gas dependency checker bug [forwarded message from Xavier Leroy]

Attached below is a bug report for gas. I did verify that gas indeed fails to detect the RAW hazard on r2 in the sample code.

	--david

Hello,

I'm taking the liberty to contact you directly, as you seem to be the main author of the IA64 port of the GNU assembler. If I should have used some mailing list instead, please accept my apologies.

While retargeting the Objective Caml compiler for the IA64, I've found what looks like two bugs in the way "as" record dependencies between instructions.

The problems were observed with version 2.9-ia64-000216-final of the GNU assembler, but seem to be still there in the working sources available from Cygnus' CVS repository.

1- "addl" vs. "adds"

Consider:

.global f#
.proc f#
f:
mov r2 = r32
addl r3 = 12345, r2
br.ret.sptk b0

.endp f#

Once assembled in -xauto -xdebug mode, we get:

[xleroy@tl2 xleroy]$ as -xauto -xdebug deps.s Checking mov for violations (line 4, 8/2) Registering 'mov' resource usage
Adding RAW 'GR%, % in 1 - 127' (2)
Adding WAW 'GR%, % in 1 - 127' (2)
Checking addl for violations (line 5, 8/2) Registering 'addl' resource usage
Adding RAW 'GR%, % in 1 - 127' (3)
Adding WAW 'GR%, % in 1 - 127' (3)
Checking br.ret.sptk for violations (line 7, 27/13) Registering 'br.ret.sptk' resource usage
Clearing register values
Insn group break (w/stop)
Insn group break (w/stop)
Insn group break (w/stop)
Insn group break (w/stop)
[xleroy@tl2 xleroy]$ objdump -d a.out

a.out: file format elf64-ia64-little

Disassembly of section .text:

0000000000000000 <f>:

   0:   11 10 00 40 00 21       [MIB]       mov r2=r32
   6:   30 c8 09 c0 48 80                   addl r3=12345,r2
   c:   00 00 84 00                         br.ret.sptk.few b0;;

This is incorrect, because the "addl" depends on the previous "mov", so both instructions cannot be executed in parallel.

However, if the "addl" is replaced by an "adds", everything works as expected:

.global f#
.proc f#
f:
mov r2 = r32
adds r3 = 123, r2
br.ret.sptk b0

.endp f#

[xleroy@tl2 xleroy]$ as -xauto -xdebug deps.s
Checking mov for violations (line 4, 8/2)
Registering 'mov' resource usage
Adding RAW 'GR%, % in 1 - 127' (2)
Adding WAW 'GR%, % in 1 - 127' (2)
Checking adds for violations (line 6, 8/2)
Use of 'adds' violates RAW dependency 'GR%, % in 1 - 127' (impliedf), specific resource number is 2 @ deps.s:4 Inserting stop
Insn group break (w/stop)
Removing RAW 'GR%, % in 1 - 127' (2)
Removing WAW 'GR%, % in 1 - 127' (2)
Registering 'adds' resource usage
Adding RAW 'GR%, % in 1 - 127' (3)
Adding WAW 'GR%, % in 1 - 127' (3)
Checking br.ret.sptk for violations (line 7, 27/13)
Registering 'br.ret.sptk' resource usage
Clearing register values
Insn group break (w/stop)
Insn group break (w/stop)
Insn group break (w/stop)
Insn group break (w/stop)
[xleroy@tl2 xleroy]$ objdump -d a.out

a.out: file format elf64-ia64-little

Disassembly of section .text:

0000000000000000 <f>:

   0:   0a 10 00 40 00 21       [MMI]       mov r2=r32;;
   6:   30 d8 0b 00 42 00                   adds r3=123,r2
   c:   00 00 04 00                         nop.i 0x0
  10:   11 00 00 00 01 00       [MIB]       nop.m 0x0
  16:   00 00 00 02 00 80                   nop.i 0x0
  1c:   00 00 84 00                         br.ret.sptk.few b0;;

2- Loads and stores with postincrement.

A similar issue (missing dependencies) occur with the post-increment forms of loads and stores. Consider:

.global f#
.proc f#
f:
ld8 r2 = [r32], 8
mov r8 = r32
br.ret.sptk b0

.endp f#

[xleroy@tl2 xleroy]$ as -xdebug -xauto postinc.s Checking ld8 for violations (line 4, 32/4) Registering 'ld8' resource usage
Adding RAW 'GR%, % in 1 - 127' (2)
Adding WAW 'GR%, % in 1 - 127' (2)
Checking mov for violations (line 5, 8/2) Registering 'mov' resource usage
Adding RAW 'GR%, % in 1 - 127' (8)
Adding WAW 'GR%, % in 1 - 127' (8)
Checking br.ret.sptk for violations (line 6, 27/13) Registering 'br.ret.sptk' resource usage
Clearing register values
Insn group break (w/stop)
Insn group break (w/stop)
Insn group break (w/stop)
Insn group break (w/stop)

[xleroy@tl2 xleroy]$ objdump -d a.out

a.out: file format elf64-ia64-little

Disassembly of section .text:

0000000000000000 <f>:

   0:   11 10 20 40 18 14       [MIB]       ld8 r2=[r32],8
   6:   80 00 80 00 42 80                   mov r8=r32
   c:   00 00 84 00                         br.ret.sptk.few b0;;

Again, I believe it's incorrect to execute the ld8 and the mov in parallel, because the mov should get the updated value of r32 (after the post-increment was performed).

The problem with postincrement isn't too serious for my application (I can avoid generating postincremented loads and stores), but the first problem with addl is a real show-stopper, due to code of the form

<restore gp after a call>
addl <tempreg> = @ltoff(some_symbol#), gp

which gets incorrectly scheduled with -xauto. So, any help you could provide will be most appreciated!

Best regards,

- Xavier Leroy