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MC: Support aligned COMMON symbols for COFF

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link.exe:
Fuzz testing has shown that COMMON symbols with size > 32 will always
have an alignment of at least 32 and all symbols with size < 32 will
have an alignment of at least the largest power of 2 less than the size
of the symbol.

binutils:
The BFD linker essentially work like the link.exe behavior but with
alignment 4 instead of 32.  The BFD linker also supports an extension to
COFF which adds an -aligncomm argument to the .drectve section which
permits specifying a precise alignment for a variable but MC currently
doesn't support editing .drectve in this way.

With all of this in mind, we decide to play a little trick: we can
ensure that the alignment will be respected by bumping the size of the
global to it's alignment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218201 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
David Majnemer 2014-09-21 09:18:07 +00:00
parent 1ca1e33c3a
commit 31b080d57f
4 changed files with 31 additions and 8 deletions
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4
lib/MC/MCObjectFileInfo.cpp
View File

@ -595,9 +595,7 @@ void MCObjectFileInfo::InitELFMCObjectFileInfo(Triple T) {
void MCObjectFileInfo::InitCOFFMCObjectFileInfo(Triple T) {
bool IsWoA = T.getArch() == Triple::arm || T.getArch() == Triple::thumb;
// The object file format cannot represent common symbols with explicit
// alignments.
CommDirectiveSupportsAlignment = false;
CommDirectiveSupportsAlignment = true;
// COFF
BSSSection =

17
lib/MC/WinCOFFStreamer.cpp
View File

@ -183,8 +183,21 @@ void MCWinCOFFStreamer::EmitCommonSymbol(MCSymbol *Symbol, uint64_t Size,
Symbol->getSection().getVariant() == MCSection::SV_COFF) &&
"Got non-COFF section in the COFF backend!");
if (ByteAlignment > 32)
report_fatal_error("alignment is limited to 32-bytes");
const Triple &T = getContext().getObjectFileInfo()->getTargetTriple();
if (T.isKnownWindowsMSVCEnvironment()) {
if (ByteAlignment > 32)
report_fatal_error("alignment is limited to 32-bytes");
} else {
// The bfd linker from binutils only supports alignments less than 4 bytes
// without inserting -aligncomm arguments into the .drectve section.
// TODO: Support inserting -aligncomm into the .drectve section.
if (ByteAlignment > 4)
report_fatal_error("alignment is limited to 4-bytes");
}
// Round size up to alignment so that we will honor the alignment request.
// TODO: We don't need to do this if we are targeting the bfd linker once we
// add support for adding -aligncomm into the .drectve section.
Size = std::max(Size, static_cast<uint64_t>(ByteAlignment));
AssignSection(Symbol, nullptr);

4
test/MC/COFF/comm.ll
View File

@ -9,5 +9,5 @@
; CHECK: .lcomm _a,1
; CHECK: .lcomm _b,8,8
; .comm uses log2 alignment
; CHECK: .comm _c,1
; CHECK: .comm _d,8
; CHECK: .comm _c,1,0
; CHECK: .comm _d,8,3

14
test/MC/COFF/comm.s
View File

@ -1,7 +1,9 @@
// RUN: llvm-mc -filetype=obj -triple i686-pc-win32 %s | llvm-readobj -t | FileCheck %s
.lcomm _a,4,4
.comm _b, 4
.comm _b, 4, 2
// _c has size 1 but align 32, the value field is the max of size and align.
.comm _c, 1, 5
// CHECK: Symbol {
@ -23,3 +25,13 @@
// CHECK-NEXT: StorageClass: External
// CHECK-NEXT: AuxSymbolCount: 0
// CHECK-NEXT: }
// CHECK: Symbol {
// CHECK: Name: _c
// CHECK-NEXT: Value: 32
// CHECK-NEXT: Section: IMAGE_SYM_UNDEFINED (0)
// CHECK-NEXT: BaseType: Null
// CHECK-NEXT: ComplexType: Null
// CHECK-NEXT: StorageClass: External
// CHECK-NEXT: AuxSymbolCount: 0
// CHECK-NEXT: }