[clang] [flang] [flang] Enable alias tags pass by default (PR #73111)

[Andrzej Warzyński via cfe-commits]

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@@ -142,6 +142,24 @@ void Flang::addCodegenOptions(const ArgList &Args,
   if (shouldLoopVersion(Args))
     CmdArgs.push_back("-fversion-loops-for-stride");
 
+  Arg *aliasAnalysis = Args.getLastArg(options::OPT_falias_analysis,
+                                       options::OPT_fno_alias_analysis);
+  Arg *optLevel =
+      Args.getLastArg(options::OPT_Ofast, options::OPT_O, options::OPT_O4);
+  if (aliasAnalysis) {
+    bool faliasAnalysis =
+        aliasAnalysis->getOption().matches(options::OPT_falias_analysis);
+    // only pass on the argument if it does not match that implied by the
+    // optimization level
+    if (optLevel && !faliasAnalysis) {
+      CmdArgs.push_back("-fno-alias-analysis");
+    } else {
+      if (faliasAnalysis)
+        // requested alias analysis but no optimization enabled
+        CmdArgs.push_back("-falias-analysis");
+    }
+  }
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banach-space wrote:

[nit] It would be worth adding a comment here explaining the intent (e.g. "Only forward `-f{no}-alias-analysis if that makes sense considering other optimisation flags"). Alternatively, you could rename `optLevel` as `optLevelForSpeed` to communicate that. Or both. 

Just to make things clear for our future selves ;-) 

https://github.com/llvm/llvm-project/pull/73111