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IRSymTab: Record _GLOBAL_OFFSET_TABLE_ for ELF x86 #89463

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IRSymTab: Record _GLOBAL_OFFSET_TABLE_ for ELF x86 #89463

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[𝘀𝗽𝗿] initial version
MaskRay Apr 19, 2024
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add comments to tests
MaskRay Apr 19, 2024
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fix typo
MaskRay Apr 23, 2024
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fix a comment typo
MaskRay Apr 23, 2024
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29 changes: 29 additions & 0 deletions lld/test/ELF/lto/i386-global-offset-table.ll
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; REQUIRES: x86
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What is this test testing?

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Thanks for the review. Added a file-level comment.

;; LTO-generated felocatable files may reference _GLOBAL_OFFSET_TABLE_ while
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relocatable

;; the IR does not mention _GLOBAL_OFFSET_TABLE_.
;; Test that there is no spurious "undefined symbol" error.

; RUN: rm -rf %t && mkdir %t && cd %t
; RUN: llvm-as %s -o a.bc
; RUN: ld.lld -pie a.bc -o a
; RUN: llvm-nm a | FileCheck %s

; CHECK: d _GLOBAL_OFFSET_TABLE_

target datalayout = "e-m:e-p:32:32-p270:32:32-p271:32:32-p272:64:64-i128:128-f64:32:64-f80:32-n8:16:32-S128"
target triple = "i386-pc-linux-gnu"

@i = global i32 0

define dso_local void @_start() {
entry:
%0 = load i32, ptr @i
%inc = add nsw i32 %0, 1
store i32 %inc, ptr @i
ret void
}

!llvm.module.flags = !{!0, !1}

!0 = !{i32 8, !"PIC Level", i32 2}
!1 = !{i32 7, !"PIE Level", i32 2}
71 changes: 71 additions & 0 deletions lld/test/ELF/lto/x86-64-global-offset-table.ll
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; REQUIRES: x86
;; LTO-generated relocatable files may reference _GLOBAL_OFFSET_TABLE_ while
;; the IR does not mention _GLOBAL_OFFSET_TABLE_.
;; Test that there is no spurious "undefined symbol" error.

; RUN: rm -rf %t && split-file %s %t && cd %t
; RUN: opt -module-summary b.ll -o b.bc

;; Test Thin LTO.
; RUN: cat a.ll medium.ll | opt -module-summary - -o medium.bc
; RUN: ld.lld -pie --no-relax medium.bc b.bc -o medium
; RUN: llvm-objdump -dt medium | FileCheck %s

;; Test regular LTO.
; RUN: cat a.ll large.ll | llvm-as - -o large.bc
; RUN: ld.lld -pie large.bc b.bc -o large
; RUN: llvm-objdump -dt large | FileCheck %s

;; Explicit reference of _GLOBAL_OFFSET_TABLE_ is fine.
; RUN: cat a.ll medium.ll ref.ll | opt -module-summary - -o ref.bc
; RUN: ld.lld -pie -u ref ref.bc b.bc -y _GLOBAL_OFFSET_TABLE_ -o ref 2>&1 | FileCheck %s --check-prefix=TRACE
; RUN: llvm-objdump -dt ref | FileCheck %s

; TRACE: ref.bc: reference to _GLOBAL_OFFSET_TABLE_
; TRACE-NEXT: ref.bc: reference to _GLOBAL_OFFSET_TABLE_
; TRACE-NEXT: <internal>: definition of _GLOBAL_OFFSET_TABLE_
; TRACE-NEXT: ref.lto.ref.o: reference to _GLOBAL_OFFSET_TABLE_

;; The IR symbol table references _GLOBAL_OFFSET_TABLE_, which causes lld to define the symbol.
; CHECK: .got.plt 0000000000000000 .hidden _GLOBAL_OFFSET_TABLE_
; CHECK: movabsq
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What is this checking for related to the GLOBAL_OFFSET_TABLE? Can you add a comment?

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Added The IR symbol table references _GLOBAL_OFFSET_TABLE_, which causes lld to define the symbol.


;--- a.ll
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@i = external global i32

define dso_local void @_start() {
entry:
%0 = load i32, ptr @i
%inc = add nsw i32 %0, 1
store i32 %inc, ptr @i
ret void
}

!llvm.module.flags = !{!0, !1, !2, !3}

!0 = !{i32 8, !"PIC Level", i32 2}
!1 = !{i32 7, !"PIE Level", i32 2}
!2 = !{i32 1, !"Large Data Threshold", i64 0}

