CodeGen: Stick constant pool entries in COMDAT sections for WinCOFF

COFF lacks a feature that other object file formats support: mergeable
sections.

To work around this, MSVC sticks constant pool entries in special COMDAT
sections so that each constant is in it's own section.  This permits
unused constants to be dropped and it also allows duplicate constants in
different translation units to get merged together.

This fixes PR20262.

Differential Revision: http://reviews.llvm.org/D4482

llvm-svn: 213006

1 files changed, 21 insertions, 7 deletions

diff --git a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
index 4c3e6aa4732..424e759caa8 100644
--- a/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
@@ -1064,7 +1064,11 @@ void AsmPrinter::EmitConstantPool() {
 
     SectionKind Kind = CPE.getSectionKind(TM.getDataLayout());
 
-    const MCSection *S = getObjFileLowering().getSectionForConstant(Kind);
+    const Constant *C = nullptr;
+    if (!CPE.isMachineConstantPoolEntry())
+      C = CPE.Val.ConstVal;
+
+    const MCSection *S = getObjFileLowering().getSectionForConstant(Kind, C);
 
     // The number of sections are small, just do a linear search from the
     // last section to the first.
@@ -1087,13 +1091,22 @@ void AsmPrinter::EmitConstantPool() {
   }
 
   // Now print stuff into the calculated sections.
+  const MCSection *CurSection = nullptr;
+  unsigned Offset = 0;
   for (unsigned i = 0, e = CPSections.size(); i != e; ++i) {
-    OutStreamer.SwitchSection(CPSections[i].S);
-    EmitAlignment(Log2_32(CPSections[i].Alignment));
-
-    unsigned Offset = 0;
     for (unsigned j = 0, ee = CPSections[i].CPEs.size(); j != ee; ++j) {
       unsigned CPI = CPSections[i].CPEs[j];
+      MCSymbol *Sym = GetCPISymbol(CPI);
+      if (!Sym->isUndefined())
+        continue;
+
+      if (CurSection != CPSections[i].S) {
+        OutStreamer.SwitchSection(CPSections[i].S);
+        EmitAlignment(Log2_32(CPSections[i].Alignment));
+        CurSection = CPSections[i].S;
+        Offset = 0;
+      }
+
       MachineConstantPoolEntry CPE = CP[CPI];
 
       // Emit inter-object padding for alignment.
@@ -1103,8 +1116,8 @@ void AsmPrinter::EmitConstantPool() {
 
       Type *Ty = CPE.getType();
       Offset = NewOffset + TM.getDataLayout()->getTypeAllocSize(Ty);
-      OutStreamer.EmitLabel(GetCPISymbol(CPI));
 
+      OutStreamer.EmitLabel(Sym);
       if (CPE.isMachineConstantPoolEntry())
         EmitMachineConstantPoolValue(CPE.Val.MachineCPVal);
       else
@@ -1142,7 +1155,8 @@ void AsmPrinter::EmitJumpTableInfo() {
   } else {
     // Otherwise, drop it in the readonly section.
     const MCSection *ReadOnlySection =
-      getObjFileLowering().getSectionForConstant(SectionKind::getReadOnly());
+        getObjFileLowering().getSectionForConstant(SectionKind::getReadOnly(),
+                                                   /*C=*/nullptr);
     OutStreamer.SwitchSection(ReadOnlySection);
     JTInDiffSection = true;
   }