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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
79 lines
3.1 KiB
LLVM
79 lines
3.1 KiB
LLVM
; RUN: llc -mtriple=x86_64-apple-darwin %s -o %t
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; RUN: FileCheck %s < %t
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; RUN: FileCheck %s -check-prefix=GOT-EQUIV < %t
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; GOT equivalent globals references can be replaced by the GOT entry of the
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; final symbol instead.
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%struct.data = type { i32, %struct.anon }
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%struct.anon = type { i32, i32 }
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; Check that these got equivalent symbols are never emitted or used
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; GOT-EQUIV-NOT: _localgotequiv
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; GOT-EQUIV-NOT: _extgotequiv
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@localfoo = global i32 42
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@localgotequiv = private unnamed_addr constant i32* @localfoo
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@extfoo = external global i32
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@extgotequiv = private unnamed_addr constant i32* @extfoo
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; Don't replace GOT equivalent usage within instructions and emit the GOT
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; equivalent since it can't be replaced by the GOT entry. @bargotequiv is
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; used by an instruction inside @t0.
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;
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; CHECK: l_bargotequiv:
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; CHECK-NEXT: .quad _extbar
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@extbar = external global i32
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@bargotequiv = private unnamed_addr constant i32* @extbar
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@table = global [4 x %struct.data] [
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; CHECK-LABEL: _table
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%struct.data { i32 1, %struct.anon { i32 2, i32 3 } },
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; Test GOT equivalent usage inside nested constant arrays.
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; CHECK: .long 5
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; CHECK-NOT: .long _localgotequiv-(_table+20)
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; CHECK-NEXT: .long _localfoo@GOTPCREL+4
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%struct.data { i32 4, %struct.anon { i32 5,
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i32 trunc (i64 sub (i64 ptrtoint (i32** @localgotequiv to i64),
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i64 ptrtoint (i32* getelementptr inbounds ([4 x %struct.data]* @table, i32 0, i64 1, i32 1, i32 1) to i64))
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to i32)}
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},
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; CHECK: .long 5
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; CHECK-NOT: _extgotequiv-(_table+32)
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; CHECK-NEXT: .long _extfoo@GOTPCREL+4
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%struct.data { i32 4, %struct.anon { i32 5,
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i32 trunc (i64 sub (i64 ptrtoint (i32** @extgotequiv to i64),
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i64 ptrtoint (i32* getelementptr inbounds ([4 x %struct.data]* @table, i32 0, i64 2, i32 1, i32 1) to i64))
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to i32)}
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},
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; Test support for arbitrary constants into the GOTPCREL offset
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; CHECK: .long 5
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; CHECK-NOT: _extgotequiv-(_table+44)
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; CHECK-NEXT: .long _extfoo@GOTPCREL+28
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%struct.data { i32 4, %struct.anon { i32 5,
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i32 add (i32 trunc (i64 sub (i64 ptrtoint (i32** @extgotequiv to i64),
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i64 ptrtoint (i32* getelementptr inbounds ([4 x %struct.data]* @table, i32 0, i64 3, i32 1, i32 1) to i64))
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to i32), i32 24)}
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}
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], align 16
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; Test multiple uses of GOT equivalents.
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; CHECK-LABEL: _delta
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; CHECK: .long _extfoo@GOTPCREL+4
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@delta = global i32 trunc (i64 sub (i64 ptrtoint (i32** @extgotequiv to i64),
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i64 ptrtoint (i32* @delta to i64))
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to i32)
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; CHECK-LABEL: _deltaplus:
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; CHECK: .long _localfoo@GOTPCREL+59
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@deltaplus = global i32 add (i32 trunc (i64 sub (i64 ptrtoint (i32** @localgotequiv to i64),
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i64 ptrtoint (i32* @deltaplus to i64))
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to i32), i32 55)
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define i32 @t0(i32 %a) {
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%x = add i32 trunc (i64 sub (i64 ptrtoint (i32** @bargotequiv to i64),
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i64 ptrtoint (i32 (i32)* @t0 to i64))
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to i32), %a
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ret i32 %x
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}
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