[flang-commits] [flang] [flang][Driver] Enable -Os and -Oz in flang (PR #164707)

[Tarun Prabhu via flang-commits]

https://github.com/tarunprabhu updated https://github.com/llvm/llvm-project/pull/164707

>From 2d755d71187888356af0a394a6d1d2d6cfeb88e0 Mon Sep 17 00:00:00 2001
From: Tarun Prabhu <tarun.prabhu at gmail.com>
Date: Wed, 22 Oct 2025 14:44:31 -0600
Subject: [PATCH] [flang] Enable -Os and -Oz in flang

The implementation adheres closely to the implementation in clang. The effect
on the pass pipelines have been tested.
---
 .../include/flang/Frontend/CodeGenOptions.def |  2 ++
 flang/lib/Frontend/CompilerInvocation.cpp     | 23 +++++++++++++++++++
 flang/lib/Frontend/FrontendActions.cpp        | 11 ++++++++-
 .../Driver/default-optimization-pipelines.f90 | 14 ++++++++++-
 4 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/flang/include/flang/Frontend/CodeGenOptions.def b/flang/include/flang/Frontend/CodeGenOptions.def
index dc3da7ba5c7f3..5892d3ef24e5a 100644
--- a/flang/include/flang/Frontend/CodeGenOptions.def
+++ b/flang/include/flang/Frontend/CodeGenOptions.def
@@ -20,6 +20,8 @@ CODEGENOPT(Name, Bits, Default)
 #endif
 
 CODEGENOPT(OptimizationLevel, 2, 0) ///< The -O[0-3] option specified.
+/// The -Os (==1) or -Oz (==2) option is specified.
+CODEGENOPT(OptimizeSize, 2, 0) 
 
 CODEGENOPT(DebugPassManager, 1, 0) ///< Prints debug information for the new
                                    ///< pass manager.
diff --git a/flang/lib/Frontend/CompilerInvocation.cpp b/flang/lib/Frontend/CompilerInvocation.cpp
index 548ca675db5ea..6dfff3b001aea 100644
--- a/flang/lib/Frontend/CompilerInvocation.cpp
+++ b/flang/lib/Frontend/CompilerInvocation.cpp
@@ -114,6 +114,10 @@ static unsigned getOptimizationLevel(llvm::opt::ArgList &args,
 
     assert(a->getOption().matches(clang::driver::options::OPT_O));
 
+    llvm::StringRef s(a->getValue());
+    if (s == "s" || s == "z")
+      return 2;
+
     return getLastArgIntValue(args, clang::driver::options::OPT_O, defaultOpt,
                               diags);
   }
@@ -121,6 +125,24 @@ static unsigned getOptimizationLevel(llvm::opt::ArgList &args,
   return defaultOpt;
 }
 
+/// Extracts the size-optimization level from \a args
+static unsigned getOptimizationLevelSize(llvm::opt::ArgList &args) {
+  if (llvm::opt::Arg *a =
+          args.getLastArg(clang::driver::options::OPT_O_Group)) {
+    if (a->getOption().matches(clang::driver::options::OPT_O)) {
+      switch (a->getValue()[0]) {
+      default:
+        return 0;
+      case 's':
+        return 1;
+      case 'z':
+        return 2;
+      }
+    }
+  }
+  return 0;
+}
+
 bool Fortran::frontend::parseDiagnosticArgs(clang::DiagnosticOptions &opts,
                                             llvm::opt::ArgList &args) {
   opts.ShowColors = parseShowColorsArgs(args);
@@ -273,6 +295,7 @@ static void parseCodeGenArgs(Fortran::frontend::CodeGenOptions &opts,
                              llvm::opt::ArgList &args,
                              clang::DiagnosticsEngine &diags) {
   opts.OptimizationLevel = getOptimizationLevel(args, diags);
+  opts.OptimizeSize = getOptimizationLevelSize(args);
 
   if (args.hasFlag(clang::driver::options::OPT_fdebug_pass_manager,
                    clang::driver::options::OPT_fno_debug_pass_manager, false))
diff --git a/flang/lib/Frontend/FrontendActions.cpp b/flang/lib/Frontend/FrontendActions.cpp
index 0c630d2ba876d..d6fb98bc930c6 100644
--- a/flang/lib/Frontend/FrontendActions.cpp
+++ b/flang/lib/Frontend/FrontendActions.cpp
@@ -602,7 +602,16 @@ mapToLevel(const Fortran::frontend::CodeGenOptions &opts) {
   case 1:
     return llvm::OptimizationLevel::O1;
   case 2:
-    return llvm::OptimizationLevel::O2;
+    switch (opts.OptimizeSize) {
+    default:
+      llvm_unreachable("Invalid optimization level for size!");
+    case 0:
+      return llvm::OptimizationLevel::O2;
+    case 1:
+      return llvm::OptimizationLevel::Os;
+    case 2:
+      return llvm::OptimizationLevel::Oz;
+    }
   case 3:
     return llvm::OptimizationLevel::O3;
   }
diff --git a/flang/test/Driver/default-optimization-pipelines.f90 b/flang/test/Driver/default-optimization-pipelines.f90
index 08e407f73da5c..18108cd632220 100644
--- a/flang/test/Driver/default-optimization-pipelines.f90
+++ b/flang/test/Driver/default-optimization-pipelines.f90
@@ -14,10 +14,16 @@
 ! RUN: %flang_fc1 -S -O2 %s -flto=full -fdebug-pass-manager -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-O2-LTO
 ! RUN: %flang_fc1 -S -O2 %s -flto=thin -fdebug-pass-manager -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-O2-THINLTO
 
-! Verify that only the left-most `-O{n}` is used
+! Verify that only the right-most `-O{n}` is used
 ! RUN: %flang -S -O2 -O0 %s -Xflang -fdebug-pass-manager -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-O0
 ! RUN: %flang_fc1 -S -O2 -O0 %s -fdebug-pass-manager -o /dev/null 2>&1 | FileCheck %s --check-prefix=CHECK-O0
 
+! Verify that passing -Os/-Oz have the desired effect on the pass pipelines.
+! RUN: %flang -S -Os %s -Xflang -fdebug-pass-manager -o /dev/null 2>&1 \
+! RUN:     | FileCheck %s --check-prefix=CHECK-OSIZE
+! RUN: %flang -S -Oz %s -Xflang -fdebug-pass-manager -o /dev/null 2>&1 \
+! RUN:     | FileCheck %s --check-prefix=CHECK-OSIZE
+
 ! CHECK-O0-NOT: Running pass: SimplifyCFGPass on simple_loop_
 ! CHECK-O0: Running analysis: TargetLibraryAnalysis on simple_loop_
 ! CHECK-O0-ANYLTO: Running pass: CanonicalizeAliasesPass on [module]
@@ -33,6 +39,12 @@
 ! CHECK-O2-THINLTO: Running pass: CanonicalizeAliasesPass on [module]
 ! CHECK-O2-THINLTO: Running pass: NameAnonGlobalPass on [module]
 
+! -Os/-Oz imply -O2, so check that a pass that runs on O2 is run. Then check
+! that passes like LibShrinkWrap, that should not be run when optimizing for
+! size, are not run (see llvm/lib/Passes/PassBuilderPipelines.cpp).
+! CHECK-OSIZE: Running pass: SimplifyCFGPass on simple_loop_
+! CHECK-OSIZE-NOT: Running pass: LibCallsShrinkWrapPass on simple_loop_
+
 subroutine simple_loop
   integer :: i
   do i=1,5