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AsmPrinter: Remove ELF's special lowerRelativeReference for unnamed_addr function; use lowerDSOLocalEquivalent in more cases #134781

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Merged
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Apr 8, 2025
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AsmPrinter: Remove ELF's special lowerRelativeReference for unnamed_addr function; use lowerDSOLocalEquivalent in more cases #134781

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[𝘀𝗽𝗿] initial version
MaskRay Apr 8, 2025
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MaskRay Apr 8, 2025
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4 changes: 0 additions & 4 deletions llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
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Expand Up @@ -125,10 +125,6 @@ class TargetLoweringObjectFileELF : public TargetLoweringObjectFile {
lowerSymbolDifference(const MCSymbol *LHS, const MCSymbol *RHS,
int64_t Addend,
std::optional<int64_t> PCRelativeOffset) const;
const MCExpr *lowerRelativeReference(const GlobalValue *LHS,
const GlobalValue *RHS, int64_t Addend,
std::optional<int64_t> PCRelativeOffset,
const TargetMachine &TM) const override;

const MCExpr *lowerDSOLocalEquivalent(const MCSymbol *LHS,
const MCSymbol *RHS, int64_t Addend,
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10 changes: 6 additions & 4 deletions llvm/lib/CodeGen/AsmPrinter/AsmPrinter.cpp
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Expand Up @@ -3496,10 +3496,12 @@ const MCExpr *AsmPrinter::lowerConstant(const Constant *CV,
LHSGV, RHSGV, Addend, PCRelativeOffset, TM);

// (ELF-specific) If the generic symbol difference does not apply, and
// LHS is a dso_local_equivalent of a dso_preemptable function,
// reference the PLT entry instead.
if (DSOEquiv && TM.getTargetTriple().isOSBinFormatELF() &&
!(LHSGV->isDSOLocal() || LHSGV->isImplicitDSOLocal()))
// LHS is a dso_local_equivalent of a function, reference the PLT entry
// instead. Note: A default visibility symbol is by default preemptible
// during linking, and should not be referenced with PC-relative
// relocations. Therefore, use a PLT relocation even if the function is
// dso_local.
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For AArch64, this change makes relocations for relative vtable entries always be PLT32, though PREL32 relocation also works (for dso_local) and its target symbol could get changed from function to section-plus-offset (in hope to omit the function symbol in local symbol table).

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Are you talking about the ld -r/--emit-reelocs --discard-all or objcopy --discard-all behavior?
It's true that the PLT-generating relocation, if present, keeps the symbol alive under --discard-all.
However, the advantage is that even in the object file, the dso_local_equivalent semantics are retained: we don't take the address of the symbol.

% cat x.s
f1:
f2:

.data
.long f1@plt - .
.long f2 - .
% llvm-mc -triple=aarch64 -filetype=obj x.s -o x.o && ld.lld x.o -o x -r --discard-all && readelf -W -s x
Symbol table '.symtab' contains 4 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT    1 f1
     2: 0000000000000000     0 SECTION LOCAL  DEFAULT    1 .text
     3: 0000000000000000     0 SECTION LOCAL  DEFAULT    2 .data

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I was referring to the logic in this function, where if it is PREL32 and the target symbol is dso_local, we may update relocation target from "func + vtable_slot_offset" to "section + offset". So if the dso_local function is not referenced elsewhere, we can omit its emission in local symtab.

if (DSOEquiv && TM.getTargetTriple().isOSBinFormatELF())
Res = getObjFileLowering().lowerDSOLocalEquivalent(
LHSSym, RHSSym, Addend, PCRelativeOffset, TM);

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18 changes: 0 additions & 18 deletions llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
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Expand Up @@ -1233,24 +1233,6 @@ const MCExpr *TargetLoweringObjectFileELF::lowerSymbolDifference(
return Res;
}

const MCExpr *TargetLoweringObjectFileELF::lowerRelativeReference(
const GlobalValue *LHS, const GlobalValue *RHS, int64_t Addend,
std::optional<int64_t> PCRelativeOffset, const TargetMachine &TM) const {
// We may only use a PLT-relative relocation to refer to unnamed_addr
// functions.
if (!LHS->hasGlobalUnnamedAddr() || !LHS->getValueType()->isFunctionTy())
return nullptr;

// Basic correctness checks.
if (LHS->getType()->getPointerAddressSpace() != 0 ||
RHS->getType()->getPointerAddressSpace() != 0 || LHS->isThreadLocal() ||
RHS->isThreadLocal())
return nullptr;

return lowerSymbolDifference(TM.getSymbol(LHS), TM.getSymbol(RHS), Addend,
PCRelativeOffset);
}

// Reference the PLT entry of a function, optionally with a subtrahend (`RHS`).
const MCExpr *TargetLoweringObjectFileELF::lowerDSOLocalEquivalent(
const MCSymbol *LHS, const MCSymbol *RHS, int64_t Addend,
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5 changes: 3 additions & 2 deletions llvm/test/CodeGen/ARM/plt-relative-reloc.ll → llvm/test/CodeGen/ARM/relative-reloc.ll
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Expand Up @@ -10,7 +10,8 @@ declare void @fn1() unnamed_addr
declare void @fn2() unnamed_addr
declare void @fn3()

