forked from OSchip/llvm-project
24492b057e
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
|
||
|---|---|---|
| .. | ||
| ARMException.cpp | ||
| AddressPool.cpp | ||
| AddressPool.h | ||
| AsmPrinter.cpp | ||
| AsmPrinterDwarf.cpp | ||
| AsmPrinterHandler.h | ||
| AsmPrinterInlineAsm.cpp | ||
| ByteStreamer.h | ||
| CMakeLists.txt | ||
| DIE.cpp | ||
| DIEHash.cpp | ||
| DIEHash.h | ||
| DbgValueHistoryCalculator.cpp | ||
| DbgValueHistoryCalculator.h | ||
| DebugLocEntry.h | ||
| DebugLocList.h | ||
| DwarfAccelTable.cpp | ||
| DwarfAccelTable.h | ||
| DwarfCFIException.cpp | ||
| DwarfCompileUnit.cpp | ||
| DwarfCompileUnit.h | ||
| DwarfDebug.cpp | ||
| DwarfDebug.h | ||
| DwarfException.h | ||
| DwarfExpression.cpp | ||
| DwarfExpression.h | ||
| DwarfFile.cpp | ||
| DwarfFile.h | ||
| DwarfStringPool.cpp | ||
| DwarfStringPool.h | ||
| DwarfUnit.cpp | ||
| DwarfUnit.h | ||
| EHStreamer.cpp | ||
| EHStreamer.h | ||
| ErlangGCPrinter.cpp | ||
| LLVMBuild.txt | ||
| Makefile | ||
| OcamlGCPrinter.cpp | ||
| Win64Exception.cpp | ||
| Win64Exception.h | ||
| WinCodeViewLineTables.cpp | ||
| WinCodeViewLineTables.h | ||