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smaller PLT entries for aarch64 (64-bit ARM)
- From: John Reiser <jreiser at bitwagon dot com>
- To: Binutils <binutils at sourceware dot org>
- Date: Mon, 15 Jan 2018 13:45:52 -0800
- Subject: smaller PLT entries for aarch64 (64-bit ARM)
- Authentication-results: sourceware.org; auth=none
Most of the PLT entries (Program Linkage Table) that I see on aarch64
(64-bit ARM) contain code like (from binutils-gdb commit origin/master
e09efd5931daf7eede1f4da46313a1aaadd4dbfa of Mon Jan 8 00:00:33 2018 +0000):
===== binutils/bfd/elfnn-aarch64.c near line 299
0x11, 0x02, 0x40, 0xf9, /* ldr x17, [x16, PLTGOT + n * 8] */
0x10, 0x02, 0x00, 0x91, /* add x16, x16, :lo12:PLTGOT + n * 8 */
=====
but this can be implemented one instruction shorter as:
0x11, 0x0e, 0x41, 0xf8, /* ldr x17, [x16, PLTGOT + n * 8]! */
where the exclamation point '!' denotes a write-back of the entire
effective address (including the displacement) into the base register.
Here is a standalone demonstration that can be single-stepped under gdb:
===== gcc -o demo -nostartfiles -nostdlib demo.S
.globl _start
_start:
nop
sub sp,sp,#4*8
mov x0,0x1234; str x0,[sp,#0*8]
mov x0,0x5678; str x0,[sp,#1*8]
mov x0,0x9abc; str x0,[sp,#2*8]
mov x0,0xdef0; str x0,[sp,#3*8]
mov x16,sp
ldr x17,[x16,#2*8]! // x17 = 0x9abc; x16 += 2*8;
nop
=====
--