Force-disable BMI2 instruction set when building snappy.lib

.github/workflows/bcny-firebase.yml

@@ -119,6 +119,25 @@ jobs:
                 -D CMAKE_MSVC_DEBUG_INFORMATION_FORMAT=Embedded `
                 -D FIREBASE_PYTHON_HOST_EXECUTABLE:FILEPATH=${{ steps.python.outputs.python-path }} `
                 -D FLATBUFFERS_FLATC_EXECUTABLE=${{ github.workspace }}/BinaryCache/flatbuffers/Release/flatc.exe
+
+      # The step below is necessary since Snappy detects if it should use BMI2 instructions on x64 based only on its ability to
+      # compile BMI2 code on the host machine, but we need to disable BMI2 instructions so that the resulting nuget package
+      # can be used on target machines without BMI2 support. Unfortunately Snappy is pulled indirectly from
+      # another dependency (firebase-ios-sdk) in a very involved dependency chain and there is no good way to
+      # set SNAPPY_HAVE_BMI2 without patching its code since compiler directives have less precendence than
+      # the result of the host machine's check_cxx_source_compiles (which ends up in the config.h file).
+      # So, the less messy solution seems to be to patch the config.h file after the configure step, which is what we do below.
+      - name: Change SNAPPY_HAVE_BMI2 in snappy config.h
+        # BMI2 instructions are only relevant in amd64
+        if: ${{ matrix.arch }} == 'amd64'
+        run: |
+          $snappy_config = "${{ github.workspace }}\BinaryCache\firebase\external\src\firestore-build\external\src\snappy-build\config.h"
+          if (-not (Select-String -Path $snappy_config -Pattern "#define SNAPPY_HAVE_BMI2 1")) {
+            Write-Error "String '#define SNAPPY_HAVE_BMI2 1' expected but not found in $snappy_config"
+            exit 1
+          }
+          (Get-Content $snappy_config) -replace '#define SNAPPY_HAVE_BMI2 1', '#define SNAPPY_HAVE_BMI2 0' | Set-Content $snappy_config
+
       - name: Build firebase
         run: cmake --build ${{ github.workspace }}/BinaryCache/firebase --config RelWithDebInfo
       - name: Install firebase
@@ -137,6 +156,24 @@ jobs:
             Copy-Item -Path $library.FullName -Destination $destination -Force
             Write-Host "... copied ${destination}"
           }
+
+      # We need this library to be used on CPUs without BMI2 support, so we check that snappy.lib was built correctly,
+      # and fail if it contains BMI2 instructions.
+      - name: Check for Snappy BMI2 instructions
+        # BMI2 instructions are only relevant in amd64
+        if: ${{ matrix.arch }} == 'amd64'
+        run: |
+          Write-Host "Checking for BMI2 instructions on Snappy.lib..."
+          $snappy_lib = "${{ github.workspace }}/BuildRoot/Library/firebase/usr/libs/windows/snappy.lib"
+          $output = dumpbin /DISASM "$snappy_lib" | Select-String -Pattern "bzhi"
+          if ($output) {
+            Write-Host $output
+            Write-Error "ERROR: A BMI2 instruction ('bzhi') was not supposed to be found in $snappy_lib."
+            exit 1
+          } else {
+            Write-Output "Success! No BMI2 instructions were found in snappy.lib."
+          }
+
       - uses: actions/upload-artifact@v3
         with:
           name: firebase-windows-${{ matrix.arch }}