[MIPS] Disassemble the 0xefefefef trap padding used by LLD

[arichardson]

Summary

Previously using llvm-objdump would just print a lot of 0xefefefef unknown
instructions. If we disassemble this as trap_ri it should be obvious that
it is a explicit trap padding rather than some instruction that is not
available because the wrong -triple was passed to llvm-objdump.

I am not sure if there trap_ri is the best mnemonic to use here, I just
decided to go for that since it will always cause a reserved instruction
trap.

[atanasyan]

In fact, 0xef (111011b to be precise) is a valid start for the swc3 mips instruction. Take a look at LLVM test\MC\Disassembler\Mips\mips2\valid-mips2.txt. Right now I do not know why llvm-objdump does not show a correct instruction instead of "<unknown>". By the way GNU objdump shows "swc3 $15,-4113(ra)" for "efefefef".

[ruiu]

IIRC, 0xefefefef is chosen by Theo de Raadt.

[sdardis]

Right now I do not know why llvm-objdump does not show a correct instruction instead of "<unknown>". By the way GNU objdump shows "swc3 $15,-4113(ra)" for "efefefef".

It's not being disassembled as the disassembler sets the internal state to be mips32. The COP3 opcodes are only disassembled for MIPS-I and MIPS-II.

That opcode was repurposed in MIPSR6:

8: efefefef lwpc ra,0xffffbfc4

For micromips(r6):

10:	ef ef 	li16	$7, 111

Which is not particularly useful as trap instructions.

I'm a little uncertain as to what specific opcode we should use here. If we have a pure microMIPS or pure MIPS environment, we can use the corresponding 'sigrie' opcode from MIPSR6 to cause a reserved instruction trap. In revisions prior to revision 6, those instructions trap anyway.

In a mixed environment as we have a problem where we don't know what ISA mode we're operating in, so we'd have to alternate 'sigrie's as the microMIPSR6 'sigrie' corresponds to a valid MIPS64 no-op instruction and to ensure that the processor sees both if we've hit padding.

I've looked at the instruction tables and I believe that the MIPSR6 sigrie with the operand 1 is the best choice. Modulo a bug in upstream binutils, for microMIPS it disassembles to:

0:	04 17 	subu16	$3, $16, $17
2:	00 01  <unknown>
4:	04 17 	subu16	$3, $16, $17
6:	00 01  <unknown>

Which I believe is the simplest solution. That just requires a small change to lld, and providing an implementation of sigrie for MIPS in llvm.

[atanasyan]

Let's abandon this review because we need to implement the SIGRIE instruction support and use it for a trap padding in the LLD. I'm going to implement both parts of the plan.

[emaste]

In D41821#1152337, @atanasyan wrote:

I'm going to implement both parts of the plan.

Thank you. (Please add a reference to this review in the new reviews so there's a link to find them.)

[atanasyan]

Support of SIGRIE instruction is implemented at rL346230. LLD related patch with new trap instruction is D54154.

lld/test/ELF/mips-got16-relocatable.s

 # REQUIRES: mips
 
 # OBJ:      Disassembly of section .text:
 # OBJ-NEXT: .text:
 # OBJ-NEXT:        0:       8f 99 00 00     lw      $25, 0($gp)
 # OBJ-NEXT:                         00000000:  R_MIPS_GOT16 .data
 # OBJ-NEXT:        4:       27 24 00 00     addiu   $4, $25, 0
 # OBJ-NEXT:                         00000004:  R_MIPS_LO16  .data
-# OBJ-NEXT:        8:       ef ef ef ef     <unknown>
-# OBJ-NEXT:        c:       ef ef ef ef     <unknown>
+# OBJ-NEXT:        8:       ef ef ef ef     trap_ri
+# OBJ-NEXT:        c:       ef ef ef ef     trap_ri
 # OBJ-NEXT:       10:       8f 99 00 00     lw      $25, 0($gp)
 # OBJ-NEXT:                         00000010:  R_MIPS_GOT16 .data
 # OBJ-NEXT:       14:       27 24 00 10     addiu   $4, $25, 16
 # OBJ-NEXT:                         00000014:  R_MIPS_LO16  .data
 
