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6bf5b2b094
Front-ends could use global unnamed_addr to hold pointers to other
symbols, like @gotequivalent below:
@foo = global i32 42
@gotequivalent = private unnamed_addr constant i32* @foo
@delta = global i32 trunc (i64 sub (i64 ptrtoint (i32** @gotequivalent to i64),
i64 ptrtoint (i32* @delta to i64))
to i32)
The global @delta holds a data "PC"-relative offset to @gotequivalent,
an unnamed pointer to @foo. The darwin/x86-64 assembly output for this follows:
.globl _foo
_foo:
.long 42
.globl _gotequivalent
_gotequivalent:
.quad _foo
.globl _delta
_delta:
.long _gotequivalent-_delta
Since unnamed_addr indicates that the address is not significant, only
the content, we can optimize the case above by replacing pc-relative
accesses to "GOT equivalent" globals, by a PC relative access to the GOT
entry of the final symbol instead. Therefore, "delta" can contain a pc
relative relocation to foo's GOT entry and we avoid the emission of
"gotequivalent", yielding the assembly code below:
.globl _foo
_foo:
.long 42
.globl _delta
_delta:
.long _foo@GOTPCREL+4
There are a couple of advantages of doing this: (1) Front-ends that need
to emit a great deal of data to store pointers to external symbols could
save space by not emitting such "got equivalent" globals and (2) IR
constructs combined with this opt opens a way to represent GOT pcrel
relocations by using the LLVM IR, which is something we previously had
no way to express.
Differential Revision: http://reviews.llvm.org/D6922
rdar://problem/18534217
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230264 91177308-0d34-0410-b5e6-96231b3b80d8
65 lines
2.5 KiB
C++
65 lines
2.5 KiB
C++
//===-- X86TargetObjectFile.h - X86 Object Info -----------------*- C++ -*-===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_LIB_TARGET_X86_X86TARGETOBJECTFILE_H
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#define LLVM_LIB_TARGET_X86_X86TARGETOBJECTFILE_H
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#include "llvm/CodeGen/TargetLoweringObjectFileImpl.h"
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#include "llvm/Target/TargetLoweringObjectFile.h"
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namespace llvm {
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/// X86_64MachoTargetObjectFile - This TLOF implementation is used for Darwin
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/// x86-64.
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class X86_64MachoTargetObjectFile : public TargetLoweringObjectFileMachO {
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public:
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X86_64MachoTargetObjectFile();
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const MCExpr *
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getTTypeGlobalReference(const GlobalValue *GV, unsigned Encoding,
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Mangler &Mang, const TargetMachine &TM,
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MachineModuleInfo *MMI,
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MCStreamer &Streamer) const override;
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// getCFIPersonalitySymbol - The symbol that gets passed to
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// .cfi_personality.
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MCSymbol *getCFIPersonalitySymbol(const GlobalValue *GV, Mangler &Mang,
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const TargetMachine &TM,
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MachineModuleInfo *MMI) const override;
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const MCExpr *
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getIndirectSymViaGOTPCRel(const MCSymbol *Sym,
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int64_t Offset) const override;
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};
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/// X86LinuxTargetObjectFile - This implementation is used for linux x86
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/// and x86-64.
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class X86LinuxTargetObjectFile : public TargetLoweringObjectFileELF {
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void Initialize(MCContext &Ctx, const TargetMachine &TM) override;
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/// \brief Describe a TLS variable address within debug info.
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const MCExpr *getDebugThreadLocalSymbol(const MCSymbol *Sym) const override;
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};
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/// \brief This implementation is used for Windows targets on x86 and x86-64.
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class X86WindowsTargetObjectFile : public TargetLoweringObjectFileCOFF {
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const MCExpr *
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getExecutableRelativeSymbol(const ConstantExpr *CE, Mangler &Mang,
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const TargetMachine &TM) const override;
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/// \brief Given a mergeable constant with the specified size and relocation
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/// information, return a section that it should be placed in.
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const MCSection *getSectionForConstant(SectionKind Kind,
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const Constant *C) const override;
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};
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} // end namespace llvm
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#endif
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