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[Linker] Support weak symbols in nodeduplicate COMDAT group

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When a nodeduplicate COMDAT group contains a weak symbol, choose
a non-weak symbol (or one of the weak ones) rather than reporting
an error. This should address issue PR51394.

With the current IR representation, a generic comdat nodeduplicate
semantics is not representable for LTO. In the linker, sections and
symbols are separate concepts. A dropped weak symbol does not force the
defining input section to be dropped as well (though it can be collected
by GC). In the IR, when a weak linkage symbol is dropped, its associate
section content is dropped as well.

For InstrProfiling, which is where ran into this issue in PR51394, the
deduplication semantic is a sufficient workaround.

Differential Revision: https://reviews.llvm.org/D108689
This commit is contained in:
Petr Hosek 2021-08-31 23:10:30 -07:00 committed by Tom Stellard
parent e415eb692a
commit 683bdb08a0
2 changed files with 29 additions and 4 deletions
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22
lib/Linker/LinkModules.cpp
View File

@ -177,9 +177,25 @@ bool ModuleLinker::computeResultingSelectionKind(StringRef ComdatName,
// Go with Dst. // Go with Dst.
LinkFromSrc = false; LinkFromSrc = false;
break; break;
case Comdat::SelectionKind::NoDeduplicate: case Comdat::SelectionKind::NoDeduplicate: {
return emitError("Linking COMDATs named '" + ComdatName + const GlobalVariable *DstGV;
"': nodeduplicate has been violated!"); const GlobalVariable *SrcGV;
if (getComdatLeader(DstM, ComdatName, DstGV) ||
getComdatLeader(*SrcM, ComdatName, SrcGV))
return true;
if (SrcGV->isWeakForLinker()) {
// Go with Dst.
LinkFromSrc = false;
} else if (DstGV->isWeakForLinker()) {
// Go with Src.
LinkFromSrc = true;
} else {
return emitError("Linking COMDATs named '" + ComdatName +
"': nodeduplicate has been violated!");
}
break;
}
case Comdat::SelectionKind::ExactMatch: case Comdat::SelectionKind::ExactMatch:
case Comdat::SelectionKind::Largest: case Comdat::SelectionKind::Largest:
case Comdat::SelectionKind::SameSize: { case Comdat::SelectionKind::SameSize: {

11
test/Linker/comdat4.ll
View File

@ -1,5 +1,14 @@
; RUN: not llvm-link %s %p/Inputs/comdat3.ll -S -o - 2>&1 | FileCheck %s ; RUN: split-file %s %t.dir
; RUN: not llvm-link %t.dir/global.ll %t.dir/global.ll -S -o - 2>&1 | FileCheck %s
; RUN: llvm-link %t.dir/global.ll %t.dir/weak.ll -S -o - 2>&1
; RUN: llvm-link %t.dir/weak.ll %t.dir/global.ll -S -o - 2>&1
; RUN: llvm-link %t.dir/weak.ll %t.dir/weak.ll -S -o - 2>&1
;--- global.ll
$foo = comdat nodeduplicate $foo = comdat nodeduplicate
@foo = global i64 43, comdat($foo) @foo = global i64 43, comdat($foo)
; CHECK: Linking COMDATs named 'foo': nodeduplicate has been violated! ; CHECK: Linking COMDATs named 'foo': nodeduplicate has been violated!
;--- weak.ll
$foo = comdat nodeduplicate
@foo = weak global i64 43, comdat($foo)