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ARM: Don't emit R_ARM_NONE relocations to compact unwinding decoders in .ARM.exidx on Android.
These relocations are specified by the ARM EHABI (section 6.3). As I understand it, their purpose is to accommodate unwinder implementations that wish to reduce code size by placing the implementations of the compact unwinding decoders in a separate translation unit, and using extern weak symbols to refer to them from the main unwinder implementation, so that they are only linked when something in the binary needs them in order to unwind. However, neither of the unwinders used on Android (libgcc, LLVM libunwind) use this technique, and in fact emitting these relocations ends up being counterproductive to code size because they cause a copy of the unwinder to be statically linked into most binaries, regardless of whether it is actually needed. Furthermore, these relocations create circular dependencies (between libc and the unwinder) in cases where the unwinder is dynamically linked and libc contains compact unwind info. Therefore, deviate from the EHABI here and stop emitting these relocations on Android. Differential Revision: https://reviews.llvm.org/D70027
This commit is contained in:
Peter Collingbourne
parent
db1604061b
commit
9930bdcc8f
@ -101,7 +101,7 @@ MCELFStreamer *createARMELFStreamer(MCContext &Context,
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std::unique_ptr<MCAsmBackend> TAB,
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std::unique_ptr<MCObjectWriter> OW,
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std::unique_ptr<MCCodeEmitter> Emitter,
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bool RelaxAll, bool IsThumb);
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bool RelaxAll, bool IsThumb, bool IsAndroid);
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} // end namespace llvm
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@ -441,10 +441,12 @@ public:
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friend class ARMTargetELFStreamer;
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ARMELFStreamer(MCContext &Context, std::unique_ptr<MCAsmBackend> TAB,
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std::unique_ptr<MCObjectWriter> OW, std::unique_ptr<MCCodeEmitter> Emitter,
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bool IsThumb)
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: MCELFStreamer(Context, std::move(TAB), std::move(OW), std::move(Emitter)),
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IsThumb(IsThumb) {
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std::unique_ptr<MCObjectWriter> OW,
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std::unique_ptr<MCCodeEmitter> Emitter, bool IsThumb,
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bool IsAndroid)
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: MCELFStreamer(Context, std::move(TAB), std::move(OW),
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std::move(Emitter)),
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IsThumb(IsThumb), IsAndroid(IsAndroid) {
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EHReset();
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}
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@ -687,6 +689,7 @@ private:
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void EmitFixup(const MCExpr *Expr, MCFixupKind Kind);
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bool IsThumb;
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bool IsAndroid;
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int64_t MappingSymbolCounter = 0;
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DenseMap<const MCSection *, std::unique_ptr<ElfMappingSymbolInfo>>
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@ -1269,7 +1272,12 @@ void ARMELFStreamer::emitFnEnd() {
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// Emit the exception index table entry
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SwitchToExIdxSection(*FnStart);
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if (PersonalityIndex < ARM::EHABI::NUM_PERSONALITY_INDEX)
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// The EHABI requires a dependency preserving R_ARM_NONE relocation to the
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// personality routine to protect it from an arbitrary platform's static
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// linker garbage collection. We disable this for Android where the unwinder
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// is either dynamically linked or directly references the personality
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// routine.
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if (PersonalityIndex < ARM::EHABI::NUM_PERSONALITY_INDEX && !IsAndroid)
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EmitPersonalityFixup(GetAEABIUnwindPersonalityName(PersonalityIndex));
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const MCSymbolRefExpr *FnStartRef =
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@ -1504,9 +1512,11 @@ MCELFStreamer *createARMELFStreamer(MCContext &Context,
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std::unique_ptr<MCAsmBackend> TAB,
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std::unique_ptr<MCObjectWriter> OW,
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std::unique_ptr<MCCodeEmitter> Emitter,
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bool RelaxAll, bool IsThumb) {
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ARMELFStreamer *S = new ARMELFStreamer(Context, std::move(TAB), std::move(OW),
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std::move(Emitter), IsThumb);
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bool RelaxAll, bool IsThumb,
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bool IsAndroid) {
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ARMELFStreamer *S =
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new ARMELFStreamer(Context, std::move(TAB), std::move(OW),
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std::move(Emitter), IsThumb, IsAndroid);
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// FIXME: This should eventually end up somewhere else where more
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// intelligent flag decisions can be made. For now we are just maintaining
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// the status quo for ARM and setting EF_ARM_EABI_VER5 as the default.