;--- medium.ll
!3 = !{i32 1, !"Code Model", i32 3}

;--- large.ll
!3 = !{i32 1, !"Code Model", i32 4}

;--- ref.ll
@_GLOBAL_OFFSET_TABLE_ = external global [0 x i8]

define dso_local ptr @ref() {
entry:
ret ptr @_GLOBAL_OFFSET_TABLE_
}

;--- b.ll
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@i = global i32 0
14 changes: 14 additions & 0 deletions llvm/lib/Object/ModuleSymbolTable.cpp
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Expand Up @@ -175,6 +175,20 @@ void ModuleSymbolTable::CollectAsmSymbols(
AsmSymbol(Key, BasicSymbolRef::Flags(Res));
}
});

// In ELF, object code generated for x86-32 and some code models of x86-64 may
// reference the special symbol _GLOBAL_OFFSET_TABLE_ that is not used in the
// IR. Record it like inline asm symbols.
Triple TT(M.getTargetTriple());
if (!TT.isOSBinFormatELF() || !TT.isX86())
return;
auto CM = M.getCodeModel();
if (TT.getArch() == Triple::x86 || CM == CodeModel::Medium ||
CM == CodeModel::Large) {
AsmSymbol("_GLOBAL_OFFSET_TABLE_",
BasicSymbolRef::Flags(BasicSymbolRef::SF_Undefined |
BasicSymbolRef::SF_Global));
}
}

void ModuleSymbolTable::CollectAsmSymvers(
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2 changes: 1 addition & 1 deletion llvm/test/LTO/X86/codemodel-2.ll
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; RUN: llvm-as %s -o %t.o
; RUN: llvm-lto2 run -r %t.o,_start,px %t.o -o %t.s
; RUN: llvm-lto2 run -r %t.o,_start,px -r %t.o,_GLOBAL_OFFSET_TABLE_, %t.o -o %t.s
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Why is it needed here and in other tests now?

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For x86-64 medium/large code model, the IR symbol table now references _GLOBAL_OFFSET_TABLE_, which requires llvm-lto2 to provide it to avoid a missing symbol resolution for error

; RUN: llvm-objdump --no-print-imm-hex -d %t.s.0 | FileCheck %s --check-prefix=CHECK-LARGE

target triple = "x86_64-unknown-linux-gnu"
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3 changes: 2 additions & 1 deletion llvm/test/LTO/X86/codemodel-3.ll
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@@ -1,6 +1,7 @@
; RUN: llvm-as %s -o %t0.o
; RUN: llvm-as < %p/Inputs/codemodel-3.ll > %t1.o
; RUN: not llvm-lto2 run -r %t0.o,_start,px -r %t1.o,bar,px %t0.o %t1.o -o %t2.s 2>&1 | FileCheck %s
; RUN: not llvm-lto2 run -r %t0.o,_start,px -r %t1.o,bar,px -r %t0.o,_GLOBAL_OFFSET_TABLE_, \
; RUN: -r %t1.o,_GLOBAL_OFFSET_TABLE_, %t0.o %t1.o -o %t2.s 2>&1 | FileCheck %s

target triple = "x86_64-unknown-linux-gnu"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
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2 changes: 1 addition & 1 deletion llvm/test/LTO/X86/largedatathreshold-1.ll
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; RUN: llvm-as %s -o %t.o
; RUN: llvm-lto2 run -r %t.o,_start,px %t.o -o %t.s
; RUN: llvm-lto2 run -r %t.o,_start,px -r %t.o,_GLOBAL_OFFSET_TABLE_, %t.o -o %t.s
; RUN: llvm-objdump -d %t.s.0 | FileCheck %s

target triple = "x86_64-unknown-linux-gnu"
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2 changes: 1 addition & 1 deletion llvm/test/LTO/X86/largedatathreshold-2.ll
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@@ -1,5 +1,5 @@
; RUN: llvm-as %s -o %t.o
; RUN: llvm-lto2 run -r %t.o,_start,px %t.o -o %t.s
; RUN: llvm-lto2 run -r %t.o,_start,px -r %t.o,_GLOBAL_OFFSET_TABLE_, %t.o -o %t.s
; RUN: llvm-objdump -d %t.s.0 | FileCheck %s

target triple = "x86_64-unknown-linux-gnu"
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3 changes: 2 additions & 1 deletion llvm/test/LTO/X86/largedatathreshold-3.ll
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@@ -1,6 +1,7 @@
; RUN: llvm-as %s -o %t0.o
; RUN: llvm-as < %p/Inputs/largedatathreshold.ll > %t1.o
; RUN: not llvm-lto2 run -r %t0.o,_start,px -r %t1.o,bar,px %t0.o %t1.o -o %t2.s 2>&1 | FileCheck %s
; RUN: not llvm-lto2 run -r %t0.o,_start,px -r %t1.o,bar,px -r %t0.o,_GLOBAL_OFFSET_TABLE_, \
; RUN: -r %t1.o,_GLOBAL_OFFSET_TABLE_, %t0.o %t1.o -o %t2.s 2>&1 | FileCheck %s

; CHECK: 'Large Data Threshold': IDs have conflicting values

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