[AIX] Default AIX to using -fno-use-cxa-atexit

[Xiangling_L]

Summary

On AIX, we use __atexit to register dtor functions rather than __cxa_atexit.
So a driver change is needed to default AIX to using -fno-use-cxa-atexit.

Windows platform does not uses __cxa_atexit either. Following its precedent, we remove the assertion for when -fuse-cxa-atexit is specified by the user, do not produce a message and silently default to -fno-use-cxa-atexit behavior.

[hubert.reinterpretcast]

LGTM with minor comments.

clang/lib/CodeGen/ItaniumCXXABI.cpp

4532

Okay, the observation is that the Windows implementation doesn't implement a similar assertion for when -fuse-cxa-atexit is specified by the user and it neither uses __cxa_atexit nor produces a message about the option. This provides some amount of rationale for why AIX does not need to take extra precautions.

Can you update the patch description to cover the removal of said precautions and a bit of the rationale (that Windows didn't take similar precautions)?

clang/test/Driver/cxa-atexit.cpp

20

Can we add an extra line for 64-bit AIX?

clang/lib/CodeGen/ItaniumCXXABI.cpp

void XLCXXABI::registerGlobalDtor(CodeGenFunction &CGF, const VarDecl &D,
                                   llvm::FunctionCallee dtor,
                                   llvm::Constant *addr) {
   if (D.getTLSKind() != VarDecl::TLS_None)
     llvm::report_fatal_error("thread local storage not yet implemented on AIX");
 
   // Create __dtor function for the var decl.
   llvm::Function *dtorStub = CGF.createAtExitStub(D, dtor, addr);
 
-  if (CGM.getCodeGenOpts().CXAAtExit)
-    llvm::report_fatal_error("using __cxa_atexit unsupported on AIX");
   // Register above __dtor with atexit().
   CGF.registerGlobalDtorWithAtExit(dtorStub);
 
   // Emit __finalize function to unregister __dtor and (as appropriate) call
   // __dtor.
   emitCXXStermFinalizer(D, dtorStub, addr);
 }
 

clang/lib/Driver/ToolChains/Clang.cpp

if (Args.hasFlag(options::OPT_fshort_enums, options::OPT_fno_short_enums,
                    TC.getArch() == llvm::Triple::hexagon))
     CmdArgs.push_back("-fshort-enums");
 
   RenderCharacterOptions(Args, AuxTriple ? *AuxTriple : RawTriple, CmdArgs);
 
   // -fuse-cxa-atexit is default.
   if (!Args.hasFlag(
           options::OPT_fuse_cxa_atexit, options::OPT_fno_use_cxa_atexit,
-          !RawTriple.isOSWindows() &&
+          !RawTriple.isOSAIX() && !RawTriple.isOSWindows() &&
               TC.getArch() != llvm::Triple::xcore &&
               ((RawTriple.getVendor() != llvm::Triple::MipsTechnologies) ||
                RawTriple.hasEnvironment())) ||
       KernelOrKext)
     CmdArgs.push_back("-fno-use-cxa-atexit");
 
   if (Args.hasFlag(options::OPT_fregister_global_dtors_with_atexit,
                    options::OPT_fno_register_global_dtors_with_atexit,

clang/test/CodeGenCXX/aix-static-init.cpp

 // RUN: %clang_cc1 -triple powerpc-ibm-aix-xcoff -S -emit-llvm -x c++ \
-// RUN:     -fno-use-cxa-atexit -std=c++2a < %s | \
+// RUN:     -std=c++2a < %s | \
 // RUN:   FileCheck --check-prefixes=CHECK,CHECK32 %s
 
 // RUN: %clang_cc1 -triple powerpc64-ibm-aix-xcoff -S -emit-llvm -x c++ \
-// RUN:     -fno-use-cxa-atexit -std=c++2a < %s | \
+// RUN:     -std=c++2a < %s | \
 // RUN:   FileCheck --check-prefixes=CHECK,CHECK64 %s
 
 namespace test1 {
   struct Test1 {
     Test1();
     ~Test1();
   } t1, t2;
 } // namespace test1

clang/test/Driver/cxa-atexit.cpp

 // RUN: %clang -### -target x86_64-unknown-windows-cygnus -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-WINDOWS
 // RUN: %clang -### -target hexagon-unknown-none -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-HEXAGON
 // RUN: %clang -### -target xcore-unknown-none -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-XCORE
 // RUN: %clang -### -target armv7-mti-none -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-MTI
 // RUN: %clang -### -target mips-unknown-none -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-MIPS
 // RUN: %clang -### -target mips-unknown-none-gnu -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-MIPS
 // RUN: %clang -### -target mips-mti-none-gnu -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-MIPS
 // RUN: %clang -### -target sparc-sun-solaris -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-SOLARIS
+// RUN: %clang -### -target powerpc-ibm-aix-xcoff -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-AIX
+// RUN: %clang -### -target powerpc64-ibm-aix-xcoff -c %s -o /dev/null 2>&1 | FileCheck %s -check-prefix CHECK-AIX
 
 // CHECK-WINDOWS: "-fno-use-cxa-atexit"
 // CHECK-SOLARIS-NOT: "-fno-use-cxa-atexit"
 // CHECK-HEXAGON-NOT: "-fno-use-cxa-atexit"
 // CHECK-XCORE: "-fno-use-cxa-atexit"
 // CHECK-MTI: "-fno-use-cxa-atexit"
 // CHECK-MIPS-NOT: "-fno-use-cxa-atexit"
+// CHECK-AIX: "-fno-use-cxa-atexit"
 
 // RUN: %clang -target x86_64-apple-darwin -fregister-global-dtors-with-atexit -fno-register-global-dtors-with-atexit -c -### %s 2>&1 | \
 // RUN: FileCheck --check-prefix=WITHOUTATEXIT %s
 // RUN: %clang -target x86_64-apple-darwin -fno-register-global-dtors-with-atexit -fregister-global-dtors-with-atexit -c -### %s 2>&1 | \
 // RUN: FileCheck --check-prefix=WITHATEXIT %s
 // RUN: %clang -target x86_64-apple-darwin -c -### %s 2>&1 | \
 // RUN: FileCheck --check-prefix=WITHATEXIT %s
 // RUN: %clang -target x86_64-apple-darwin -c -mkernel -### %s 2>&1 | \