 # SO:      Disassembly of section .text:
 # SO-NEXT: .text:
 # SO-NEXT:    10000:       8f 99 80 18     lw      $25, -32744($gp)
 # SO-NEXT:    10004:       27 24 00 00     addiu   $4, $25, 0
-# SO-NEXT:    10008:       ef ef ef ef     <unknown>
-# SO-NEXT:    1000c:       ef ef ef ef     <unknown>
+# SO-NEXT:    10008:       ef ef ef ef     trap_ri
+# SO-NEXT:    1000c:       ef ef ef ef     trap_ri
 # SO-NEXT:    10010:       8f 99 80 18     lw      $25, -32744($gp)
 # SO-NEXT:    10014:       27 24 00 10     addiu   $4, $25, 16
 
   .text
   lw     $t9, %got(.data)($gp)
   addiu  $a0, $t9, %lo(.data)
 
   .data
 data:
   .word 0

lld/test/ELF/mips-npic-call-pic-os.s

 # CHECK-NEXT:    20020:       00 00 00 00     nop
 # CHECK: foo1b:
 # CHECK-NEXT:    20024:       00 00 00 00     nop
 # CHECK: __LA25Thunk_foo2:
 # CHECK-NEXT:    20028:       3c 19 00 02     lui     $25, 2
 # CHECK-NEXT:    2002c:       08 00 80 10     j       131136 <foo2>
 # CHECK-NEXT:    20030:       27 39 00 40     addiu   $25, $25, 64
 # CHECK-NEXT:    20034:       00 00 00 00     nop
-# CHECK-NEXT:    20038:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2003c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20038:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2003c:       ef ef ef ef     trap_ri
 # CHECK: foo2:
 # CHECK-NEXT:    20040:       00 00 00 00     nop
 # CHECK: __LA25Thunk_fpic:
 # CHECK-NEXT:    20044:       3c 19 00 02     lui     $25, 2
 # CHECK-NEXT:    20048:       08 00 80 18     j       131168 <fpic>
 # CHECK-NEXT:    2004c:       27 39 00 60     addiu   $25, $25, 96
 # CHECK-NEXT:    20050:       00 00 00 00     nop
-# CHECK-NEXT:    20054:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20058:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2005c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20054:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20058:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2005c:       ef ef ef ef     trap_ri
 # CHECK: fpic:
 # CHECK-NEXT:    20060:       00 00 00 00     nop
-# CHECK-NEXT:    20064:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20068:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2006c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20064:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20068:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2006c:       ef ef ef ef     trap_ri
 # CHECK: fnpic:
 # CHECK-NEXT:    20070:       00 00 00 00     nop
 # CHECK-NEXT: Disassembly of section differentos:
 # CHECK-NEXT: __start:
 # CHECK-NEXT:    20074:       0c 00 80 00     jal     131072 <__LA25Thunk_foo1a>
 # CHECK-NEXT:    20078:       00 00 00 00     nop
 # CHECK-NEXT:    2007c:       0c 00 80 0a     jal     131112 <__LA25Thunk_foo2>
 # CHECK-NEXT:    20080:       00 00 00 00     nop

