transfer of
                      Call -> 
+                     -- see Note [llvm non-tail calls]
                        if hscTarget dflags == HscLlvm
                          then ([], widthInBytes (wordWidth dflags))
                          else