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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
2021-08-25 01:12:19 -07:00
parent c2162e4d89
commit 92f54e1c75
2 changed files with 25 additions and 7 deletions
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22
llvm/lib/Linker/LinkModules.cpp
View File

@@ -179,9 +179,25 @@ bool ModuleLinker::computeResultingSelectionKind(StringRef ComdatName,
// Go with Dst.
From = LinkFrom::Dst;
break;
case Comdat::SelectionKind::NoDeduplicate:
return emitError("Linking COMDATs named '" + ComdatName +
"': nodeduplicate has been violated!");
case Comdat::SelectionKind::NoDeduplicate: {
const GlobalVariable *DstGV;
const GlobalVariable *SrcGV;
if (getComdatLeader(DstM, ComdatName, DstGV) ||
getComdatLeader(*SrcM, ComdatName, SrcGV))
return true;
if (SrcGV->isWeakForLinker()) {
// Go with Dst.
From = LinkFrom::Dst;
} else if (DstGV->isWeakForLinker()) {
// Go with Src.
From = LinkFrom::Src;
} else {
return emitError("Linking COMDATs named '" + ComdatName +
"': nodeduplicate has been violated!");
}
break;
}
case Comdat::SelectionKind::ExactMatch:
case Comdat::SelectionKind::Largest:
case Comdat::SelectionKind::SameSize: {

10
llvm/test/Linker/comdat-nodeduplicate.ll
View File

@@ -3,14 +3,16 @@
; CHECK: error: Linking COMDATs named 'foo': nodeduplicate has been violated!
; RUN: not llvm-link -S %t/2.ll %t/2-aux.ll 2>&1 | FileCheck %s --check-prefix=CHECK2
; RUN: not llvm-link -S %t/2-aux.ll %t/2.ll 2>&1 | FileCheck %s --check-prefix=CHECK2
; RUN: llvm-link -S %t/2.ll %t/2-aux.ll | FileCheck %s --check-prefix=CHECK2
; RUN: llvm-link -S %t/2-aux.ll %t/2.ll | FileCheck %s --check-prefix=CHECK2
; CHECK2: error: Linking COMDATs named 'foo'
; CHECK2-DAG: @foo = global i64 2, section "data", comdat, align 8
; CHECK2-DAG: @bar = weak global i64 0, section "cnts", comdat($foo)
; CHECK2-DAG: @qux = weak_odr global i64 4, comdat($foo)
; RUN: not llvm-link -S %t/non-var.ll %t/non-var.ll 2>&1 | FileCheck %s --check-prefix=NONVAR
; NONVAR: error: Linking COMDATs named 'foo': nodeduplicate has been violated!
; NONVAR: error: Linking COMDATs named 'foo': GlobalVariable required for data dependent selection!
;--- 1.ll
$foo = comdat nodeduplicate