Bruno Cardoso Lopes 6bf5b2b094 [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 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230264 91177308-0d34-0410-b5e6-96231b3b80d8		2015-02-23 21:26:18 +00:00
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llvm	[AsmPrinter] Access pointers to globals via pcrel GOT entries	2015-02-23 21:26:18 +00:00
llvm-c	[BDCE] Add a bit-tracking DCE pass	2015-02-17 01:36:59 +00:00