[AArch64][ELF] Prefer to lower MC_GlobalAddress operands to .Lfoo$local

[MaskRay]

Summary

Similar to X86 D73230 & 46788a21f9152be3950e57dc526454655682bdd4

With this change, we can set dso_local in clang's -fpic -fno-semantic-interposition mode,
for default visibility external linkage non-ifunc-non-COMDAT definitions.

For such dso_local definitions, variable access/taking the address of a
function/calling a function will go through a local alias to avoid GOT/PLT.

Note: the 'S' inline assembly constraint refers to an absolute symbolic address
or a label reference (D46745).

[MaskRay]

improve tests

[efriedma]

I'm not sure I understand the practical effect of this. I mean, I get the immediate effect on the object file, but I'm not sure I understand how this impacts linking. Without this patch, will we end up with runtime relocations in some cases? If so, which cases?

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

592

This is target-independent code; what targets does it affect?

[MaskRay]

In D101872#2740815, @efriedma wrote:

I'm not sure I understand the practical effect of this. I mean, I get the immediate effect on the object file, but I'm not sure I understand how this impacts linking. Without this patch, will we end up with runtime relocations in some cases? If so, which cases?

Currently clang -fpic -fno-semantic-interposition is a no-op on non-x86.

On x86, clang -fpic -fno-semantic-interposition sets dso_local on default visibility external linkage definitions. (Non-x86 targets don't set dso_local on such definitions.)

This change by itself does not affect clang behavior. (dso_local definitions with changed behavior are not emitted by Clang) With D101873, -fpic -fno-semantic-interposition will be effective on aarch64.

If D101873 is enabled without this patch, clang may produce assembly like

; CHECK-NEXT:    adrp x0, gv0  # instead of .Lgv0$local
; CHECK-NEXT:    ldr w0, [x0, :lo12:gv0]

This will be rejected by {ld.bfd,gold,ld.lld} -shared because in ELF, gv0 (STV_DEFAULT STB_GLOBAL) is by default preemptible and direct access relocations referencing a preemptible symbol cannot be used in -shared mode. By switching from gv0 to .Lgv0$local, we can make linkers happy.

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

592

Only aarch64. No-op for others.

With D101873, aarch64 will behave like x86: -fpic -fno-semantic-interposition will set dso_local on default visibility external linkage definitions.

[efriedma]

In D101872#2740867, @MaskRay wrote:

If D101873 is enabled without this patch, clang may produce assembly like

; CHECK-NEXT:    adrp x0, gv0  # instead of .Lgv0$local
; CHECK-NEXT:    ldr w0, [x0, :lo12:gv0]

This will be rejected by {ld.bfd,gold,ld.lld} -shared because in ELF, gv0 (STV_DEFAULT STB_GLOBAL) is by default preemptible and direct access relocations referencing a preemptible symbol cannot be used in -shared mode.

So this is basically a bugfix for dso_local handling? Okay.

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

592

Do you mean this code is unreachable for non-aarch64 targets? Or that getSymbolPreferLocal() does the same thing as getSymbol() on other targets? Or that clang won't emit IR that's impacted by this change?

[MaskRay]

In D101872#2740878, @efriedma wrote:

In D101872#2740867, @MaskRay wrote:

If D101873 is enabled without this patch, clang may produce assembly like

; CHECK-NEXT:    adrp x0, gv0  # instead of .Lgv0$local
; CHECK-NEXT:    ldr w0, [x0, :lo12:gv0]

This will be rejected by {ld.bfd,gold,ld.lld} -shared because in ELF, gv0 (STV_DEFAULT STB_GLOBAL) is by default preemptible and direct access relocations referencing a preemptible symbol cannot be used in -shared mode.

So this is basically a bugfix for dso_local handling? Okay.

I may perceive this as an unimplemented feature (lowering of default visibility external linkage dso_local definitions).

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

592

On other targets, clang doesn't emit IR which may observe different behavior due to getSymbol -> getSymbolPreferLocal.

On aarch64, D101873 will cause clang to emit IR which may observe different behavior.

[efriedma]

LGTM

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

592

Okay.

