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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@227431 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 84862811a9c52e7643bfc9a6728ae91df01fda37..41fb0645c813ac8e052557f1b3db10af17d01e25 100644 (file)
--- a/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/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/test/CodeGen/X86/global-sections.ll b/test/CodeGen/X86/global-sections.ll
index 24be27161f4a9524244898f89a75bd1debae2448..29bf9963b2d5e520da2261959d779ee75deff424 100644 (file)
--- a/test/CodeGen/X86/global-sections.ll
+++ b/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