;; Create a PC-relative relocation that the linker might decline if the addend symbol is preemptible.
; CHECK: .long 0
; CHECK-NEXT: .long fn1(prel31)-vtable-4
; CHECK-NEXT: .long fn2(prel31)-vtable-4
; CHECK-NEXT: .long fn1-vtable-4
; CHECK-NEXT: .long fn2-vtable-4
; CHECK-NEXT: .long fn3-vtable-4
4 changes: 2 additions & 2 deletions llvm/test/CodeGen/RISCV/dso_local_equivalent.ll
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Expand Up @@ -26,9 +26,9 @@ declare void @extern_func()
; CHECK-NEXT: .word 0 # 0x0
; CHECK-NEXT: .word %pltpcrel(f0)
; CHECK-NEXT: .word %pltpcrel(f1+4)
; CHECK-NEXT: .word f2-_ZTV1B-8
; CHECK-NEXT: .word %pltpcrel(f2+8)
; CHECK-NEXT: .word %pltpcrel(f3+12)
; CHECK-NEXT: .word f4-_ZTV1B-8
; CHECK-NEXT: .word %pltpcrel(f4+16)
; CHECK-NEXT: .size _ZTV1B, 28
declare void @f0()
declare void @f1()
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5 changes: 3 additions & 2 deletions .../test/CodeGen/RISCV/plt-relative-reloc.ll → llvm/test/CodeGen/RISCV/relative-reloc.ll
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Expand Up @@ -12,10 +12,11 @@ declare void @fn2() unnamed_addr
declare void @fn3()
@global4 = external unnamed_addr global i8

;; Create a PC-relative relocation that the linker might decline if the addend symbol is preemptible.
; CHECK: vtable:
; CHECK-NEXT: .word 0 # 0x0
; CHECK-NEXT: .word %pltpcrel(fn1)
; CHECK-NEXT: .word %pltpcrel(fn2+4)
; CHECK-NEXT: .word fn1-vtable-4
; CHECK-NEXT: .word fn2-vtable-4
; CHECK-NEXT: .word fn3-vtable-4
; CHECK-NEXT: .word global4-vtable-4
; CHECK-NEXT: .size vtable, 20
26 changes: 21 additions & 5 deletions llvm/test/CodeGen/X86/dso_local_equivalent.ll
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Expand Up @@ -207,11 +207,27 @@ define void @call_no_name_extern() {

declare hidden void @0()
declare void @1()
declare void @forward_extern_func()

;; Note that we keep this at the very end because llc emits this after all the
;; functions.
; CHECK: const:
; CHECK: .long forward_extern_func@PLT
@const = constant i32 trunc (i64 ptrtoint (ptr dso_local_equivalent @forward_extern_func to i64) to i32)

declare void @forward_extern_func()
; CHECK-LABEL: _ZTV1B:
; CHECK: .long 0
; CHECK-NEXT: .long 0
; CHECK-NEXT: .long f0@PLT-_ZTV1B-8
; CHECK-NEXT: .long f1@PLT-_ZTV1B-8
; CHECK-NEXT: .long f2@PLT-_ZTV1B-8

@_ZTV1B = dso_local constant { [5 x i32] } { [5 x i32] [
i32 0,
i32 0,
i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @f0 to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [7 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32),
i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @f1 to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [7 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32),
i32 trunc (i64 sub (i64 ptrtoint (ptr dso_local_equivalent @f2 to i64), i64 ptrtoint (ptr getelementptr inbounds ({ [7 x i32] }, ptr @_ZTV1B, i32 0, i32 0, i32 2) to i64)) to i32)
] }

declare void @f0()
declare void @f1()
define dso_local void @f2() {
ret void
}
4 changes: 2 additions & 2 deletions ...est/CodeGen/X86/x86-plt-relative-reloc.ll → llvm/test/CodeGen/X86/relative-reloc-32.ll
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Expand Up @@ -11,6 +11,6 @@ declare void @fn2() unnamed_addr
declare void @fn3()

; CHECK: .long 0
; CHECK-NEXT: .long fn1@PLT-vtable-4
; CHECK-NEXT: .long fn2@PLT-vtable-4
; CHECK-NEXT: .long fn1-vtable-4
; CHECK-NEXT: .long fn2-vtable-4
; CHECK-NEXT: .long fn3-vtable-4
5 changes: 3 additions & 2 deletions .../CodeGen/X86/x86-64-plt-relative-reloc.ll → llvm/test/CodeGen/X86/relative-reloc-64.ll
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Expand Up @@ -12,8 +12,9 @@ declare void @fn2() unnamed_addr
declare void @fn3()
@global4 = external unnamed_addr global i8

;; Create a PC-relative relocation that the linker might decline if the addend symbol is preemptible.
; CHECK: .long 0
; CHECK-NEXT: .long fn1@PLT-vtable-4
; CHECK-NEXT: .long fn2@PLT-vtable-4
; CHECK-NEXT: .long fn1-vtable-4
; CHECK-NEXT: .long fn2-vtable-4
; CHECK-NEXT: .long fn3-vtable-4
; CHECK-NEXT: .long global4-vtable-4
2 changes: 1 addition & 1 deletion llvm/test/CodeGen/X86/relptr-rodata.ll
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Expand Up @@ -24,7 +24,7 @@ target triple = "x86_64-unknown-linux-gnu"
; CHECK-NEXT: .globl obj
; CHECK: obj:
; CHECK: .long 0
; CHECK: .long hidden_func-obj-4
; CHECK: .long hidden_func@PLT-obj-4

declare hidden void @hidden_func()

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