MC-COFF: Don't emit local storage with .bss$linkonce

[llvmbot]

Bugzilla Link	8354
Resolution	FIXED
Resolved on	Aug 11, 2014 12:36
Version	trunk
OS	Windows XP
Blocks	llvm/llvm-bugzilla-archive#8390
Reporter	LLVM Bugzilla Contributor
CC	@rnk

Extended Description

.bss$linkonce_foobar would be unified by linker.
static and anonymous variables should not emitted to linkonce.

//////// foo.cc
int foo;
static int bar;

namespace {
int qux;
static int quux;
}

void fooset(int a0, int a1, int a2, int a3)
{
foo = a0;
bar = a1;
qux = a2;
quux = a3;
}

int fooget()
{
return foo + bar + qux + quux;
}

//////// bar.cc
extern int foo;
static int bar;

namespace {
int qux;
static int quux;
}

void barset(int a0, int a1, int a2, int a3)
{
foo = a0;
bar = a1;
qux = a2;
quux = a3;
}

int barget()
{
return foo + bar + qux + quux;
}

========

$ clang++ -O3 -S -emit-llvm foo.cc -o foo.ll
(snip)
@​foo = global i32 0, align 4
@​_ZL3bar = internal global i32 0, align 4
@​_ZN12_GLOBAL__N_13quxE = internal global i32 0, align 4
@​_ZN12_GLOBAL__N_1L4quuxE = internal global i32 0, align 4

$ clang++ -O3 -S -emit-llvm bar.cc -o bar.ll
(snip)
@​foo = external global i32
@​_ZL3bar = internal global i32 0, align 4
@​_ZN12_GLOBAL__N_13quxE = internal global i32 0, align 4
@​_ZN12_GLOBAL__N_1L4quuxE = internal global i32 0, align 4

========

$ llc -filetype=obj foo.ll
$ llc -filetype=obj bar.ll
$ ld foo.o bar.o -o foobar.exe

$ objdump -h foobar.exe
Sections:
Idx Name Size VMA LMA File off Algn
0 .text 000000a8 00401000 00401000 00000400 24
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .data 00000004 00402000 00402000 00000600 22
CONTENTS, ALLOC, LOAD, DATA
2 .bss 0000000c 00403000 00403000 00000000 22
ALLOC
3 .idata 00000014 00404000 00404000 00000800 22
CONTENTS, ALLOC, LOAD, DATA

!!! size of .bss should be 0x18(4 * 6)!

$ nm -n --demangle foobar.exe

00402000 D data_start_
00402000 D foo
00402004 D data_end_
00403000 B bss_start_
00403000 B bss_end_
00403000 b .bss$linkonce__ZL3bar
00403000 b bar
00403004 b .bss$linkonce__ZN12_GLOBAL__N_13quxE
00403004 b (anonymous namespace)::qux
00403008 b .bss$linkonce__ZN12_GLOBAL__N_1L4quuxE
00403008 b (anonymous namespace)::quux

[llvmbot]

assigned to @Bigcheese

[llvmbot]

FYI, with MS link.exe emits below;

Sections:
Idx Name Size VMA LMA File off Algn
0 .text 00000098 00401000 00401000 00000200 22
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .data 0000001c 00402000 00402000 00000400 22
CONTENTS, ALLOC, LOAD, DATA

foo.o:
401000: 8b 44 24 04 mov 0x4(%esp),%eax
401004: a3 00 20 40 00 mov %eax,0x402000
401009: 8b 44 24 08 mov 0x8(%esp),%eax
40100d: a3 04 20 40 00 mov %eax,0x402004
401012: 8b 44 24 0c mov 0xc(%esp),%eax
401016: a3 0c 20 40 00 mov %eax,0x40200c
40101b: 8b 44 24 10 mov 0x10(%esp),%eax
40101f: a3 14 20 40 00 mov %eax,0x402014
401024: c3 ret

401030: a1 04 20 40 00 mov 0x402004,%eax
401035: 03 05 00 20 40 00 add 0x402000,%eax
40103b: 03 05 0c 20 40 00 add 0x40200c,%eax
401041: 03 05 14 20 40 00 add 0x402014,%eax
401047: c3 ret

bar.o:
401050: 8b 44 24 04 mov 0x4(%esp),%eax
401054: a3 00 20 40 00 mov %eax,0x402000
401059: 8b 44 24 08 mov 0x8(%esp),%eax
40105d: a3 08 20 40 00 mov %eax,0x402008
401062: 8b 44 24 0c mov 0xc(%esp),%eax
401066: a3 10 20 40 00 mov %eax,0x402010
40106b: 8b 44 24 10 mov 0x10(%esp),%eax
40106f: a3 18 20 40 00 mov %eax,0x402018
401074: c3 ret

401080: a1 08 20 40 00 mov 0x402008,%eax
401085: 03 05 00 20 40 00 add 0x402000,%eax
40108b: 03 05 10 20 40 00 add 0x402010,%eax
401091: 03 05 18 20 40 00 add 0x402018,%eax
401097: c3 ret

I don't know why .bss$* is not allocated to .bss.

[llvmbot]

a trivial patch.

--- a/lib/MC/WinCOFFStreamer.cpp
+++ b/lib/MC/WinCOFFStreamer.cpp
@@ -129,8 +129,11 @@ void WinCOFFStreamer::AddCommonSymbol(MCSymbol *Symbol, uint64_t Size,
unsigned ByteAlignment, bool External) {
assert(!Symbol->isInSection() && "Symbol must not already have a section!");

• std::string SectionName(".bss$linkonce");
• SectionName.append(Symbol->getName().begin(), Symbol->getName().end());

• std::string SectionName(".bss");

• if (External) {

• SectionName.append("$");

• SectionName.append(Symbol->getName().begin() + 1, Symbol->getName().end());

• }

  MCSymbolData &SymbolData = getAssembler().getOrCreateSymbolData(*Symbol);

I got executable opt.exe and clang.exe ;)

[rnk]

This was fixed somewhere along the way. We no longer emit anything like .bss$linkonce.

[llvmbot]

mentioned in issue llvm/llvm-bugzilla-archive#8390