[clang][XCOFF] Indicate that XCOFF does not support COMDATs

[daltenty]

Summary

XCOFF doesn't support COMDATs, so clang shouldn't emit them.

[daltenty]

• Add missing newline

[sfertile]

1. We should probably update the COMDAT section of the lang ref to mention XCOFF doens't support COMDATS.
2. Will we report an error somewhere in the backend if we encounter a COMDAT?

[Xiangling_L]

In D74631#1876995, @sfertile wrote:

Will we report an error somewhere in the backend if we encounter a COMDAT?

I hit this when I was doing my static init patch as well: error in backend: COMDAT not yet supported by AIX.

[daltenty]

In D74631#1876995, @sfertile wrote:

1. We should probably update the COMDAT section of the lang ref to mention XCOFF doesn't support COMDATS.

I will update the langref as suggested.

2. Will we report an error somewhere in the backend if we encounter a COMDAT?

As mentioned by @Xiangling_L, we will run into report_fatal_error("COMDAT not yet supported by AIX.").

[daltenty]

• Update lang doc to indicate XCOFF doesn't support COMDAT

[sfertile]

LGTM.

llvm/docs/LangRef.rst

913–914

really minor nit: The current wording sounds a bit odd to my ear. Maybe either XCOFF and Mach-O platforms or XCOFF and Mach-O don't support COMDATs, ...

clang/test/CodeGen/xcoff-comdat.cpp

+// RUN: %clang_cc1 -triple powerpc-ibm-aix -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple powerpc64-ibm-aix -emit-llvm -o - %s | FileCheck %s
+
+class a {
+  virtual void d() {}
+  virtual void e();
+};
+void a::e() {}
+
+// CHECK-NOT: = comdat

llvm/docs/LangRef.rst

 ``largest``
     The linker will choose the section containing the largest COMDAT key.
 ``noduplicates``
     The linker requires that only section with this COMDAT key exist.
 ``samesize``
     The linker may choose any COMDAT key but the sections must contain the
     same amount of data.
 
-Note that the Mach-O platform doesn't support COMDATs, and ELF and WebAssembly
-only support ``any`` as a selection kind.
+Note that XCOFF and the Mach-O platform don't support COMDATs, and ELF and
+WebAssembly only support ``any`` as a selection kind.
 
 Here is an example of a COMDAT group where a function will only be selected if
 the COMDAT key's section is the largest:
 
 .. code-block:: text
 
    $foo = comdat largest
    @foo = global i32 2, comdat($foo)

llvm/include/llvm/ADT/Triple.h

public:
 
   /// Tests whether the target is VE
   bool isVE() const {
     return getArch() == Triple::ve;
   }
 
   /// Tests whether the target supports comdat
   bool supportsCOMDAT() const {
-    return !isOSBinFormatMachO();
+    return !(isOSBinFormatMachO() || isOSBinFormatXCOFF());
   }
 
   /// Tests whether the target uses emulated TLS as default.
   bool hasDefaultEmulatedTLS() const {
     return isAndroid() || isOSOpenBSD() || isWindowsCygwinEnvironment();
   }
 
   /// @}