Friendlier CMake output and ReadMe tips

CMakeLists.txt

@@ -166,6 +166,7 @@ if (BUILD_LLVM_UTILS OR BUILD_TV)
   find_package(LLVM REQUIRED CONFIG)
 
   message(STATUS "Found LLVM ${LLVM_PACKAGE_VERSION}")
+  message(STATUS "CMAKE_PREFIX_PATH: ${CMAKE_PREFIX_PATH}")
   message(STATUS "Using LLVMConfig.cmake in: ${LLVM_DIR}")
 
   if (NOT LLVM_ENABLE_RTTI)

README.md

@@ -119,11 +119,15 @@ To run translation validation on all the LLVM unit tests for IR-level
 transformations:
 
 ```
-$LLVM2_BUILD/bin/llvm-lit -vv -Dopt=$ALIVE2_HOME/alive2/build/opt-alive.sh $LLVM2_HOME/llvm/test/Transforms
+$LLVM2_BUILD/bin/llvm-lit -s -Dopt=$ALIVE2_HOME/alive2/build/opt-alive.sh $LLVM2_HOME/llvm/test/Transforms
 ```
 
 We run this command on the main LLVM branch each day, and keep track of the results
-[here](https://web.ist.utl.pt/nuno.lopes/alive2/).
+[here](https://web.ist.utl.pt/nuno.lopes/alive2/).  To detect unsound transformations in a local run:
+
+```
+fgrep -r "(unsound)" $ALIVE2_HOME/alive2/build/logs/
+```
 
 
 Running Alive2 as a Clang Plugin
@@ -307,11 +311,15 @@ should be the only user of this server.
 
 Troubleshooting
 --------
-Some combinations of Clang and MacOS versions may give link warnings 
+* Check the “LLVMConfig.cmake” and “CMAKE_PREFIX_PATH” output from CMake in case of build problems. CMake may look for configuration information in old installations of LLVM, e.g., under `/opt/`, if these are not set properly.
+* Some combinations of Clang and MacOS versions may give link warnings 
 “-undefined dynamic_lookup may not work with chained fixups,” and
 runtime errors with “symbol not found in flat namespace.”  Setting
 [CMAKE_OSX_DEPLOYMENT_TARGET](https://cmake.org/cmake/help/latest/variable/CMAKE_OSX_DEPLOYMENT_TARGET.html) as a cache entry to 11.0
-or less at the beginning of CMakeLists.txt may work around this. 
+or less at the beginning of CMakeLists.txt may work around this.
+* Building for Translation Validation requires enabling `BUILD_SHARED_LIBS`.  For LLVM forks not normally built with the option, this may interfere with CMake files’ use of `USEDLIBS` and `LLVMLIBS` and perhaps `dd_llvm_target`. 
+* Building for Translation Validation is tightly coupled to LLVM top of tree source.  Building a fork with older source may require reverting to the corresponding Alive2 commit.  This in turn may require experimentation with Clang versions and vendors.
+* Building older source on an up-to-date machine may require adjustments.  For example, the now-deleted file `scripts/rewritepass.py` depended on the deprecated Python 2; update the shebang line to `python3`.
 
 LLVM Bugs Found by Alive2
 --------