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author | Bruno Cardoso Lopes <bruno.cardoso@gmail.com> | 2015-02-23 21:26:18 +0000 |
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committer | Bruno Cardoso Lopes <bruno.cardoso@gmail.com> | 2015-02-23 21:26:18 +0000 |
commit | 24492b057ed616870a651caaac8d6a3c94378e07 (patch) | |
tree | 7c05fe3c4925dfb61015272d50fda816698832a9 /llvm/lib/Target/X86/X86TargetObjectFile.h | |
parent | 4d7aae932c48f0b1e16dc989a84a84a60befb437 (diff) | |
download | llvm-24492b057ed616870a651caaac8d6a3c94378e07.zip llvm-24492b057ed616870a651caaac8d6a3c94378e07.tar.gz llvm-24492b057ed616870a651caaac8d6a3c94378e07.tar.bz2 |
[AsmPrinter] Access pointers to globals via pcrel GOT entries
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
llvm-svn: 230264
Diffstat (limited to 'llvm/lib/Target/X86/X86TargetObjectFile.h')
-rw-r--r-- | llvm/lib/Target/X86/X86TargetObjectFile.h | 6 |
1 files changed, 6 insertions, 0 deletions
diff --git a/llvm/lib/Target/X86/X86TargetObjectFile.h b/llvm/lib/Target/X86/X86TargetObjectFile.h index 6a6988a..f7455382 100644 --- a/llvm/lib/Target/X86/X86TargetObjectFile.h +++ b/llvm/lib/Target/X86/X86TargetObjectFile.h @@ -19,6 +19,8 @@ namespace llvm { /// x86-64. class X86_64MachoTargetObjectFile : public TargetLoweringObjectFileMachO { public: + X86_64MachoTargetObjectFile(); + const MCExpr * getTTypeGlobalReference(const GlobalValue *GV, unsigned Encoding, Mangler &Mang, const TargetMachine &TM, @@ -30,6 +32,10 @@ namespace llvm { MCSymbol *getCFIPersonalitySymbol(const GlobalValue *GV, Mangler &Mang, const TargetMachine &TM, MachineModuleInfo *MMI) const override; + + const MCExpr * + getIndirectSymViaGOTPCRel(const MCSymbol *Sym, + int64_t Offset) const override; }; /// X86LinuxTargetObjectFile - This implementation is used for linux x86 |