Author: majnemer
Date: Sun Sep 21 04:18:07 2014
New Revision: 218201
URL: http://llvm.org/viewvc/llvm-project?rev=218201&view=rev
Log:
MC: Support aligned COMMON symbols for COFF
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.
Modified:
llvm/trunk/lib/MC/MCObjectFileInfo.cpp
llvm/trunk/lib/MC/WinCOFFStreamer.cpp
llvm/trunk/test/MC/COFF/comm.ll
llvm/trunk/test/MC/COFF/comm.s
Modified: llvm/trunk/lib/MC/MCObjectFileInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/MCObjectFileInfo.cpp?rev=218201&r1=218200&r2=218201&view=diff
==============================================================================
--- llvm/trunk/lib/MC/MCObjectFileInfo.cpp (original)
+++ llvm/trunk/lib/MC/MCObjectFileInfo.cpp Sun Sep 21 04:18:07 2014
@@ -595,9 +595,7 @@ void MCObjectFileInfo::InitELFMCObjectFi
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 =
Modified: llvm/trunk/lib/MC/WinCOFFStreamer.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/MC/WinCOFFStreamer.cpp?rev=218201&r1=218200&r2=218201&view=diff
==============================================================================
--- llvm/trunk/lib/MC/WinCOFFStreamer.cpp (original)
+++ llvm/trunk/lib/MC/WinCOFFStreamer.cpp Sun Sep 21 04:18:07 2014
@@ -183,8 +183,21 @@ void MCWinCOFFStreamer::EmitCommonSymbol
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);
Modified: llvm/trunk/test/MC/COFF/comm.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/COFF/comm.ll?rev=218201&r1=218200&r2=218201&view=diff
==============================================================================
--- llvm/trunk/test/MC/COFF/comm.ll (original)
+++ llvm/trunk/test/MC/COFF/comm.ll Sun Sep 21 04:18:07 2014
@@ -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
Modified: llvm/trunk/test/MC/COFF/comm.s
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/COFF/comm.s?rev=218201&r1=218200&r2=218201&view=diff
==============================================================================
--- llvm/trunk/test/MC/COFF/comm.s (original)
+++ llvm/trunk/test/MC/COFF/comm.s Sun Sep 21 04:18:07 2014
@@ -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 {
@@ -20,6 +22,16 @@
// 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: }
+
+// 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: }