Don't create multiple mergeable sections with -fdata-sections.

ELF has support for sections that can be split into fixed size or
null terminated entities.

Since these sections can be split by the linker, it is not necessary
to split them in codegen.

This reduces the combined .o size in a llvm+clang build from
202,394,570 to 173,819,098 bytes.

The time for linking clang with gold (on a VM, on a laptop) goes
from 2.250089985 to 1.383001792 seconds.

The flip side is the size of rodata in clang goes from 10,926,785
to 10,929,345 bytes.

The increase seems to be because of http://sourceware.org/bugzilla/show_bug.cgi?id=17902.

llvm-svn: 227431

diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 8486281..41fb064 100644 (file)
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -246,24 +246,25 @@ static StringRef getSectionPrefixForGlobal(SectionKind Kind) {
 const MCSection *TargetLoweringObjectFileELF::
 SelectSectionForGlobal(const GlobalValue *GV, SectionKind Kind,
                        Mangler &Mang, const TargetMachine &TM) const {
+  unsigned Flags = getELFSectionFlags(Kind);
+
   // If we have -ffunction-section or -fdata-section then we should emit the
   // global value to a uniqued section specifically for it.
-  bool EmitUniquedSection;
-  if (Kind.isText())
-    EmitUniquedSection = TM.getFunctionSections();
-  else
-    EmitUniquedSection = TM.getDataSections();
+  bool EmitUniquedSection = false;
+  if (!(Flags & ELF::SHF_MERGE) && !Kind.isCommon()) {
+    if (Kind.isText())
+      EmitUniquedSection = TM.getFunctionSections();
+    else
+      EmitUniquedSection = TM.getDataSections();
+  }
 
-  // If this global is linkonce/weak and the target handles this by emitting it
-  // into a 'uniqued' section name, create and return the section now.
-  if ((EmitUniquedSection && !Kind.isCommon()) || GV->hasComdat()) {
+  if (EmitUniquedSection || GV->hasComdat()) {
     StringRef Prefix = getSectionPrefixForGlobal(Kind);
 
     SmallString<128> Name(Prefix);
     TM.getNameWithPrefix(Name, GV, Mang, true);
 
     StringRef Group = "";
-    unsigned Flags = getELFSectionFlags(Kind);
     if (const Comdat *C = getELFComdat(GV)) {
       Flags |= ELF::SHF_GROUP;
       Group = C->getName();

diff --git a/llvm/test/CodeGen/X86/global-sections.ll b/llvm/test/CodeGen/X86/global-sections.ll
index 24be271..29bf996 100644 (file)
--- a/llvm/test/CodeGen/X86/global-sections.ll
+++ b/llvm/test/CodeGen/X86/global-sections.ll
@@ -121,7 +121,7 @@ define void @F1() {
 ; LINUX: G7:
 ; LINUX:        .asciz  "abcdefghi"
 
-; LINUX-SECTIONS: .section        .rodata.G7,"aMS",@progbits,1
+; LINUX-SECTIONS: .section        .rodata.str1.1,"aMS",@progbits,1
 ; LINUX-SECTIONS:       .globl G7
 
 ; WIN32-SECTIONS: .section        .rdata,"rd",one_only,_G7
@@ -182,7 +182,7 @@ define void @F1() {
 @G14 = private unnamed_addr constant [4 x i8] c"foo\00", align 1
 
 ; LINUX-SECTIONS:        .type   .LG14,@object           # @G14
-; LINUX-SECTIONS:        .section        .rodata..LG14,"aMS",@progbits,1
+; LINUX-SECTIONS:        .section        .rodata.str1.1,"aMS",@progbits,1
 ; LINUX-SECTIONS: .LG14:
 ; LINUX-SECTIONS:        .asciz  "foo"
 ; LINUX-SECTIONS:        .size   .LG14, 4
@@ -206,7 +206,7 @@ define void @F1() {
 ; DARWIN64: .section       __TEXT,__const
 ; DARWIN64: _G15:
 
-; LINUX-SECTIONS: .section      .rodata.G15,"aM",@progbits,8
+; LINUX-SECTIONS: .section      .rodata.cst8,"aM",@progbits,8
 ; LINUX-SECTIONS: G15:
 
 ; WIN32-SECTIONS: .section      .rdata,"rd",one_only,_G15