Put jump tables in unique sections on COFF.

If a function is going in an unique section (because of -ffunction-sections
for example), putting a jump table in .rodata will keep .rodata alive and
that will keep alive any other function that also has a jump table.

Instead, put the jump table in a unique section that is associated with the
function.

llvm-svn: 231961

llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h

@@ -147,6 +147,10 @@ public:
                            SectionKind Kind, Mangler &Mang,
                            const TargetMachine &TM) const override;
 
+  const MCSection *
+  getSectionForJumpTable(const Function &F, Mangler &Mang,
+                         const TargetMachine &TM) const override;
+
   /// Extract the dependent library name from a linker option string. Returns
   /// StringRef() if the option does not specify a library.
   StringRef getDepLibFromLinkerOpt(StringRef LinkerOption) const override;

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -948,6 +948,31 @@ SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
   return DataSection;
 }
 
+const MCSection *TargetLoweringObjectFileCOFF::getSectionForJumpTable(
+    const Function &F, Mangler &Mang, const TargetMachine &TM) const {
+  // If the function can be removed, produce a unique section so that
+  // the table doesn't prevent the removal.
+  const Comdat *C = F.getComdat();
+  bool EmitUniqueSection = TM.getFunctionSections() || C;
+  if (!EmitUniqueSection)
+    return ReadOnlySection;
+
+  // FIXME: we should produce a symbol for F instead.
+  if (F.hasPrivateLinkage())
+    return ReadOnlySection;
+
+  MCSymbol *Sym = TM.getSymbol(&F, Mang);
+  StringRef COMDATSymName = Sym->getName();
+
+  SectionKind Kind = SectionKind::getReadOnly();
+  const char *Name = getCOFFSectionNameForUniqueGlobal(Kind);
+  unsigned Characteristics = getCOFFSectionFlags(Kind);
+  Characteristics |= COFF::IMAGE_SCN_LNK_COMDAT;
+
+  return getContext().getCOFFSection(Name, Characteristics, Kind, COMDATSymName,
+                                     COFF::IMAGE_COMDAT_SELECT_ASSOCIATIVE);
+}
+
 StringRef TargetLoweringObjectFileCOFF::
 getDepLibFromLinkerOpt(StringRef LinkerOption) const {
   const char *LibCmd = "/DEFAULTLIB:";

llvm/test/CodeGen/X86/global-sections.ll

@@ -6,6 +6,7 @@
 ; RUN: llc < %s -mtriple=i386-unknown-linux-gnu -function-sections | FileCheck %s -check-prefix=LINUX-FUNC-SECTIONS
 ; RUN: llc < %s -mtriple=x86_64-pc-linux -data-sections -function-sections -relocation-model=pic | FileCheck %s -check-prefix=LINUX-SECTIONS-PIC
 ; RUN: llc < %s -mtriple=i686-pc-win32 -data-sections -function-sections | FileCheck %s -check-prefix=WIN32-SECTIONS
+; RUN: llc < %s -mtriple=i686-pc-win32 -function-sections | FileCheck %s -check-prefix=WIN32-FUNC-SECTIONS
 
 define void @F1() {
   ret void
@@ -48,6 +49,11 @@ bb5:
 ; LINUX-FUNC-SECTIONS-NEXT: .cfi_endproc
 ; LINUX-FUNC-SECTIONS-NEXT: .section        .rodata.F2,"a",@progbits
 
+; WIN32-FUNC-SECTIONS: .section        .text,"xr",one_only,_F2
+; WIN32-FUNC-SECTIONS-NOT: .section
+; WIN32-FUNC-SECTIONS: .section        .rdata,"dr",associative,_F2
+
+
 ; LINUX-SECTIONS-PIC: .section        .text.F2,"ax",@progbits
 ; LINUX-SECTIONS-PIC: .size   F2,
 ; LINUX-SECTIONS-PIC-NEXT: .cfi_endproc