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Hi everyone,
I was playing around a bit with gem5 and noticed by accident that the
O3 CPU model seems to be unable to handle jumps to the same location
(at least, that's what it looks like).
I have the following piece of x86 assembly code:
_start:
jmp _start
mov $0xDEADBEEF, %rax
mov $0xDEADBEEF, %rbx
When I execute that in FS mode with Exec tracing, I get:
355000: system.cpu T0 : @_start. 0 : JMP_I : rdip t1, %ctrl153,
: IntAlu : D=0x0000000000240005
355000: system.cpu T0 : @_start.1 : JMP_I : limm t2,
0xfffffffffffffffb : IntAlu : D=0xfffffffffffffffb
355000: system.cpu T0 : @_start.2 : JMP_I : wrip , t1, t2 :
IntAlu :
359500: system.cpu T0 : @_start.0 : JMP_I : rdip t1, %ctrl153,
: IntAlu : D=0x0000000000240008
359500: system.cpu T0 : @_start.1 : JMP_I : limm t2,
0xfffffffffffffffb : IntAlu : D=0xfffffffffffffffb
359500: system.cpu T0 : @_start.2 : JMP_I : wrip , t1, t2 :
IntAlu :
365000: system.cpu T0 : @_start+3.0 : UD2 : fault Invalid-Opcode
: No_OpClass :
376500: system.cpu T0 : @_start+3.32768 : Microcode_ROM : slli
t4, t1, 0x4 : IntAlu : D=0x0000000000000060
376500: system.cpu T0 : @_start+3.32769 : Microcode_ROM : ld t2,
IDTR:[t4 + 0x8] : MemRead : D=0x0000000000000000 A=0x68
376500: system.cpu T0 : @_start+3.32770 : Microcode_ROM : ld t4,
IDTR:[t4] : MemRead : D=0x0000000000000000 A=0x60
[...]
What happens exactly after the jump, depends on the instructions
behind it. To "fix" that problem, it is sufficient to e.g. add a nop
in the loop:
_start:
nop
jmp _start
Now everything works fine. Note that the problem does not occur with
the timing CPU model.
Best regards,
Nils
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