[Flang][Driver] Add a flag to control zero initialization of global variables

Patch adds a flag to control zero initialization of global variables
without default initialization. The default is to zero initialize.

Author: kiranchandramohan

clang/include/clang/Driver/Options.td

@@ -3494,6 +3494,9 @@ def fno_struct_path_tbaa : Flag<["-"], "fno-struct-path-tbaa">, Group<f_Group>;
 def fno_strict_enums : Flag<["-"], "fno-strict-enums">, Group<f_Group>;
 def fno_strict_overflow : Flag<["-"], "fno-strict-overflow">, Group<f_Group>,
   Visibility<[ClangOption, FlangOption]>;
+def fno_zero_init_global_without_init : Flag<["-"], "fno-zero-init-global-without-init">, Group<f_Group>,
+  Visibility<[FlangOption, FC1Option]>,
+  HelpText<"Do not zero initialize globals without default initialization">;
 def fno_pointer_tbaa : Flag<["-"], "fno-pointer-tbaa">, Group<f_Group>;
 def fno_temp_file : Flag<["-"], "fno-temp-file">, Group<f_Group>,
   Visibility<[ClangOption, CC1Option, CLOption, DXCOption]>, HelpText<

clang/lib/Driver/ToolChains/Flang.cpp

@@ -153,6 +153,7 @@ void Flang::addCodegenOptions(const ArgList &Args,
   Args.addAllArgs(CmdArgs, {options::OPT_flang_experimental_hlfir,
                             options::OPT_flang_deprecated_no_hlfir,
                             options::OPT_fno_ppc_native_vec_elem_order,
+                            options::OPT_fno_zero_init_global_without_init,
                             options::OPT_fppc_native_vec_elem_order});
 }
 

flang/include/flang/Lower/LoweringOptions.def

@@ -44,5 +44,8 @@ ENUM_LOWERINGOPT(IntegerWrapAround, unsigned, 1, 0)
 /// If false, assume that the shapes/types/allocation-status match.
 ENUM_LOWERINGOPT(ReallocateLHS, unsigned, 1, 1)
 
+/// If true, initialize globals without initialization to zero.
+/// On by default.
+ENUM_LOWERINGOPT(ZeroInitGlobalsWithoutInit, unsigned, 1, 1)
 #undef LOWERINGOPT
 #undef ENUM_LOWERINGOPT

flang/lib/Frontend/CompilerInvocation.cpp

@@ -1373,6 +1373,12 @@ bool CompilerInvocation::createFromArgs(
     invoc.loweringOpts.setNoPPCNativeVecElemOrder(true);
   }
 
+  // -fno-zero-init-global-without-init
+  if (args.hasArg(
+          clang::driver::options::OPT_fno_zero_init_global_without_init)) {
+    invoc.loweringOpts.setZeroInitGlobalsWithoutInit(false);
+  }
+
   // Preserve all the remark options requested, i.e. -Rpass, -Rpass-missed or
   // -Rpass-analysis. This will be used later when processing and outputting the
   // remarks generated by LLVM in ExecuteCompilerInvocation.cpp.

flang/lib/Lower/ConvertVariable.cpp

@@ -50,7 +50,6 @@ static llvm::cl::opt<bool>
     allowAssumedRank("allow-assumed-rank",
                      llvm::cl::desc("Enable assumed rank lowering"),
                      llvm::cl::init(true));
-
 #define DEBUG_TYPE "flang-lower-variable"
 
 /// Helper to lower a scalar expression using a specific symbol mapping.
@@ -635,7 +634,11 @@ static fir::GlobalOp defineGlobal(Fortran::lower::AbstractConverter &converter,
       global.setLinkName(builder.createCommonLinkage());
     Fortran::lower::createGlobalInitialization(
         builder, global, [&](fir::FirOpBuilder &builder) {
-          mlir::Value initValue = builder.create<fir::ZeroOp>(loc, symTy);
+          mlir::Value initValue;
+          if (converter.getLoweringOptions().getZeroInitGlobalsWithoutInit())
+            initValue = builder.create<fir::ZeroOp>(loc, symTy);
+          else
+            initValue = builder.create<fir::UndefOp>(loc, symTy);
           builder.create<fir::HasValueOp>(loc, initValue);
         });
   }

flang/test/Lower/zero_init.f90

@@ -0,0 +1,16 @@
+! RUN: %flang_fc1 -emit-hlfir -o - %s | FileCheck --check-prefix=CHECK-DEFAULT %s
+! RUN: %flang_fc1 -fno-zero-init-global-without-init -emit-hlfir -o - %s | FileCheck --check-prefix=CHECK-NO-ZERO-INIT %s
+
+module m1
+  real :: x
+end module m1
+
+!CHECK-DEFAULT: fir.global @_QMm1Ex : f32 {
+!CHECK-DEFAULT:   %[[UNDEF:.*]] = fir.zero_bits f32
+!CHECK-DEFAULT:   fir.has_value %[[UNDEF]] : f32
+!CHECK-DEFAULT: }
+
+!CHECK-NO-ZERO-INIT: fir.global @_QMm1Ex : f32 {
+!CHECK-NO-ZERO-INIT:   %[[UNDEF:.*]] = fir.undefined f32
+!CHECK-NO-ZERO-INIT:   fir.has_value %[[UNDEF]] : f32
+!CHECK-NO-ZERO-INIT: }

flang/test/Lower/zero_init_default_init.f90

@@ -0,0 +1,16 @@
+! RUN: %flang_fc1 -emit-hlfir -o - %s | FileCheck %s
+! RUN: %flang_fc1 -fno-zero-init-global-without-init -emit-hlfir -o - %s | FileCheck %s
+
+module m2
+  type val
+    integer :: my_val = 1
+  end type val
+  type(val) :: v1
+end module m2
+
+!CHECK:  fir.global @_QMm2Ev1 : !fir.type<_QMm2Tval{my_val:i32}> {
+!CHECK:    %[[V1:.*]] = fir.undefined !fir.type<_QMm2Tval{my_val:i32}>
+!CHECK:    %[[ONE:.*]] = arith.constant 1 : i32
+!CHECK:    %[[V1_INIT:.*]] = fir.insert_value %[[V1]], %[[ONE]], ["my_val", !fir.type<_QMm2Tval{my_val:i32}>] : (!fir.type<_QMm2Tval{my_val:i32}>, i32) -> !fir.type<_QMm2Tval{my_val:i32}>
+!CHECK:    fir.has_value %[[V1_INIT]] : !fir.type<_QMm2Tval{my_val:i32}>
+!CHECK:  }