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[AsmPrinter][ELF] Define local aliases (.Lfoo$local) for GlobalObjects

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For `MC_GlobalAddress` operands referencing **certain** GlobalObjects,
we can lower them to STB_LOCAL aliases to avoid costs brought by
assembler/linker's conservative decisions about symbol interposition:

* An assembler conservatively assumes a global default visibility symbol interposable (ELF
  semantics). So relocations in object files are needed even if the code generator assumed
  the definition exact and non-interposable.
* The relocations can cause the creation of PLT entries on some targets for -shared links.
  A linker conservatively assumes a global default visibility symbol interposable (if not
  otherwise constrained by -Bsymbolic/--dynamic-list/VER_NDX_LOCAL/etc).

"certain" refers to GlobalObjects in the intersection of
`hasExactDefinition() and !isInterposable()`: `external`, `appending`, `internal`, `private`.
Local linkages (`internal` and `private`) cannot be interposed. `appending` is for very
few objects LLVM interpret specially.  So the set just includes `external`.

This patch emits STB_LOCAL aliases (.Lfoo$local) for such GlobalObjects, so that targets can lower
MC_GlobalAddress operands to STB_LOCAL aliases if applicable.
We may extend the scope and include GlobalAlias in the future.

LLVM's existing -fno-semantic-interposition behaviors give us license to do such optimizations:

* Various optimizations (ipconstprop, inliner, sccp, sroa, etc) treat normal ExternalLinkage
  GlobalObjects as non-interposable.
* Before D72197, MC resolved a PC-relative VK_None fixup to a non-local symbol at assembly time (no
  outstanding relocation), if the target is defined in the same section. Put it simply, even if IR
  optimizations failed to optimize and allowed interposition for the function call in
  `void foo() {} void bar() { foo(); }`, the assembler would disallow it.

This patch sets up AsmPrinter infrastructure to make -fno-semantic-interposition more so.
With and without the patch, the object file output should be identical:
`.Lfoo$local` does not take a symbol table entry.

Reviewed By: sfertile

Differential Revision: https://reviews.llvm.org/D73228
This commit is contained in:
Fangrui Song 2020-01-22 12:26:04 -08:00
parent 287f3d0352
commit 60f7be42e3
7 changed files with 51 additions and 7 deletions
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5
include/llvm/CodeGen/AsmPrinter.h
View File

@ -247,6 +247,11 @@ public:
MCSymbol *getSymbol(const GlobalValue *GV) const;
/// Similar to getSymbol() but preferred for references. On ELF, this uses a
/// local symbol if a reference to GV is guaranteed to be resolved to the
/// definition in the same module.
MCSymbol *getSymbolPreferLocal(const GlobalValue &GV) const;
//===------------------------------------------------------------------===//
// XRay instrumentation implementation.
//===------------------------------------------------------------------===//

26
lib/CodeGen/AsmPrinter/AsmPrinter.cpp
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@ -450,6 +450,21 @@ MCSymbol *AsmPrinter::getSymbol(const GlobalValue *GV) const {
return TM.getSymbol(GV);
}
MCSymbol *AsmPrinter::getSymbolPreferLocal(const GlobalValue &GV) const {
// On ELF, use .Lfoo$local if GV is a non-interposable GlobalObject with an
// exact definion (intersection of GlobalValue::hasExactDefinition() and
// !isInterposable()). These linkages include: external, appending, internal,
// private. It may be profitable to use a local alias for external. The
// assembler would otherwise be conservative and assume a global default
// visibility symbol can be interposable, even if the code generator already
// assumed it.
if (TM.getTargetTriple().isOSBinFormatELF() &&
GlobalObject::isExternalLinkage(GV.getLinkage()) && GV.isDSOLocal() &&
!GV.isDeclaration() && isa<GlobalObject>(GV))
return getSymbolWithGlobalValueBase(&GV, "$local");
return TM.getSymbol(&GV);
}
/// EmitGlobalVariable - Emit the specified global variable to the .s file.
void AsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
bool IsEmuTLSVar = TM.useEmulatedTLS() && GV->isThreadLocal();
@ -631,6 +646,9 @@ void AsmPrinter::EmitGlobalVariable(const GlobalVariable *GV) {
EmitAlignment(Alignment, GV);
OutStreamer->EmitLabel(EmittedInitSym);
MCSymbol *LocalAlias = getSymbolPreferLocal(*GV);
if (LocalAlias != EmittedInitSym)
OutStreamer->EmitLabel(LocalAlias);
EmitGlobalConstant(GV->getParent()->getDataLayout(), GV->getInitializer());
@ -767,7 +785,13 @@ void AsmPrinter::EmitFunctionEntryLabel() {
report_fatal_error("'" + Twine(CurrentFnSym->getName()) +
"' label emitted multiple times to assembly file");
return OutStreamer->EmitLabel(CurrentFnSym);
OutStreamer->EmitLabel(CurrentFnSym);
if (TM.getTargetTriple().isOSBinFormatELF()) {
MCSymbol *Sym = getSymbolPreferLocal(MF->getFunction());
if (Sym != CurrentFnSym)
OutStreamer->EmitLabel(Sym);
}
}
/// emitComments - Pretty-print comments for instructions.

