COFF: Do not assign linker-weak symbols to selectany comdat sections.

[pcc]

Summary

It is mandatory to specify a comdat in order to receive comdat semantics
for a symbol. We were previously getting this wrong in -function-sections
mode; linker-weak symbols were being emitted in a selectany comdat. This
change causes such symbols to use a noduplicates comdat instead, fixing
the inconsistency.

Also correct an inaccuracy in the docs.

[majnemer]

@rafael, what do you think?

[• rafael]

2015-06-30 2:36 GMT-04:00 David Majnemer <[e-mail]>:

@rafael, what do you think?

If I remember correctly, the only reason for using comdats at all with
-function-sections was so that link.exe could gc the sections.

Does that still work with IMAGE_COMDAT_SELECT_NODUPLICATES? If so this
LGTM, otherwise we should probably stop using comdats for
-ffunction-sections.

Cheers,
Rafael

[pcc]

I have verified (with the /map flag) that link.exe will GC unused noduplicates comdat sections.

[• rafael]

LGTM

docs/LangRef.rst

.. code-block:: llvm
    @g1 = global i32 42, section "sec", comdat($foo)
    @g2 = global i32 42, section "sec", comdat($bar)
 
 From the object file perspective, this requires the creation of two sections
 with the same name.  This is necessary because both globals belong to different
 COMDAT groups and COMDATs, at the object file level, are represented by
 sections.
 
-Note that certain IR constructs like global variables and functions may create
-COMDATs in the object file in addition to any which are specified using COMDAT
-IR.  This arises, for example, when a global variable has linkonce_odr linkage.
+Note that certain IR constructs like global variables and functions may
+create COMDATs in the object file in addition to any which are specified using
+COMDAT IR.  This arises when the code generator is configured to emit globals
+in individual sections (e.g. when `-data-sections` or `-function-sections`
+is supplied to `llc`).
 
 .. _namedmetadatastructure:
 
 Named Metadata
 --------------
 
 Named metadata is a collection of metadata. :ref:`Metadata
 nodes <metadata>` (but not metadata strings) are the only valid

lib/CodeGen/TargetLoweringObjectFileImpl.cpp

if (ComdatKey == GV) {
       case Comdat::NoDuplicates:
         return COFF::IMAGE_COMDAT_SELECT_NODUPLICATES;
       case Comdat::SameSize:
         return COFF::IMAGE_COMDAT_SELECT_SAME_SIZE;
       }
     } else {
       return COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE;
     }
-  } else if (GV->isWeakForLinker()) {
-    return COFF::IMAGE_COMDAT_SELECT_ANY;
   }
   return 0;
 }
 
 MCSection *TargetLoweringObjectFileCOFF::getExplicitSectionGlobal(
     const GlobalValue *GV, SectionKind Kind, Mangler &Mang,
     const TargetMachine &TM) const {
   int Selection = 0;

test/CodeGen/X86/coff-weak.ll

+; RUN: llc -function-sections -o - %s | FileCheck %s
+
+target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+; CHECK: .section{{.*}}one_only
+define linkonce_odr void @foo() {
+  ret void
+}