 # REVERSE-NEXT:    20020:       00 00 00 00     nop
 # REVERSE: foo1b:
 # REVERSE-NEXT:    20024:       00 00 00 00     nop
 # REVERSE: __LA25Thunk_foo2:
 # REVERSE-NEXT:    20028:       3c 19 00 02     lui     $25, 2
 # REVERSE-NEXT:    2002c:       08 00 80 10     j       131136 <foo2>
 # REVERSE-NEXT:    20030:       27 39 00 40     addiu   $25, $25, 64
 # REVERSE-NEXT:    20034:       00 00 00 00     nop
-# REVERSE-NEXT:    20038:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    2003c:       ef ef ef ef     <unknown>
+# REVERSE-NEXT:    20038:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    2003c:       ef ef ef ef     trap_ri
 # REVERSE: foo2:
 # REVERSE-NEXT:    20040:       00 00 00 00     nop
-# REVERSE-NEXT:    20044:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    20048:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    2004c:       ef ef ef ef     <unknown>
+# REVERSE-NEXT:    20044:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    20048:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    2004c:       ef ef ef ef     trap_ri
 # REVERSE: __LA25Thunk_fpic:
 # REVERSE-NEXT:    20050:       3c 19 00 02     lui     $25, 2
 # REVERSE-NEXT:    20054:       08 00 80 18     j       131168 <fpic>
 # REVERSE-NEXT:    20058:       27 39 00 60     addiu   $25, $25, 96
 # REVERSE-NEXT:    2005c:       00 00 00 00     nop
 # REVERSE: fpic:
 # REVERSE-NEXT:    20060:       00 00 00 00     nop
-# REVERSE-NEXT:    20064:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    20068:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    2006c:       ef ef ef ef     <unknown>
+# REVERSE-NEXT:    20064:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    20068:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    2006c:       ef ef ef ef     trap_ri
 # REVERSE: fnpic:
 # REVERSE-NEXT:    20070:       00 00 00 00     nop
 # REVERSE-NEXT: Disassembly of section differentos:
 # REVERSE-NEXT: __start:
 # REVERSE-NEXT:    20074:       0c 00 80 00     jal     131072 <__LA25Thunk_foo1a>
 # REVERSE-NEXT:    20078:       00 00 00 00     nop
 # REVERSE-NEXT:    2007c:       0c 00 80 0a     jal     131112 <__LA25Thunk_foo2>
 # REVERSE-NEXT:    20080:       00 00 00 00     nop

lld/test/ELF/mips-npic-call-pic-script.s

 # CHECK-NEXT:    20020:       00 00 00 00     nop
 # CHECK: foo1b:
 # CHECK-NEXT:    20024:       00 00 00 00     nop
 # CHECK: __LA25Thunk_foo2:
 # CHECK-NEXT:    20028:       3c 19 00 02     lui     $25, 2
 # CHECK-NEXT:    2002c:       08 00 80 10     j       131136 <foo2>
 # CHECK-NEXT:    20030:       27 39 00 40     addiu   $25, $25, 64
 # CHECK-NEXT:    20034:       00 00 00 00     nop
-# CHECK-NEXT:    20038:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2003c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20038:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2003c:       ef ef ef ef     trap_ri
 # CHECK: foo2:
 # CHECK-NEXT:    20040:       00 00 00 00     nop
-# CHECK-NEXT:    20044:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20048:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2004c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20050:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20054:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20058:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2005c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20060:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20064:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20068:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2006c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20070:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20074:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20078:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2007c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20080:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20084:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20088:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2008c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20090:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20094:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20098:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2009c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200a0:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200a4:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200a8:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200ac:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200b0:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200b4:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200b8:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200bc:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200c0:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200c4:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200c8:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200cc:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200d0:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200d4:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200d8:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200dc:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200e0:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200e4:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200e8:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200ec:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200f0:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200f4:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200f8:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    200fc:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20100:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20104:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20108:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2010c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20110:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20114:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20118:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2011c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20120:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20124:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20128:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2012c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20130:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20134:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20138:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2013c:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20140:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20144:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20148:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2014c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20044:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20048:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2004c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20050:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20054:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20058:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2005c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20060:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20064:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20068:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2006c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20070:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20074:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20078:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2007c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20080:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20084:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20088:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2008c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20090:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20094:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20098:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2009c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200a0:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200a4:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200a8:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200ac:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200b0:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200b4:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200b8:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200bc:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200c0:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200c4:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200c8:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200cc:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200d0:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200d4:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200d8:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200dc:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200e0:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200e4:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200e8:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200ec:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200f0:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200f4:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200f8:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    200fc:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20100:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20104:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20108:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2010c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20110:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20114:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20118:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2011c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20120:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20124:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20128:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2012c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20130:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20134:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20138:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2013c:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20140:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20144:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20148:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2014c:       ef ef ef ef     trap_ri
 # CHECK: __start:
 # CHECK-NEXT:    20150:       0c 00 80 00     jal     131072 <__LA25Thunk_foo1a>
 # CHECK-NEXT:    20154:       00 00 00 00     nop
 # CHECK-NEXT:    20158:       0c 00 80 0a     jal     131112 <__LA25Thunk_foo2>
 # CHECK-NEXT:    2015c:       00 00 00 00     nop
 # CHECK-NEXT:    20160:       0c 00 80 04     jal     131088 <__LA25Thunk_foo1b>
 # CHECK-NEXT:    20164:       00 00 00 00     nop
 # CHECK-NEXT:    20168:       0c 00 80 0a     jal     131112 <__LA25Thunk_foo2>
 # CHECK-NEXT:    2016c:       00 00 00 00     nop
 # CHECK-NEXT:    20170:       0c 00 80 60     jal     131456 <__LA25Thunk_fpic>
 # CHECK-NEXT:    20174:       00 00 00 00     nop
 # CHECK-NEXT:    20178:       0c 00 80 68     jal     131488 <fnpic>
 # CHECK-NEXT:    2017c:       00 00 00 00     nop
 # CHECK: __LA25Thunk_fpic:
 # CHECK-NEXT:    20180:       3c 19 00 02     lui     $25, 2
 # CHECK-NEXT:    20184:       08 00 80 64     j       131472 <fpic>
 # CHECK-NEXT:    20188:       27 39 01 90     addiu   $25, $25, 400
 # CHECK-NEXT:    2018c:       00 00 00 00     nop
 # CHECK: fpic:
 # CHECK-NEXT:    20190:       00 00 00 00     nop
-# CHECK-NEXT:    20194:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20198:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2019c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20194:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20198:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2019c:       ef ef ef ef     trap_ri
 # CHECK: fnpic:
 # CHECK-NEXT:    201a0:       00 00 00 00     nop
 