[MaskRay]

add a function call test

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

if (!strcmp(Code, "private")) {
     Msg << "Unknown special formatter '" << Code
          << "' for machine instr: " << *MI;
     report_fatal_error(Msg.str());
   }
 }
 
 void AsmPrinter::PrintSymbolOperand(const MachineOperand &MO, raw_ostream &OS) {
   assert(MO.isGlobal() && "caller should check MO.isGlobal");
-  getSymbol(MO.getGlobal())->print(OS, MAI);
+  getSymbolPreferLocal(*MO.getGlobal())->print(OS, MAI);
   printOffset(MO.getOffset(), OS);
 }
 
 /// PrintAsmOperand - Print the specified operand of MI, an INLINEASM
 /// instruction, using the specified assembler variant.  Targets should
 /// override this to format as appropriate for machine specific ExtraCodes
 /// or when the arch-independent handling would be too complex otherwise.
 bool AsmPrinter::PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,

llvm/lib/Target/AArch64/AArch64MCInstLower.cpp

AArch64MCInstLower::AArch64MCInstLower(MCContext &ctx, AsmPrinter &printer)
     : Ctx(ctx), Printer(printer) {}
 
 MCSymbol *
 AArch64MCInstLower::GetGlobalAddressSymbol(const MachineOperand &MO) const {
   const GlobalValue *GV = MO.getGlobal();
   unsigned TargetFlags = MO.getTargetFlags();
   const Triple &TheTriple = Printer.TM.getTargetTriple();
   if (!TheTriple.isOSBinFormatCOFF())
-    return Printer.getSymbol(GV);
+    return Printer.getSymbolPreferLocal(*GV);
 
   assert(TheTriple.isOSWindows() &&
          "Windows is the only supported COFF target");
 
   bool IsIndirect = (TargetFlags & (AArch64II::MO_DLLIMPORT | AArch64II::MO_COFFSTUB));
   if (!IsIndirect)
     return Printer.getSymbol(GV);
 

llvm/test/CodeGen/AArch64/elf-globals-pic.ll

define i32 @test_default_align() {
   ret i32 %val
 }
 
 define i64 @test_default_unaligned() {
   %addr = bitcast [2 x i32]* @var_default to i64*
   %val = load i64, i64* %addr
   ret i64 %val
 }
+
+@var_dsolocal = dso_local global i64 0, align 8
+
+define i64 @test_dsolocal(i64 %new) nounwind {
+  %val = load i64, i64* @var_dsolocal, align 8
+  store i64 %new, i64* @var_dsolocal
+  ret i64 %val
+; CHECK-PIC-LABEL: test_dsolocal:
+; CHECK-PIC-NEXT:  // %bb.0:
+; CHECK-PIC-NEXT:    adrp {{x[0-9]+}}, .Lvar_dsolocal$local
+; CHECK-PIC-NEXT:    ldr {{x[0-9]+}}, [{{x[0-9]+}}, :lo12:.Lvar_dsolocal$local]
+; CHECK-PIC-NEXT:    str x0, [{{x[0-9]+}}, :lo12:.Lvar_dsolocal$local]
+}

llvm/test/CodeGen/AArch64/semantic-interposition-asm.ll

+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc -mtriple=aarch64 -relocation-model=pic < %s | FileCheck %s
+
+;; Test that we use the local alias for dso_local globals in inline assembly.
+
+@gv0 = dso_local global i32 0
+@gv1 = dso_preemptable global i32 1
+
+define i32 @load() nounwind {
+; CHECK-LABEL: load:
+; CHECK:       // %bb.0: // %entry
+; CHECK-NEXT:    //APP
+; CHECK-NEXT:    adrp x0, .Lgv0$local
+; CHECK-NEXT:    ldr w0, [x0, :lo12:.Lgv0$local]
+; CHECK-NEXT:    adrp x8, gv1
+; CHECK-NEXT:    ldr w8, [x8, :lo12:gv1]
+; CHECK-NEXT:    add x0, x8, x0
+; CHECK-NEXT:    //NO_APP
+; CHECK-NEXT:    // kill: def $w0 killed $w0 killed $x0
+; CHECK-NEXT:    ret
+entry:
+  %0 = tail call i64 asm "adrp $0, $1\0Aldr ${0:w}, [$0, :lo12:$1]\0Aadrp x8, $2\0Aldr w8, [x8, :lo12:$2]\0Aadd $0,x8,$0", "=r,S,S,~{x8}"(i32* nonnull @gv0, i32* nonnull @gv1)
+  %conv = trunc i64 %0 to i32
+  ret i32 %conv
+}