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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
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| .. | ||
| AlignedBundling | ||
| 3DNow.s | ||
| 2011-09-06-NoNewline.s | ||
| address-size.s | ||
| avx512-encodings.s | ||
| avx512bw-encoding.s | ||
| avx512vl-encoding.s | ||
| cfi_def_cfa-crash.s | ||
| compact-unwind.s | ||
| cstexpr-gotpcrel.ll | ||
| fde-reloc.s | ||
| fixup-cpu-mode.s | ||
| gnux32-dwarf-gen.s | ||
| index-operations.s | ||
| intel-syntax-2.s | ||
| intel-syntax-ambiguous.s | ||
| intel-syntax-avx512.s | ||
| intel-syntax-bitwise-ops.s | ||
| intel-syntax-directional-label.s | ||
| intel-syntax-encoding.s | ||
| intel-syntax-error.s | ||
| intel-syntax-hex.s | ||
| intel-syntax-invalid-basereg.s | ||
| intel-syntax-invalid-scale.s | ||
| intel-syntax-ptr-sized.s | ||
| intel-syntax-unsized-memory.s | ||
| intel-syntax.s | ||
| lit.local.cfg | ||
| macho-uleb.s | ||
| no-elf-compact-unwind.s | ||
| padlock.s | ||
| relax-insn.s | ||
| reloc-macho.s | ||
| reloc-undef-global.s | ||
| ret.s | ||
| sgx-encoding.s | ||
| shuffle-comments.s | ||
| stackmap-nops.ll | ||
| validate-inst-att.s | ||
| validate-inst-intel.s | ||
| variant-diagnostics.s | ||
| x86_64-avx-clmul-encoding.s | ||
| x86_64-avx-encoding.s | ||
| x86_64-bmi-encoding.s | ||
| x86_64-encoding.s | ||
| x86_64-fma3-encoding.s | ||
| x86_64-fma4-encoding.s | ||
| x86_64-hle-encoding.s | ||
| x86_64-imm-widths.s | ||
| x86_64-rand-encoding.s | ||
| x86_64-rtm-encoding.s | ||
| x86_64-signed-reloc.s | ||
| x86_64-sse4a.s | ||
| x86_64-tbm-encoding.s | ||
| x86_64-xop-encoding.s | ||
| x86_directives.s | ||
| x86_errors.s | ||
| x86_long_nop.s | ||
| x86_nop.s | ||
| x86_operands.s | ||
| x86-16.s | ||
| x86-32-avx.s | ||
| x86-32-coverage.s | ||
| x86-32-fma3.s | ||
| x86-32-ms-inline-asm.s | ||
| x86-32.s | ||
| x86-64-avx512bw_vl.s | ||
| x86-64-avx512bw.s | ||
| x86-64-avx512dq_vl.s | ||
| x86-64-avx512dq.s | ||
| x86-64-avx512f_vl.s | ||
| x86-64.s | ||
| x86-itanium.ll | ||
| x86-target-directives.s | ||
| x86-windows-itanium-libcalls.ll | ||