Add -fexternal-dynamic-refs

Summary:
The `-dynamic` flag does two things:

* In the code generator, it generates code designed to link against
  external shared libraries.  References outside of the current module
  go through platform-specific indirection tables (e.g. the GOT on ELF).

* It enables a "way", which changes which hi files we look
  for (`Foo.dyn_hi`) and which libraries we link against.

Some specialised applications want the first of these without the
second. (I could go into detail here but it's probably not all that
important).

This diff splits out the code-generation effects of `-dynamic` from the
"way" parts of its behaviour, via a new flag `-fexternal-dynamic-refs`.

Test Plan: validate

Reviewers: niteria, bgamari, erikd

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4477

8 files changed, 48 insertions, 18 deletions

diff --git a/compiler/cmm/CLabel.hs b/compiler/cmm/CLabel.hs
index 9170ee0410..a37ce7e87f 100644
--- a/compiler/cmm/CLabel.hs
+++ b/compiler/cmm/CLabel.hs
@@ -982,17 +982,19 @@ labelDynamic :: DynFlags -> Module -> CLabel -> Bool
 labelDynamic dflags this_mod lbl =
   case lbl of
    -- is the RTS in a DLL or not?
-   RtsLabel _           -> (WayDyn `elem` ways dflags) && (this_pkg /= rtsUnitId)
+   RtsLabel _ ->
+     (gopt Opt_ExternalDynamicRefs dflags) && (this_pkg /= rtsUnitId)
 
-   IdLabel n _ _        -> isDllName dflags this_mod n
+   IdLabel n _ _ ->
+     isDllName dflags this_mod n
 
    -- When compiling in the "dyn" way, each package is to be linked into
    -- its own shared library.
    CmmLabel pkg _ _
     | os == OSMinGW32 ->
-       (WayDyn `elem` ways dflags) && (this_pkg /= pkg)
+       (gopt Opt_ExternalDynamicRefs dflags) && (this_pkg /= pkg)
     | otherwise ->
-       True
+       gopt Opt_ExternalDynamicRefs dflags
 
    LocalBlockLabel _    -> False
 
@@ -1010,14 +1012,15 @@ labelDynamic dflags this_mod lbl =
             -- When compiling in the "dyn" way, each package is to be
             -- linked into its own DLL.
             ForeignLabelInPackage pkgId ->
-                (WayDyn `elem` ways dflags) && (this_pkg /= pkgId)
+                (gopt Opt_ExternalDynamicRefs dflags) && (this_pkg /= pkgId)
 
        else -- On Mac OS X and on ELF platforms, false positives are OK,
             -- so we claim that all foreign imports come from dynamic
             -- libraries
             True
 
-   HpcTicksLabel m        -> (WayDyn `elem` ways dflags) && this_mod /= m
+   HpcTicksLabel m ->
+     (gopt Opt_ExternalDynamicRefs dflags) && this_mod /= m
 
    -- Note that DynamicLinkerLabels do NOT require dynamic linking themselves.
    _                 -> False
diff --git a/compiler/main/DynFlags.hs b/compiler/main/DynFlags.hs
index 99a798e936..7b9cb13254 100644
--- a/compiler/main/DynFlags.hs
+++ b/compiler/main/DynFlags.hs
@@ -539,6 +539,7 @@ data GeneralFlag
    | Opt_PIC                         -- ^ @-fPIC@
    | Opt_PIE                         -- ^ @-fPIE@
    | Opt_PICExecutable               -- ^ @-pie@
+   | Opt_ExternalDynamicRefs
    | Opt_SccProfilingOn
    | Opt_Ticky
    | Opt_Ticky_Allocd
@@ -1564,7 +1565,7 @@ wayGeneralFlags :: Platform -> Way -> [GeneralFlag]
 wayGeneralFlags _ (WayCustom {}) = []
 wayGeneralFlags _ WayThreaded = []
 wayGeneralFlags _ WayDebug    = []
-wayGeneralFlags _ WayDyn      = [Opt_PIC]
+wayGeneralFlags _ WayDyn      = [Opt_PIC, Opt_ExternalDynamicRefs]
     -- We could get away without adding -fPIC when compiling the
     -- modules of a program that is to be linked with -dynamic; the
     -- program itself does not need to be position-independent, only
@@ -3905,6 +3906,7 @@ fFlagsDeps = [
   flagSpec "error-spans"                      Opt_ErrorSpans,
   flagSpec "excess-precision"                 Opt_ExcessPrecision,
   flagSpec "expose-all-unfoldings"            Opt_ExposeAllUnfoldings,
+  flagSpec "external-dynamic-refs"            Opt_ExternalDynamicRefs,
   flagSpec "external-interpreter"             Opt_ExternalInterpreter,
   flagSpec "flat-cache"                       Opt_FlatCache,
   flagSpec "float-in"                         Opt_FloatIn,
diff --git a/compiler/nativeGen/PIC.hs b/compiler/nativeGen/PIC.hs
index 834aa1b86c..b28e0fc68f 100644
--- a/compiler/nativeGen/PIC.hs
+++ b/compiler/nativeGen/PIC.hs
@@ -178,7 +178,8 @@ cmmMakePicReference dflags lbl
                                 (platformOS     $ targetPlatform dflags)
                                 lbl ]
 
-        | (positionIndependent dflags || WayDyn `elem` ways dflags) && absoluteLabel lbl
+        | (positionIndependent dflags || gopt Opt_ExternalDynamicRefs dflags)
+            && absoluteLabel lbl
         = CmmMachOp (MO_Add (wordWidth dflags))
                 [ CmmReg (CmmGlobal PicBaseReg)
                 , CmmLit $ picRelative
@@ -238,7 +239,7 @@ howToAccessLabel
 howToAccessLabel dflags _ OSMinGW32 this_mod _ lbl
 