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@ -212,7 +212,8 @@ static MCStreamer *createELFStreamer(const Triple &T, MCContext &Ctx,
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bool RelaxAll) {
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return createARMELFStreamer(
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Ctx, std::move(MAB), std::move(OW), std::move(Emitter), false,
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(T.getArch() == Triple::thumb || T.getArch() == Triple::thumbeb));
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(T.getArch() == Triple::thumb || T.getArch() == Triple::thumbeb),
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T.isAndroid());
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}
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static MCStreamer *
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@ -1,7 +1,9 @@
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@ RUN: llvm-mc %s -triple=armv7-unknown-linux-gnueabi -filetype=obj -o - \
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@ RUN: | llvm-readobj -S --sd --sr > %t
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@ RUN: FileCheck %s < %t
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@ RUN: FileCheck --check-prefix=RELOC %s < %t
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@ RUN: FileCheck --check-prefixes=RELOC,RELOC-NOAND %s < %t
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@ RUN: llvm-mc %s -triple=armv7-unknown-linux-androideabi -filetype=obj -o - \
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@ RUN: | llvm-readobj -S --sd --sr | FileCheck --check-prefix=RELOC %s
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@ Check the compact pr0 model
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@ -61,18 +63,16 @@ func2:
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@ CHECK: )
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@ CHECK: }
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@-------------------------------------------------------------------------------
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@ The first word should be relocated to .TEST1 section. Besides, there is
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@ another relocation entry for __aeabi_unwind_cpp_pr0, so that the linker
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@ will keep __aeabi_unwind_cpp_pr0.
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@ The first word should be relocated to .TEST1 section. Besides, on non-Android
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@ there is another relocation entry for __aeabi_unwind_cpp_pr0, so that the
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@ linker will keep __aeabi_unwind_cpp_pr0.
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@-------------------------------------------------------------------------------
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@ RELOC: Section {
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@ RELOC: Name: .rel.ARM.exidx.TEST1
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@ RELOC: Relocations [
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@ RELOC: 0x0 R_ARM_NONE __aeabi_unwind_cpp_pr0 0x0
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@ RELOC: 0x0 R_ARM_PREL31 .TEST1 0x0
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@ RELOC: ]
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@ RELOC: }
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@ RELOC: Section {
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@ RELOC: Name: .rel.ARM.exidx.TEST1
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@ RELOC: Relocations [
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@ RELOC-NOAND-NEXT: 0x0 R_ARM_NONE __aeabi_unwind_cpp_pr0 0x0
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@ RELOC-NEXT: 0x0 R_ARM_PREL31 .TEST1 0x0
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@ RELOC-NEXT: ]
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@-------------------------------------------------------------------------------
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@ Check .TEST2 section
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@ -98,14 +98,13 @@ func2:
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@ CHECK: )
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@ CHECK: }
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@-------------------------------------------------------------------------------
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@ The first word should be relocated to .TEST2 section. Besides, there is
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@ another relocation entry for __aeabi_unwind_cpp_pr0, so that the linker
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@ will keep __aeabi_unwind_cpp_pr0.
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@ The first word should be relocated to .TEST2 section. Besides, on non-Android
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@ there is another relocation entry for __aeabi_unwind_cpp_pr0, so that the
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@ linker will keep __aeabi_unwind_cpp_pr0.
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@-------------------------------------------------------------------------------
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@ RELOC: Section {
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@ RELOC: Name: .rel.ARM.exidx.TEST2
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@ RELOC: Relocations [
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@ RELOC: 0x0 R_ARM_NONE __aeabi_unwind_cpp_pr0 0x0
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@ RELOC: 0x0 R_ARM_PREL31 .TEST2 0x0
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@ RELOC: ]
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@ RELOC: }
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@ RELOC: Section {
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@ RELOC: Name: .rel.ARM.exidx.TEST2
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@ RELOC: Relocations [
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@ RELOC-NOAND-NEXT: 0x0 R_ARM_NONE __aeabi_unwind_cpp_pr0 0x0
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@ RELOC-NEXT: 0x0 R_ARM_PREL31 .TEST2 0x0
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@ RELOC-NEXT: ]
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