   .text
   .globl __start
 __start:
   jal foo1a
   jal foo2

 # ORPH1-NEXT:    2002c:       00 00 00 00     nop
 # ORPH1: __LA25Thunk_fpic:
 # ORPH1-NEXT:    20030:       3c 19 00 02     lui     $25, 2
 # ORPH1-NEXT:    20034:       08 00 80 10     j       131136 <fpic>
 # ORPH1-NEXT:    20038:       27 39 00 40     addiu   $25, $25, 64
 # ORPH1-NEXT:    2003c:       00 00 00 00     nop
 # ORPH1: fpic:
 # ORPH1-NEXT:    20040:       00 00 00 00     nop
-# ORPH1-NEXT:    20044:       ef ef ef ef     <unknown>
-# ORPH1-NEXT:    20048:       ef ef ef ef     <unknown>
-# ORPH1-NEXT:    2004c:       ef ef ef ef     <unknown>
+# ORPH1-NEXT:    20044:       ef ef ef ef     trap_ri
+# ORPH1-NEXT:    20048:       ef ef ef ef     trap_ri
+# ORPH1-NEXT:    2004c:       ef ef ef ef     trap_ri
 # ORPH1: fnpic:
 # ORPH1-NEXT:    20050:       00 00 00 00     nop
 # ORPH1: __LA25Thunk_foo1a:
 # ORPH1-NEXT:    20054:       3c 19 00 02     lui     $25, 2
 # ORPH1-NEXT:    20058:       08 00 80 20     j       131200 <foo1a>
 # ORPH1-NEXT:    2005c:       27 39 00 80     addiu   $25, $25, 128
 # ORPH1-NEXT:    20060:       00 00 00 00     nop
 # ORPH1: __LA25Thunk_foo1b:
 # ORPH1-NEXT:    20064:       3c 19 00 02     lui     $25, 2
 # ORPH1-NEXT:    20068:       08 00 80 21     j       131204 <foo1b>
 # ORPH1-NEXT:    2006c:       27 39 00 84     addiu   $25, $25, 132
 # ORPH1-NEXT:    20070:       00 00 00 00     nop
-# ORPH1-NEXT:    20074:       ef ef ef ef     <unknown>
-# ORPH1-NEXT:    20078:       ef ef ef ef     <unknown>
-# ORPH1-NEXT:    2007c:       ef ef ef ef     <unknown>
+# ORPH1-NEXT:    20074:       ef ef ef ef     trap_ri
+# ORPH1-NEXT:    20078:       ef ef ef ef     trap_ri
+# ORPH1-NEXT:    2007c:       ef ef ef ef     trap_ri
 # ORPH1: foo1a:
 # ORPH1-NEXT:    20080:       00 00 00 00     nop
 # ORPH1: foo1b:
 # ORPH1-NEXT:    20084:       00 00 00 00     nop
 # ORPH1: __LA25Thunk_foo2:
 # ORPH1-NEXT:    20088:       3c 19 00 02     lui     $25, 2
 # ORPH1-NEXT:    2008c:       08 00 80 28     j       131232 <foo2>
 # ORPH1-NEXT:    20090:       27 39 00 a0     addiu   $25, $25, 160
 # ORPH1-NEXT:    20094:       00 00 00 00     nop
-# ORPH1-NEXT:    20098:       ef ef ef ef     <unknown>
-# ORPH1-NEXT:    2009c:       ef ef ef ef     <unknown>
+# ORPH1-NEXT:    20098:       ef ef ef ef     trap_ri
+# ORPH1-NEXT:    2009c:       ef ef ef ef     trap_ri
 # ORPH1: foo2:
 # ORPH1-NEXT:    200a0:       00 00 00 00     nop
 
