AsmPrinter: Remove ELF's special lowerRelativeReference for unnamed_addr function

[MaskRay]

https://reviews.llvm.org/D17938 introduced lowerRelativeReference to
give ConstantExpr sub (A-B) special semantics in ELF: when A is an
unnamed_addr function, create a PLT-generating relocation. This was
intended for C++ relative vtables, but C++ relative vtable ended up
using DSOLocalEquivalent (lowerDSOLocalEquivalent).

This special treatment of unnamed_addr seems unusual.
Let's remove it. Only COFF needs an overload to generate a @IMGREL32
relocation specifier (llvm/test/MC/COFF/cross-section-relative.ll).

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: Fangrui Song (MaskRay)

Changes

https://reviews.llvm.org/D17938 introduced lowerRelativeReference to
give ConstantExpr sub (A-B) special semantics in ELF: when A is an
unnamed_addr function, create a PLT-generating relocation. This was
intended for C++ relative vtables, but C++ relative vtable ended up
using DSOLocalEquivalent (lowerDSOLocalEquivalent).

This special treatment of unnamed_addr seems unusual.
Let's remove it. Only COFF needs an overload to generate a @IMGREL32
relocation specifier (llvm/test/MC/COFF/cross-section-relative.ll).

---

Full diff: https://github.com/llvm/llvm-project/pull/132684.diff

4 Files Affected:

• (modified) llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h (-4)
• (modified) llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp (-20)
• (modified) llvm/test/CodeGen/X86/x86-64-plt-relative-reloc.ll (+3-2)
• (modified) llvm/test/CodeGen/X86/x86-plt-relative-reloc.ll (+2-2)

diff --git a/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h b/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
index 76571690eeda0..293aa9d1faf7d 100644
--- a/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
+++ b/llvm/include/llvm/CodeGen/TargetLoweringObjectFileImpl.h
@@ -112,10 +112,6 @@ class TargetLoweringObjectFileELF : public TargetLoweringObjectFile {
   MCSection *getStaticDtorSection(unsigned Priority,
                                   const MCSymbol *KeySym) const override;
 
-  const MCExpr *lowerRelativeReference(const GlobalValue *LHS,
-                                       const GlobalValue *RHS,
-                                       const TargetMachine &TM) const override;
-
   const MCExpr *lowerDSOLocalEquivalent(const DSOLocalEquivalent *Equiv,
                                         const TargetMachine &TM) const override;
 
diff --git a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
index 0e44acdd1dccc..0f6d0001087d9 100644
--- a/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp
@@ -1174,26 +1174,6 @@ MCSection *TargetLoweringObjectFileELF::getStaticDtorSection(
                                   KeySym);
 }
 
-const MCExpr *TargetLoweringObjectFileELF::lowerRelativeReference(
-    const GlobalValue *LHS, const GlobalValue *RHS,
-    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 MCBinaryExpr::createSub(
-      MCSymbolRefExpr::create(TM.getSymbol(LHS), PLTRelativeSpecifier,
-                              getContext()),
-      MCSymbolRefExpr::create(TM.getSymbol(RHS), getContext()), getContext());
-}
-
 const MCExpr *TargetLoweringObjectFileELF::lowerDSOLocalEquivalent(
     const DSOLocalEquivalent *Equiv, const TargetMachine &TM) const {
   assert(supportDSOLocalEquivalentLowering());
diff --git a/llvm/test/CodeGen/X86/x86-64-plt-relative-reloc.ll b/llvm/test/CodeGen/X86/x86-64-plt-relative-reloc.ll
index f949c83efd03f..02b012cf18199 100644
--- a/llvm/test/CodeGen/X86/x86-64-plt-relative-reloc.ll
+++ b/llvm/test/CodeGen/X86/x86-64-plt-relative-reloc.ll
@@ -12,8 +12,9 @@ declare void @fn2() unnamed_addr
 declare void @fn3()
 @global4 = external unnamed_addr global i8
 
+;; Generate a PC-relative relocation, which might be rejected by the linker if the referenced 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
diff --git a/llvm/test/CodeGen/X86/x86-plt-relative-reloc.ll b/llvm/test/CodeGen/X86/x86-plt-relative-reloc.ll
index 8c86cd29d1c81..ea5dd5b5a83a1 100644
--- a/llvm/test/CodeGen/X86/x86-plt-relative-reloc.ll
+++ b/llvm/test/CodeGen/X86/x86-plt-relative-reloc.ll
@@ -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