[XCOFF] Enable -fdata-sections on AIX

Summary:
Some design decision worth noting about:

I've noticed a recent mailing discussing about why string literal is
not affected by -fdata-sections for ELF target:
http://lists.llvm.org/pipermail/llvm-dev/2020-September/145121.html

But on AIX, our linker could not split the mergeable string like other target.
So I think it would make more sense for us to emit separate csect for
every mergeable string in -fdata-sections mode,
as there might not be other ways for linker to do garbage collection
on unused mergeable string.

Reviewed By: daltenty, hubert.reinterpretcast

Differential Revision: https://reviews.llvm.org/D88339

1 files changed, 29 insertions, 21 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 676a465..92ffbec 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -2055,11 +2055,10 @@ MCSection *TargetLoweringObjectFileWasm::getStaticDtorSection(
 MCSymbol *
 TargetLoweringObjectFileXCOFF::getTargetSymbol(const GlobalValue *GV,
                                                const TargetMachine &TM) const {
-  if (TM.getDataSections())
-    report_fatal_error("XCOFF unique data sections not yet implemented");
-
   // We always use a qualname symbol for a GV that represents
   // a declaration, a function descriptor, or a common symbol.
+  // If a GV represents a GlobalVariable and -fdata-sections is enabled, we
+  // also return a qualname so that a label symbol could be avoided.
   // It is inherently ambiguous when the GO represents the address of a
   // function, as the GO could either represent a function descriptor or a
   // function entry point. We choose to always return a function descriptor
@@ -2074,15 +2073,12 @@ TargetLoweringObjectFileXCOFF::getTargetSymbol(const GlobalValue *GV,
       return cast<MCSectionXCOFF>(
                  getSectionForFunctionDescriptor(cast<Function>(GO), TM))
           ->getQualNameSymbol();
-    if (GOKind.isCommon() || GOKind.isBSSLocal())
+    if (TM.getDataSections() || GOKind.isCommon() || GOKind.isBSSLocal())
       return cast<MCSectionXCOFF>(SectionForGlobal(GO, GOKind, TM))
           ->getQualNameSymbol();
   }
 
   // For all other cases, fall back to getSymbol to return the unqualified name.
-  // This could change for a GV that is a GlobalVariable when we decide to
-  // support -fdata-sections since we could avoid having label symbols if the
-  // linkage name is applied to the csect symbol.
   return nullptr;
 }
 
@@ -2107,9 +2103,6 @@ MCSection *TargetLoweringObjectFileXCOFF::getSectionForExternalReference(
 
 MCSection *TargetLoweringObjectFileXCOFF::SelectSectionForGlobal(
     const GlobalObject *GO, SectionKind Kind, const TargetMachine &TM) const {
-  assert(!TM.getDataSections() &&
-         "XCOFF unique data sections not yet implemented.");
-
   // Common symbols go into a csect with matching name which will get mapped
   // into the .bss section.
   if (Kind.isBSSLocal() || Kind.isCommon()) {
@@ -2129,6 +2122,9 @@ MCSection *TargetLoweringObjectFileXCOFF::SelectSectionForGlobal(
     SmallString<128> Name;
     Name = SizeSpec + utostr(Alignment.value());
 
+    if (TM.getDataSections())
+      getNameWithPrefix(Name, GO, TM);
+
     return getContext().getXCOFFSection(Name, XCOFF::XMC_RO, XCOFF::XTY_SD,
                                         Kind, /*BeginSymbolName*/ nullptr);
   }
@@ -2141,20 +2137,32 @@ MCSection *TargetLoweringObjectFileXCOFF::SelectSectionForGlobal(
     return TextSection;
   }
 
-  if (Kind.isData() || Kind.isReadOnlyWithRel())
-    // TODO: We may put this under option control, because user may want to
-    // have read-only data with relocations placed into a read-only section by
-    // the compiler.
-    return DataSection;
-
-  // Zero initialized data must be emitted to the .data section because external
-  // linkage control sections that get mapped to the .bss section will be linked
-  // as tentative defintions, which is only appropriate for SectionKind::Common.
-  if (Kind.isBSS())
+  // TODO: We may put Kind.isReadOnlyWithRel() under option control, because
+  // user may want to have read-only data with relocations placed into a
+  // read-only section by the compiler.
+  // For BSS kind, zero initialized data must be emitted to the .data section
+  // because external linkage control sections that get mapped to the .bss
+  // section will be linked as tentative defintions, which is only appropriate
+  // for SectionKind::Common.
+  if (Kind.isData() || Kind.isReadOnlyWithRel() || Kind.isBSS()) {
+    if (TM.getDataSections()) {
+      SmallString<128> Name;
+      getNameWithPrefix(Name, GO, TM);
+      return getContext().getXCOFFSection(Name, XCOFF::XMC_RW, XCOFF::XTY_SD,
+                                          SectionKind::getData());
+    }
     return DataSection;
+  }
 
-  if (Kind.isReadOnly())
+  if (Kind.isReadOnly()) {
+    if (TM.getDataSections()) {
+      SmallString<128> Name;
+      getNameWithPrefix(Name, GO, TM);
+      return getContext().getXCOFFSection(Name, XCOFF::XMC_RO, XCOFF::XTY_SD,
+                                          SectionKind::getReadOnly());
+    }
     return ReadOnlySection;
+  }
 
   report_fatal_error("XCOFF other section types not yet implemented.");
 }