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[LLD] [COFF] Don't preserve unnecessary __imp_ prefixed symbols #72989

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Merged
merged 1 commit into from
Dec 4, 2023
Merged

[LLD] [COFF] Don't preserve unnecessary __imp_ prefixed symbols #72989

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19 changes: 13 additions & 6 deletions lld/COFF/InputFiles.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1042,6 +1042,19 @@ void BitcodeFile::parse() {
sym = ctx.symtab.addUndefined(symName, this, false);
if (objSym.isWeak())
sym->deferUndefined = true;
// If one LTO object file references (i.e. has an undefined reference to)
// a symbol with an __imp_ prefix, the LTO compilation itself sees it
// as unprefixed but with a dllimport attribute instead, and doesn't
// understand the relation to a concrete IR symbol with the __imp_ prefix.
//
// For such cases, mark the symbol as used in a regular object (i.e. the
// symbol must be retained) so that the linker can associate the
// references in the end. If the symbol is defined in an import library
// or in a regular object file, this has no effect, but if it is defined
// in another LTO object file, this makes sure it is kept, to fulfill
// the reference when linking the output of the LTO compilation.
if (symName.starts_with("__imp_"))
sym->isUsedInRegularObj = true;
} else if (objSym.isCommon()) {
sym = ctx.symtab.addCommon(this, symName, objSym.getCommonSize());
} else if (objSym.isWeak() && objSym.isIndirect()) {
Expand All @@ -1063,12 +1076,6 @@ void BitcodeFile::parse() {
} else {
sym = ctx.symtab.addRegular(this, symName, nullptr, fakeSC, 0,
objSym.isWeak());
// Model all symbols with the __imp_ prefix as having external
// references. If one LTO object defines a __imp_<foo> symbol, and
// another LTO object refers to <foo> with dllimport, make sure the
// __imp_ symbol is kept.
if (symName.starts_with("__imp_"))
sym->isUsedInRegularObj = true;
}
symbols.push_back(sym);
if (objSym.isUsed())
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8 changes: 2 additions & 6 deletions lld/test/COFF/lto-imp-prefix.ll
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Original file line number Diff line number Diff line change
Expand Up @@ -8,13 +8,9 @@

; RUN: lld-link /entry:entry %t.main.obj %t.other1.obj /out:%t1.exe /subsystem:console /debug:symtab

;; The current implementation for handling __imp_ symbols retains all of them.
;; Observe that this currently produces __imp_unusedFunc even if nothing
;; references unusedFunc in any form.

;; Check that we don't retain __imp_ prefixed symbols we don't need.
; RUN: llvm-nm %t1.exe | FileCheck %s

; CHECK: __imp_unusedFunc
; CHECK-NOT: __imp_unusedFunc

; RUN: lld-link /entry:entry %t.main.obj %t.other2.obj /out:%t2.exe /subsystem:console

Expand Down