 # Test script with orphans added to new OutputSection, the .text.1 and
 # .text.2 sections will form a new OutputSection .text
 # RUN: echo "SECTIONS { .out 0x20000 : { *(.text) }  }" > %t3.script
 # RUN: ld.lld --script %t3.script %t-npic.o %t-pic.o %t-sto-pic.o -o %t3.exe
 # RUN: llvm-objdump -d %t3.exe | FileCheck -check-prefix=ORPH2 %s

 # ORPH2-NEXT:    2002c:       00 00 00 00     nop
 # ORPH2: __LA25Thunk_fpic:
 # ORPH2-NEXT:    20030:       3c 19 00 02     lui     $25, 2
 # ORPH2-NEXT:    20034:       08 00 80 10     j       131136 <fpic>
 # ORPH2-NEXT:    20038:       27 39 00 40     addiu   $25, $25, 64
 # ORPH2-NEXT:    2003c:       00 00 00 00     nop
 # ORPH2: fpic:
 # ORPH2-NEXT:    20040:       00 00 00 00     nop
-# ORPH2-NEXT:    20044:       ef ef ef ef     <unknown>
-# ORPH2-NEXT:    20048:       ef ef ef ef     <unknown>
-# ORPH2-NEXT:    2004c:       ef ef ef ef     <unknown>
+# ORPH2-NEXT:    20044:       ef ef ef ef     trap_ri
+# ORPH2-NEXT:    20048:       ef ef ef ef     trap_ri
+# ORPH2-NEXT:    2004c:       ef ef ef ef     trap_ri
 # ORPH2: fnpic:
 # ORPH2-NEXT:    20050:       00 00 00 00     nop
 # ORPH2-NEXT: Disassembly of section .text:
 # ORPH2-NEXT: __LA25Thunk_foo1a:
 # ORPH2-NEXT:    20060:       3c 19 00 02     lui     $25, 2
 # ORPH2-NEXT:    20064:       08 00 80 20     j       131200 <foo1a>
 # ORPH2-NEXT:    20068:       27 39 00 80     addiu   $25, $25, 128
 # ORPH2-NEXT:    2006c:       00 00 00 00     nop
 # ORPH2: __LA25Thunk_foo1b:
 # ORPH2-NEXT:    20070:       3c 19 00 02     lui     $25, 2
 # ORPH2-NEXT:    20074:       08 00 80 21     j       131204 <foo1b>
 # ORPH2-NEXT:    20078:       27 39 00 84     addiu   $25, $25, 132
 # ORPH2-NEXT:    2007c:       00 00 00 00     nop
 # ORPH2: foo1a:
 # ORPH2-NEXT:    20080:       00 00 00 00     nop
 # ORPH2: foo1b:
 # ORPH2-NEXT:    20084:       00 00 00 00     nop
 # ORPH2: __LA25Thunk_foo2:
 # ORPH2-NEXT:    20088:       3c 19 00 02     lui     $25, 2
 # ORPH2-NEXT:    2008c:       08 00 80 28     j       131232 <foo2>
 # ORPH2-NEXT:    20090:       27 39 00 a0     addiu   $25, $25, 160
 # ORPH2-NEXT:    20094:       00 00 00 00     nop
-# ORPH2-NEXT:    20098:       ef ef ef ef     <unknown>
-# ORPH2-NEXT:    2009c:       ef ef ef ef     <unknown>
+# ORPH2-NEXT:    20098:       ef ef ef ef     trap_ri
+# ORPH2-NEXT:    2009c:       ef ef ef ef     trap_ri
 # ORPH2: foo2:
 # ORPH2-NEXT:    200a0:       00 00 00 00     nop

