[llvm] r227431 - Don't create multiple mergeable sections with -fdata-sections.

[Nico Weber]

I built clang at r227430 and r227431 and used each to build chrome on linux
in static library build (so most code goes into the final executable,
instead of having many .so files).

Without the patch, rebuilding after touching a small cc file takes 14.0s
(min of 3 builds following 1 build to warm up caches). With the patch, it's
13.8s. Binary size goes from 144,060,288 to 144,342,192, a 282 kB increase.
I didn't look at combined .o size, since that doesn't seem like an
interesting metric. So this looks more like a regression from over here.
(This is with GNU gold (GNU Binutils 2.24) 1.11).

Nico

On Thu, Jan 29, 2015 at 8:57 AM, David Blaikie <dblaikie at gmail.com> wrote:

>
> On Jan 29, 2015 4:46 AM, "Rafael Espindola" <rafael.espindola at gmail.com>
> wrote:
> >
> > Author: rafael
> > Date: Thu Jan 29 06:43:28 2015
> > New Revision: 227431
> >
> > URL: http://llvm.org/viewvc/llvm-project?rev=227431&view=rev
> > Log:
> > 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.
>
> Nice! - though the linked binary size is essentially the same (bar the
> rodata you mentioned below - hmm, chrome is fussy about executable
> size...)?
>
> >
> > The time for linking clang with gold (on a VM, on a laptop) goes
> > from 2.250089985 to 1.383001792 seconds.
>
> That sounds pretty awesome! Though I thought my gold links even on my
> non-virtual beefy desktop were longer than that. Perhaps not.
>
> >
> > 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.
> >
> > Modified:
> >     llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
> >     llvm/trunk/test/CodeGen/X86/global-sections.ll
> >
> > Modified: llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp?rev=227431&r1=227430&r2=227431&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp (original)
> > +++ llvm/trunk/lib/CodeGen/TargetLoweringObjectFileImpl.cpp Thu Jan 29
> 06:43:28 2015
> > @@ -246,24 +246,25 @@ static StringRef getSectionPrefixForGlob
> >  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();
> >
> > Modified: llvm/trunk/test/CodeGen/X86/global-sections.ll
> > URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/global-sections.ll?rev=227431&r1=227430&r2=227431&view=diff
> >
> ==============================================================================
> > --- llvm/trunk/test/CodeGen/X86/global-sections.ll (original)
> > +++ llvm/trunk/test/CodeGen/X86/global-sections.ll Thu Jan 29 06:43:28
> 2015
> > @@ -121,7 +121,7 @@ define void @F1() {
> >  ; LINUX: G7:
> >  ; LINUX:        .asciz  "abcdefghi"
> >
> > -; LINUX-SECTIONS: .section        .rodata.G7,"aMS", at progbits,1
> > +; LINUX-SECTIONS: .section        .rodata.str1.1,"aMS", at 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, at object           # @G14
> > -; LINUX-SECTIONS:        .section        .rodata..LG14,"aMS", at progbits,1
> > +; LINUX-SECTIONS:        .section
> .rodata.str1.1,"aMS", at 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", at progbits,8
> > +; LINUX-SECTIONS: .section      .rodata.cst8,"aM", at progbits,8
> >  ; LINUX-SECTIONS: G15:
> >
> >  ; WIN32-SECTIONS: .section      .rdata,"rd",one_only,_G15
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
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