Fix finding LLD on Homebrew when multiple versions are installed.

.github/workflows/presubmit.yml

@@ -33,7 +33,7 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Install clang-tidy
-        run: brew install llvm@19 ninja lld
+        run: brew install llvm@19 ninja lld@19
       - name: Run clang-tidy
         run: ./run-clang-tidy.sh
         env:

cmake/FindHalide_LLVM.cmake

@@ -28,13 +28,38 @@ endif ()
 option(Halide_LLVM_SHARED_LIBS "Enable to link to shared libLLVM" "${Halide_LLVM_SHARED_LIBS}")
 
 if (LLVM_FOUND)
-    find_package(Clang HINTS "${LLVM_INSTALL_PREFIX}" "${LLVM_DIR}/../clang" "${LLVM_DIR}/../lib/cmake/clang")
+    # LLVM sadly doesn't provide *ConfigVersion.cmake files for its
+    # sub-components, including Clang and LLD. Consequently, attempting to
+    # constrain the version to LLVM_PACKAGE_VERSION prevents these find_package
+    # calls from succeeding at all. Worse, package maintainers have some
+    # "interesting" ideas as to how they should lay out the -dev packages,
+    # especially when they want to support multiple parallel versions. These
+    # hints take effect at a lower precedence than Halide_LLVM_ROOT
+    # or CMAKE_PREFIX_PATH (which are the standard ways of setting up the
+    # dependency search), but at a higher precedence than the system-wide
+    # fallback locations.
+    find_package(
+        Clang
+        HINTS
+        "${LLVM_INSTALL_PREFIX}" # Same root as the LLVM we found
+        "${LLVM_DIR}/../clang" # LLVM found in $ROOT/lib/cmake/llvm
+        "${LLVM_DIR}/../lib/cmake/clang" # LLVM found in $ROOT/cmake
+    )
 
     foreach (comp IN LISTS LLVM_TARGETS_TO_BUILD)
         if (comp STREQUAL "WebAssembly")
             set(Halide_LLVM_${comp}_FOUND 0)
 
-            find_package(LLD HINTS "${LLVM_INSTALL_PREFIX}" "${LLVM_DIR}/../lld" "${LLVM_DIR}/../lib/cmake/lld")
+            find_package(
+                LLD HINTS
+                "${LLVM_INSTALL_PREFIX}"
+                # Homebrew split the LLVM and LLD packages as of version 19, so
+                # having multiple LLVM versions installed leads to the newest
+                # LLD being found without this hint.
+                "${LLVM_INSTALL_PREFIX}/../lld@${LLVM_VERSION_MAJOR}"
+                "${LLVM_DIR}/../lld"
+                "${LLVM_DIR}/../lib/cmake/lld"
+            )
             if (NOT LLD_FOUND)
                 string(APPEND REASON_FAILURE_MESSAGE
                        "WebAssembly was not found because liblld is missing. "