lld/test/ELF/mips-npic-call-pic.s

 # CHECK: foo1b:
 # CHECK-NEXT:    20054:       00 00 00 00     nop
 
 # CHECK: __LA25Thunk_foo2:
 # CHECK-NEXT:    20058:       3c 19 00 02     lui     $25, 2
 # CHECK-NEXT:    2005c:       08 00 80 1c     j       131184 <foo2>
 # CHECK-NEXT:    20060:       27 39 00 70     addiu   $25, $25, 112
 # CHECK-NEXT:    20064:       00 00 00 00     nop
-# CHECK-NEXT:    20068:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2006c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20068:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2006c:       ef ef ef ef     trap_ri
 
 # CHECK: foo2:
 # CHECK-NEXT:    20070:       00 00 00 00     nop
 
 # CHECK: __LA25Thunk_fpic:
 # CHECK-NEXT:    20074:       3c 19 00 02     lui     $25, 2
 # CHECK-NEXT:    20078:       08 00 80 24     j       131216 <fpic>
 # CHECK-NEXT:    2007c:       27 39 00 90     addiu   $25, $25, 144
 # CHECK-NEXT:    20080:       00 00 00 00     nop
-# CHECK-NEXT:    20084:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20088:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2008c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20084:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20088:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2008c:       ef ef ef ef     trap_ri
 
 # CHECK: fpic:
 # CHECK-NEXT:    20090:       00 00 00 00     nop
-# CHECK-NEXT:    20094:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    20098:       ef ef ef ef     <unknown>
-# CHECK-NEXT:    2009c:       ef ef ef ef     <unknown>
+# CHECK-NEXT:    20094:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    20098:       ef ef ef ef     trap_ri
+# CHECK-NEXT:    2009c:       ef ef ef ef     trap_ri
 
 # CHECK: fnpic:
 # CHECK-NEXT:    200a0:       00 00 00 00     nop
 
 # Make sure the thunks are created properly no matter how
 # objects are laid out.
 #
 # RUN: ld.lld %t-pic.o %t-npic.o %t-sto-pic.o -o %t.exe