         -- Assume all symbols will be in the same PE, so just access them directly.
-        | WayDyn `notElem` ways dflags
+        | not (gopt Opt_ExternalDynamicRefs dflags)
         = AccessDirectly
 
         -- If the target symbol is in another PE we need to access it via the
@@ -339,7 +340,8 @@ howToAccessLabel dflags _ os _ _ _
         --           if we don't dynamically link to Haskell code,
         --           it actually manages to do so without messing things up.
         | osElfTarget os
-        , not (positionIndependent dflags) && WayDyn `notElem` ways dflags
+        , not (positionIndependent dflags) &&
+          not (gopt Opt_ExternalDynamicRefs dflags)
         = AccessDirectly
 
 howToAccessLabel dflags arch os this_mod DataReference lbl
@@ -470,7 +472,7 @@ needImportedSymbols dflags arch os
         -- PowerPC Linux: -fPIC or -dynamic
         | osElfTarget os
         , arch  == ArchPPC
-        = positionIndependent dflags || WayDyn `elem` ways dflags
+        = positionIndependent dflags || gopt Opt_ExternalDynamicRefs dflags
 
         -- PowerPC 64 Linux: always
         | osElfTarget os
@@ -480,7 +482,8 @@ needImportedSymbols dflags arch os
         -- i386 (and others?): -dynamic but not -fPIC
         | osElfTarget os
         , arch /= ArchPPC_64 ELF_V1 && arch /= ArchPPC_64 ELF_V2
-        = WayDyn `elem` ways dflags && not (positionIndependent dflags)
+        = gopt Opt_ExternalDynamicRefs dflags &&
+          not (positionIndependent dflags)
 
         | otherwise
         = False
diff --git a/docs/users_guide/8.6.1-notes.rst b/docs/users_guide/8.6.1-notes.rst
index 17617ac61a..1e3f509843 100644
--- a/docs/users_guide/8.6.1-notes.rst
+++ b/docs/users_guide/8.6.1-notes.rst
@@ -56,6 +56,11 @@ Compiler
 
 - GHC now supports British spelling of :extension:`GeneralizedNewtypeDeriving`.
 
+- The code-generation effects of :ghc-flag:`-dynamic` can now be
+  enabled independently by the flag
+  :ghc-flag:`-fexternal-dynamic-refs`. If you don't know why you might
+  need this, you don't need it.
+
 Runtime system
 ~~~~~~~~~~~~~~
 
diff --git a/docs/users_guide/phases.rst b/docs/users_guide/phases.rst
index ad9daf1279..a2c25c79eb 100644
--- a/docs/users_guide/phases.rst
+++ b/docs/users_guide/phases.rst
@@ -580,6 +580,15 @@ Options affecting code generation
     Windows, position-independent code is never used so the flag is a
     no-op on that platform.
 
+.. ghc-flag:: -fexternal-dynamic-refs
+    :shortdesc: Generate code for linking against dynamic libraries
+    :type: dynamic
+    :category: codegen
+
+    When generating code, assume that entities imported from a
+    different module might be dynamically linked.  This flag is enabled
+    automatically by :ghc-flag:`-dynamic`.
+
 .. ghc-flag:: -fPIE
     :shortdesc: Generate code for a position-independent executable (where available)
     :type: dynamic
@@ -596,12 +605,11 @@ Options affecting code generation
     :category: codegen
     :noindex:
 
-    When generating code, assume that entities imported from a different
-    package will be dynamically linked. This can reduce code size
-    tremendously, but may slow-down cross-package calls of non-inlined
-    functions. There can be some complications combining :ghc-flag:`-shared`
-    with this flag relating to linking in the RTS under Linux. See
-    :ghc-ticket:`10352`.
+    Build code for dynamic linking.  This can reduce code size
+    tremendously, but may slow-down cross-module calls of non-inlined
+    functions. There can be some complications combining
+    :ghc-flag:`-shared` with this flag relating to linking in the RTS
+    under Linux. See :ghc-ticket:`10352`.
 
     Note that using this option when linking causes GHC to link against
     shared libraries.
diff --git a/testsuite/tests/ghci/scripts/T9293.stdout b/testsuite/tests/ghci/scripts/T9293.stdout
index c5be11aa58..2e5adc404c 100644
--- a/testsuite/tests/ghci/scripts/T9293.stdout
+++ b/testsuite/tests/ghci/scripts/T9293.stdout
@@ -6,6 +6,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
@@ -24,6 +25,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
@@ -41,6 +43,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
@@ -60,6 +63,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
diff --git a/testsuite/tests/ghci/scripts/ghci024.stdout b/testsuite/tests/ghci/scripts/ghci024.stdout
index 1247616719..f38393bad4 100644
--- a/testsuite/tests/ghci/scripts/ghci024.stdout
+++ b/testsuite/tests/ghci/scripts/ghci024.stdout
@@ -7,6 +7,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
diff --git a/testsuite/tests/ghci/scripts/ghci057.stdout b/testsuite/tests/ghci/scripts/ghci057.stdout
index c5be11aa58..2e5adc404c 100644
--- a/testsuite/tests/ghci/scripts/ghci057.stdout
+++ b/testsuite/tests/ghci/scripts/ghci057.stdout
@@ -6,6 +6,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
@@ -24,6 +25,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
@@ -41,6 +43,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history
@@ -60,6 +63,7 @@ with the following modifiers:
 GHCi-specific dynamic flag settings:
 other dynamic, non-language, flag settings:
   -fno-diagnostics-show-caret
+  -fexternal-dynamic-refs
   -fignore-optim-changes
   -fignore-hpc-changes
   -fno-ghci-history