building/running-tests.md

@@ -583,3 +583,39 @@ respectively.
 # Problems running the testsuite
 
 1. If the test suite fails mysteriously, make sure that the `timeout` utility is working properly. This Haskell utility is compiled with the stage 1 compiler and invoked by the python driver, which does not print a nice error report if the utility fails. This can happen if, for example, the compiler produces bogus binaries. A workaround is to compile `timeout` with a stable `ghc`.
+
+# The testsuite and branches
+
+
+It is not clear what to do with the testsuite when branching a compiler; should the testsuite also be branched?
+
+
+If it is not branched then we have the problem that, given a set of tests
+
+```wiki
+test(tc1, ...)
+test(tc2, ...)
+test(tc3, ...)
+```
+
+
+if we add first one test, and then another to the HEAD
+
+```wiki
+test(tc1, ...)
+test(tc2, ...)
+test(tc3, ...)
+test(tc4, ...)
+test(tc5, ...)
+```
+
+
+and we want to merge `tc5` but not `tc4` to the branch then the merge has to be done by hand,
+as the patch for tc5 depends on the patch for tc4, although most of the files in the patches (`tc5.hs` etc) are disjoint.
+
+
+On the other hand, if it is not branched then any changes in test output mean we need to add extra logic to the test definitions, e.g.
+
+```wiki
+test(tc5, namebase_if_compiler_lt('ghc','6.9', 'tc5-6.8'), ...)
+```
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