 # REVERSE-NEXT:    20020:       00 00 00 00     nop
 # REVERSE: foo1b:
 # REVERSE-NEXT:    20024:       00 00 00 00     nop
 # REVERSE: __LA25Thunk_foo2:
 # REVERSE-NEXT:    20028:       3c 19 00 02     lui     $25, 2
 # REVERSE-NEXT:    2002c:       08 00 80 10     j       131136 <foo2>
 # REVERSE-NEXT:    20030:       27 39 00 40     addiu   $25, $25, 64
 # REVERSE-NEXT:    20034:       00 00 00 00     nop
-# REVERSE-NEXT:    20038:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    2003c:       ef ef ef ef     <unknown>
+# REVERSE-NEXT:    20038:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    2003c:       ef ef ef ef     trap_ri
 # REVERSE: foo2:
 # REVERSE-NEXT:    20040:       00 00 00 00     nop
-# REVERSE-NEXT:    20044:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    20048:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    2004c:       ef ef ef ef     <unknown>
+# REVERSE-NEXT:    20044:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    20048:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    2004c:       ef ef ef ef     trap_ri
 # REVERSE: __start:
 # REVERSE-NEXT:    20050:       0c 00 80 00     jal     131072 <__LA25Thunk_foo1a>
 # REVERSE-NEXT:    20054:       00 00 00 00     nop
 # REVERSE-NEXT:    20058:       0c 00 80 0a     jal     131112 <__LA25Thunk_foo2>
 # REVERSE-NEXT:    2005c:       00 00 00 00     nop
 # REVERSE-NEXT:    20060:       0c 00 80 04     jal     131088 <__LA25Thunk_foo1b>
 # REVERSE-NEXT:    20064:       00 00 00 00     nop
 # REVERSE-NEXT:    20068:       0c 00 80 0a     jal     131112 <__LA25Thunk_foo2>
 # REVERSE-NEXT:    2006c:       00 00 00 00     nop
 # REVERSE-NEXT:    20070:       0c 00 80 20     jal     131200 <__LA25Thunk_fpic>
 # REVERSE-NEXT:    20074:       00 00 00 00     nop
 # REVERSE-NEXT:    20078:       0c 00 80 28     jal     131232 <fnpic>
 # REVERSE-NEXT:    2007c:       00 00 00 00     nop
 # REVERSE: __LA25Thunk_fpic:
 # REVERSE-NEXT:    20080:       3c 19 00 02     lui     $25, 2
 # REVERSE-NEXT:    20084:       08 00 80 24     j       131216 <fpic>
 # REVERSE-NEXT:    20088:       27 39 00 90     addiu   $25, $25, 144
 # REVERSE-NEXT:    2008c:       00 00 00 00     nop
 # REVERSE: fpic:
 # REVERSE-NEXT:    20090:       00 00 00 00     nop
-# REVERSE-NEXT:    20094:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    20098:       ef ef ef ef     <unknown>
-# REVERSE-NEXT:    2009c:       ef ef ef ef     <unknown>
+# REVERSE-NEXT:    20094:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    20098:       ef ef ef ef     trap_ri
+# REVERSE-NEXT:    2009c:       ef ef ef ef     trap_ri
 # REVERSE: fnpic:
 # REVERSE-NEXT:    200a0:       00 00 00 00     nop
 
   .text
   .globl __start
 __start:
   jal foo1a
   jal foo2
   jal foo1b
   jal foo2
   jal fpic
   jal fnpic

llvm/lib/Target/Mips/MipsInstrFormats.td

class CACHEOP_FM<bits<6> op> : StdArch {
 
   bits<32> Inst;
 
   let Inst{31-26} = op;
   let Inst{25-21} = base;
   let Inst{20-16} = hint;
   let Inst{15-0}  = offset;
 }
+
+class TRAP_RI_FM<string asmstr> :
+    InstSE<(outs), (ins), asmstr, [], II_TRAP, FrmOther> {
+  bits<32> Inst;
+  let Inst{31-0} = 0xefefefef;
+}
+

llvm/lib/Target/Mips/MipsInstrInfo.td

 let AdditionalPredicates = [NotInMicroMips] in {
   /// Word Swap Bytes Within Halfwords
   def WSBH : MMRel, SubwordSwap<"wsbh", GPR32Opnd, II_WSBH>, SEB_FM<2, 0x20>,
              ISA_MIPS32R2;
 }
 
 /// No operation.
 def NOP : PseudoSE<(outs), (ins), []>, PseudoInstExpansion<(SLL ZERO, ZERO, 0)>;
+/// Reserved instruction trap (use as padding by LLD)
+def TRAP_RI : TRAP_RI_FM<"trap_ri">;
 
 // FrameIndexes are legalized when they are operands from load/store
 // instructions. The same not happens for stack address copies, so an
 // add op with mem ComplexPattern is used and the stack address copy
 // can be matched. It's similar to Sparc LEA_ADDRi
 def LEA_ADDiu : MMRel, EffectiveAddress<"addiu", GPR32Opnd>, LW_FM<9>;
 
 // MADD*/MSUB*

llvm/test/MC/Mips/disas-lld-trap-padding.s

+# RUN: echo "0xef 0xef 0xef 0xef" | llvm-mc -disassemble -triple=mips64-unknown-linux-gnu | FileCheck %s
+# CHECK: .text
+# CHECK-NEXT: trap_ri
+