lld-link: Allow mixing 'discard' and 'largest' comdat selections
cl.exe and clang-cl.exe put vftables in a 'discard' comdat when building with
RTTI disabled (/GR-) but in a 'largest' comdat when building with RTTI enabled.
To be able to link /GR- code with /GR code, lld-link needs to accept comdats
that have this type of comdat selection conflict.
For example, static libraries in the Visual Studio standard library are built
with /GR, and without this it's impossible to build client code with /GR- and
still link to the standard library.
link.exe also accepts merging 'discard' with 'largest', and it accepts merging
'largest' with any other selection type. lld-link is still a bit stricter since
it only allows merging 'largest' with 'discard' for symmetry.
Differential Revision: https://reviews.llvm.org/D57515
git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@352765 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/COFF/InputFiles.cpp b/COFF/InputFiles.cpp
index 8687102..ba46184 100644
--- a/COFF/InputFiles.cpp
+++ b/COFF/InputFiles.cpp
@@ -481,15 +481,23 @@
Selection = LeaderSelection;
}
- // Requiring selections to match exactly is a bit more strict than
- // link.exe which allows merging "any" and "largest" if "any" is the first
- // symbol the linker sees, and it allows merging "largest" with everything
- // (!) if "largest" is the first symbol the linker sees. Making this
- // symmetric independent of which selection is seen first seems better
- // though, and if we can get away with not allowing merging "any" and
- // "largest" that keeps things more regular too.
- // (ModuleLinker::getComdatResult() also does comdat type merging in a
- // different way and it's also a bit more permissive.)
+ if ((Selection == IMAGE_COMDAT_SELECT_ANY &&
+ LeaderSelection == IMAGE_COMDAT_SELECT_LARGEST) ||
+ (Selection == IMAGE_COMDAT_SELECT_LARGEST &&
+ LeaderSelection == IMAGE_COMDAT_SELECT_ANY)) {
+ // cl.exe picks "any" for vftables when building with /GR- and
+ // "largest" when building with /GR. To be able to link object files
+ // compiled with each flag, "any" and "largest" are merged as "largest".
+ LeaderSelection = Selection = IMAGE_COMDAT_SELECT_LARGEST;
+ }
+
+ // Other than that, comdat selections must match. This is a bit more
+ // strict than link.exe which allows merging "any" and "largest" if "any"
+ // is the first symbol the linker sees, and it allows merging "largest"
+ // with everything (!) if "largest" is the first symbol the linker sees.
+ // Making this symmetric independent of which selection is seen first
+ // seems better though.
+ // (This behavior matches ModuleLinker::getComdatResult().)
if (Selection != LeaderSelection) {
std::string Msg = ("conflicting comdat type for " + toString(*Leader) +
": " + Twine((int)LeaderSelection) + " in " +
diff --git a/test/COFF/comdat-selection.s b/test/COFF/comdat-selection.s
index b9d0301..4926173 100644
--- a/test/COFF/comdat-selection.s
+++ b/test/COFF/comdat-selection.s
@@ -6,6 +6,7 @@
# Create obj files with each selection type.
# RUN: sed -e s/SEL/discard/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.discard.obj
+# RUN: sed -e s/SEL/discard/ -e s/.long/.short/ -e s/1/2/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.discard.short.2.obj
# RUN: sed -e s/SEL/one_only/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.one_only.obj
# RUN: sed -e s/SEL/same_size/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_size.obj
# RUN: sed -e s/SEL/same_size/ -e s/.long/.short/ %s | llvm-mc -triple x86_64-pc-win32 -filetype=obj -o %t.same_size.short.obj
@@ -80,8 +81,18 @@
# RUN: not lld-link /dll /noentry /nodefaultlib %t.same_contents.obj %t.same_size.obj 2>&1 | FileCheck --check-prefix=CONTENTSSIZE %s
# CONTENTSSIZE: lld-link: error: conflicting comdat type for symbol: 4 in
+# Check that linking one 'discard' and one 'largest' has the effect of
+# 'largest'.
+# RUN: lld-link /dll /noentry /nodefaultlib %t.discard.short.2.obj %t.largest.obj
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=DISCARDLARGEST %s
+# DISCARDLARGEST: Contents of section .text:
+# DISCARDLARGEST: 180001000 01000000 ....
+# RUN: lld-link /dll /noentry /nodefaultlib %t.largest.obj %t.discard.short.2.obj
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=LARGESTDISCARD %s
+# LARGESTDISCARD: Contents of section .text:
+# LARGESTDISCARD: 180001000 01000000 ....
+
+
# These cases are accepted by link.exe but not by lld-link.
-# RUN: not lld-link /dll /noentry /nodefaultlib %t.discard.obj %t.largest.obj 2>&1 | FileCheck --check-prefix=DISCARDLARGEST %s
-# DISCARDLARGEST: lld-link: error: conflicting comdat type for symbol: 2 in
# RUN: not lld-link /dll /noentry /nodefaultlib %t.largest.obj %t.one_only.obj 2>&1 | FileCheck --check-prefix=LARGESTONE %s
# LARGESTONE: lld-link: error: conflicting comdat type for symbol: 6 in
