Align GHCi's library search order more closely with LDs

Summary:
Given a static library and an import library in the same folder. e.g.

```
libfoo.a
libfoo.dll.a
```

running `ghci -lfoo` we should prefer the import library `libfoo.dll.a`
over `libfoo.a` because we prefer having to just load the DLL.
And not having to do any linking.

This also more closely emulated the behaviour of LD, which has a search order of

```
libxxx.dll.a
xxx.dll.a
libxxx.a
cygxxx.dll (*)
libxxx.dll
xxx.dll
```

Test Plan: ./validate

Reviewers: RyanGlScott, austin, hvr, bgamari, erikd, simonmar

Reviewed By: RyanGlScott

Subscribers: thomie, #ghc_windows_task_force

Differential Revision: https://phabricator.haskell.org/D2651

GHC Trac Issues: #12771

7 files changed, 35 insertions, 3 deletions

diff --git a/compiler/ghci/Linker.hs b/compiler/ghci/Linker.hs
index a2ca1b514b..73d0fac1f5 100644
--- a/compiler/ghci/Linker.hs
+++ b/compiler/ghci/Linker.hs
@@ -1281,7 +1281,8 @@ loadFrameworks hsc_env platform pkg
 -- If it isn't present, we assume that addDLL in the RTS can find it,
 -- which generally means that it should be a dynamic library in the
 -- standard system search path.
-
+-- For GHCi we tend to prefer dynamic libraries over static ones as
+-- they are easier to load and manage, have less overhead.
 locateLib :: HscEnv -> Bool -> [FilePath] -> String -> IO LibrarySpec
 locateLib hsc_env is_hs dirs lib
   | not is_hs
@@ -1290,16 +1291,16 @@ locateLib hsc_env is_hs dirs lib
     --   then  look in library-dirs for a static library (libfoo.a)
     --   then look in library-dirs and inplace GCC for a dynamic library (libfoo.so)
     --   then  check for system dynamic libraries (e.g. kernel32.dll on windows)
-    --   then  try "gcc --print-file-name" to search gcc's search path
     --   then  try looking for import libraries on Windows (.dll.a, .lib)
+    --   then  try "gcc --print-file-name" to search gcc's search path
     --   then  look in library-dirs and inplace GCC for a static library (libfoo.a)
     --       for a dynamic library (#5289)
     --   otherwise, assume loadDLL can find it
     --
   = findDll     `orElse`
     findSysDll  `orElse`
-    tryGcc      `orElse`
     tryImpLib   `orElse`
+    tryGcc      `orElse`
     findArchive `orElse`
     assumeDll
 
diff --git a/testsuite/tests/rts/T12771/Makefile b/testsuite/tests/rts/T12771/Makefile
new file mode 100644
index 0000000000..d6960a022a
--- /dev/null
+++ b/testsuite/tests/rts/T12771/Makefile
@@ -0,0 +1,10 @@
+TOP=../../..
+include $(TOP)/mk/boilerplate.mk
+include $(TOP)/mk/test.mk
+
+T12771:
+	'$(TEST_CC)' -c foo.c -o foo.o
+	'$(AR)' rsc libfoo.a foo.o
+	'$(TEST_HC)' -shared foo_dll.c -o libfoo-1.dll
+	mv libfoo-1.dll.a libfoo.dll.a
+	echo main | '$(TEST_HC)' $(TEST_HC_OPTS_INTERACTIVE) main.hs -lfoo -L"$(PWD)"
diff --git a/testsuite/tests/rts/T12771/T12771.stdout b/testsuite/tests/rts/T12771/T12771.stdout
new file mode 100644
index 0000000000..d00491fd7e
--- /dev/null
+++ b/testsuite/tests/rts/T12771/T12771.stdout
@@ -0,0 +1 @@
+1
diff --git a/testsuite/tests/rts/T12771/all.T b/testsuite/tests/rts/T12771/all.T
new file mode 100644
index 0000000000..50933d5e67
--- /dev/null
+++ b/testsuite/tests/rts/T12771/all.T
@@ -0,0 +1,5 @@
+test('T12771', [ extra_clean(['libfoo.a', 'libfoo-1.dll', 'foo.o', 'main.o'])
+               , extra_files(['foo.c', 'main.hs', 'foo_dll.c'])
+               , unless(opsys('mingw32'), skip)
+               ],
+               run_command, ['$MAKE -s --no-print-directory T12771'])
diff --git a/testsuite/tests/rts/T12771/foo.c b/testsuite/tests/rts/T12771/foo.c
new file mode 100644
index 0000000000..af8ad9cb50
--- /dev/null
+++ b/testsuite/tests/rts/T12771/foo.c
@@ -0,0 +1,6 @@
+extern int bar();
+
+int foo ()
+{
+    return bar();
+}
diff --git a/testsuite/tests/rts/T12771/foo_dll.c b/testsuite/tests/rts/T12771/foo_dll.c
new file mode 100644
index 0000000000..8ea6c22735
--- /dev/null
+++ b/testsuite/tests/rts/T12771/foo_dll.c
@@ -0,0 +1,4 @@
+int foo()
+{
+    return 1;
+}
diff --git a/testsuite/tests/rts/T12771/main.hs b/testsuite/tests/rts/T12771/main.hs
new file mode 100644
index 0000000000..fbc8f5603c
--- /dev/null
+++ b/testsuite/tests/rts/T12771/main.hs
@@ -0,0 +1,5 @@
+module Main where
+
+foreign import ccall "foo" c_foo :: Int
+
+main = print c_foo