testsuite: Make ghc_built_by_llvm check more precise

Previously it would hackily look at the flavour name to determine
whether LLVM was used to build stage2 ghc. However, this didn't work at
all with Hadrian and would miss cases like ARM where we use the LLVM
backend by default.

See #16087 for the motivation for why ghc_built_by_llvm is needed at
all. This should catch one of the ARMv7 failures described in #17555.

3 files changed, 11 insertions, 11 deletions

diff --git a/hadrian/src/Settings/Builders/RunTest.hs b/hadrian/src/Settings/Builders/RunTest.hs
index a26092ef49..d3ce1c6fb8 100644
--- a/hadrian/src/Settings/Builders/RunTest.hs
+++ b/hadrian/src/Settings/Builders/RunTest.hs
@@ -93,9 +93,6 @@ runTestBuilderArgs = builder RunTest ? do
     let timeoutProg = root -/- timeoutPath
     statsFilesDir <- expr haddockStatsFilesDir
 
-    -- See #16087
-    let ghcBuiltByLlvm = False -- TODO: Implement this check
-
     let asBool :: String -> Bool -> String
         asBool s b = s ++ show b
 
@@ -128,7 +125,6 @@ runTestBuilderArgs = builder RunTest ? do
 
             , arg "-e", arg $ "config.ghc_dynamic_by_default=" ++ show hasDynamicByDefault
             , arg "-e", arg $ "config.ghc_dynamic=" ++ show hasDynamic
-            , arg "-e", arg $ "config.ghc_built_by_llvm=" ++ show ghcBuiltByLlvm
 
             , arg "-e", arg $ "config.top=" ++ show (top -/- "testsuite")
             , arg "-e", arg $ "config.wordsize=" ++ show wordsize
diff --git a/testsuite/config/ghc b/testsuite/config/ghc
index 5e644d5477..884cb1e953 100644
--- a/testsuite/config/ghc
+++ b/testsuite/config/ghc
@@ -179,6 +179,15 @@ def get_compiler_info():
 
     config.have_ncg = compilerInfoDict.get("Have native code generator", "NO") == "YES"
 
+    # Whether GHC itself was built using the LLVM backend. We need to know this
+    # since some tests in ext-interp fail when stage2 ghc is built using
+    # LLVM. See #16087.
+    #
+    # The condition here is a bit approximate: we assume that if stage2 doesn't
+    # have the NCG and isn't unregisterised then it must be using the LLVM
+    # backend by default.
+    config.ghc_built_by_llvm = not config.have_ncg and not config.unregisterised
+
     config.have_RTS_linker = compilerInfoDict.get("target has RTS linker", "NO") == "YES"
     # external interpreter needs RTS linker support
     # If the field is not present (GHC 8.0 and earlier), assume we don't
@@ -220,6 +229,7 @@ def get_compiler_info():
         config.ghc_plugin_way   = "normal"
 
 # Note [Replacing backward slashes in config.libdir]
+# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 #
 # We *do* need to replace backslashes in config.libdir, for the following
 # reason:
diff --git a/testsuite/mk/test.mk b/testsuite/mk/test.mk
index be6b741751..6e6def19ab 100644
--- a/testsuite/mk/test.mk
+++ b/testsuite/mk/test.mk
@@ -178,6 +178,7 @@ else
 RUNTEST_OPTS += -e ghc_with_smp=False
 endif
 
+# Does the LLVM backend work?
 ifeq "$(LLC)" ""
 RUNTEST_OPTS += -e ghc_with_llvm=False
 else ifeq "$(TargetARCH_CPP)" "powerpc"
@@ -244,13 +245,6 @@ else
 RUNTEST_OPTS += -e config.local=True
 endif
 
-# Some tests in ext-interp fail when ghc-stage2 is built using LLVM. See #16087
-ifeq "$(findstring llvm,$(BUILD_FLAVOUR))" ""
-RUNTEST_OPTS += -e config.ghc_built_by_llvm=False
-else
-RUNTEST_OPTS += -e config.ghc_built_by_llvm=True
-endif
-
 RUNTEST_OPTS +=  \
 	--rootdir=. \
 	--config-file=$(CONFIG) \