2
test/CodeGen/AArch64/emutls.ll
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@ -155,6 +155,7 @@ entry:
; ARM64: .data{{$}}
; ARM64: .globl __emutls_v.i4
; ARM64-LABEL: __emutls_v.i4:
; ARM64-NEXT: .L__emutls_v.i4$local:
; ARM64-NEXT: .xword 4
; ARM64-NEXT: .xword 4
; ARM64-NEXT: .xword 0
@ -162,6 +163,7 @@ entry:
; ARM64: .section .rodata,
; ARM64-LABEL: __emutls_t.i4:
; ARM64-NEXT: .L__emutls_t.i4$local:
; ARM64-NEXT: .word 15
; ARM64-NOT: __emutls_v.i5:

2
test/CodeGen/AArch64/machine-outliner-retaddr-sign-sp-mod.ll
View File

@ -4,6 +4,7 @@
@v = common dso_local global i32* null, align 8
; CHECK-LABEL: foo: // @foo
; CHECK-NEXT: .Lfoo$local:
; CHECK-NEXT: // %bb.0: // %entry
; CHECK-NEXT: paciasp
; CHECK-NOT: OUTLINED_FUNCTION_
@ -22,6 +23,7 @@ entry:
}
; CHECK-LABEL: bar: // @bar
; CHECK-NEXT: .Lbar$local:
; CHECK-NEXT: // %bb.0: // %entry
; CHECK-NEXT: paciasp
; CHECK-NOT: OUTLINED_FUNCTION_

2
test/CodeGen/ARM/emutls.ll
View File

@ -238,6 +238,7 @@ entry:
; ARM32: .data{{$}}
; ARM32: .globl __emutls_v.i4
; ARM32-LABEL: __emutls_v.i4:
; ARM32-NEXT: .L__emutls_v.i4$local:
; ARM32-NEXT: .long 4
; ARM32-NEXT: .long 4
; ARM32-NEXT: .long 0
@ -245,6 +246,7 @@ entry:
; ARM32: .section .rodata,
; ARM32-LABEL: __emutls_t.i4:
; ARM32-NEXT: .L__emutls_t.i4$local:
; ARM32-NEXT: .long 15
; ARM32-NOT: __emutls_v.i5:

4
test/CodeGen/X86/emutls.ll
View File

@ -275,6 +275,7 @@ entry:
; X32 .section .data.rel.local,
; X32-LABEL: __emutls_v.i4:
; X32-NEXT: .L__emutls_v.i4$local:
; X32-NEXT: .long 4
; X32-NEXT: .long 4
; X32-NEXT: .long 0
@ -282,6 +283,7 @@ entry:
; X32 .section .rodata,
; X32-LABEL: __emutls_t.i4:
; X32-NEXT: .L__emutls_t.i4$local:
; X32-NEXT: .long 15
; X32-NOT: __emutls_v.i5:
@ -336,6 +338,7 @@ entry:
; X64 .section .data.rel.local,
; X64-LABEL: __emutls_v.i4:
; X64-NEXT: .L__emutls_v.i4$local:
; X64-NEXT: .quad 4
; X64-NEXT: .quad 4
; X64-NEXT: .quad 0
@ -343,6 +346,7 @@ entry:
; X64 .section .rodata,
; X64-LABEL: __emutls_t.i4:
; X64-NEXT: .L__emutls_t.i4$local:
; X64-NEXT: .long 15
; X64-NOT: __emutls_v.i5:

17
test/CodeGen/X86/linux-preemption.ll
View File

@ -1,9 +1,9 @@
; RUN: llc -mtriple x86_64-pc-linux \
; RUN: -relocation-model=static < %s | FileCheck --check-prefix=STATIC %s
; RUN: llc -mtriple x86_64-pc-linux \
; RUN: -relocation-model=pic < %s | FileCheck %s
; RUN: llc -mtriple x86_64-pc-linux \
; RUN: -relocation-model=dynamic-no-pic < %s | FileCheck %s
; RUN: llc -mtriple x86_64-pc-linux -relocation-model=static < %s | \
; RUN: FileCheck --check-prefixes=COMMON,STATIC %s
; RUN: llc -mtriple x86_64-pc-linux -relocation-model=pic < %s | \
; RUN: FileCheck --check-prefixes=COMMON,CHECK %s
; RUN: llc -mtriple x86_64-pc-linux -relocation-model=dynamic-no-pic < %s | \
; RUN: FileCheck --check-prefixes=COMMON,CHECK %s
; 32 bits
@ -173,6 +173,8 @@ define dso_local void @strong_local_function() {
define void()* @get_strong_local_function() {
ret void()* @strong_local_function
}
; COMMON: {{^}}strong_local_function:
; COMMON-NEXT .Lstrong_local_function:
; CHECK: leaq strong_local_function(%rip), %rax
; STATIC: movl $strong_local_function, %eax
; CHECK32: leal strong_local_function@GOTOFF(%eax), %eax
@ -223,3 +225,6 @@ define void()* @get_external_preemptable_function() {
; CHECK: movq external_preemptable_function@GOTPCREL(%rip), %rax
; STATIC: movl $external_preemptable_function, %eax
; CHECK32: movl external_preemptable_function@GOT(%eax), %eax
; COMMON: {{^}}strong_local_global:
; COMMON-NEXT .Lstrong_local_global: