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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

llvm-svn: 352765
This commit is contained in:
Nico Weber 2019-01-31 16:14:33 +00:00
parent 00eaf6732e
commit 9aa55d3c66
2 changed files with 30 additions and 11 deletions
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26
lld/COFF/InputFiles.cpp
View File

@ -481,15 +481,23 @@ Optional<Symbol *> ObjFile::createDefined(
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 " +

15
lld/test/COFF/comdat-selection.s
View File

@ -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 @@ symbol:
# 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