[flang] (Re-)Enable alias tags pass by default (#74250)

Enable by default for optimization levels higher than 0 (same behavior
as clang).

For simplicity, only forward the flag to the frontend driver when it
contradicts what is implied by the optimization level.

This was first landed in
https://github.com/llvm/llvm-project/pull/73111 but was later reverted
due to a performance regression. That regression was fixed by
https://github.com/llvm/llvm-project/pull/74065.

1 files changed, 6 insertions, 4 deletions

diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index e7730d5..ec04727 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -242,10 +242,12 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
                    clang::driver::options::OPT_fno_loop_versioning, false))
     opts.LoopVersioning = 1;
 
-  opts.AliasAnalysis =
-      args.hasFlag(clang::driver::options::OPT_falias_analysis,
-                   clang::driver::options::OPT_fno_alias_analysis,
-                   /*default=*/false);
+  opts.AliasAnalysis = opts.OptimizationLevel > 0;
+  if (auto *arg =
+          args.getLastArg(clang::driver::options::OPT_falias_analysis,
+                          clang::driver::options::OPT_fno_alias_analysis))
+    opts.AliasAnalysis =
+        arg->getOption().matches(clang::driver::options::OPT_falias_analysis);
 
   for (auto *a : args.filtered(clang::driver::options::OPT_fpass_plugin_EQ))
     opts.LLVMPassPlugins.push_back(a->getValue());