[cfe-dev] Strange assembly behavior

[James Molloy]

Hi Vincent,

A jump to the next instruction is often a sideeffect of looking at a
non-relocated object with relocations to apply. Have you run this through
objdump with the -r flag (objdump -dr $FILE) and seen if there is a
relocation to apply to the jump?

I'm concerned about the lack of the code after MCCLEAN:, but it could be
that the debugger is just not showing it to you for some reason - I'd check
with objdump first as that never lies :)

Cheers,

James


On 11 March 2013 12:49, Vincent Habchi <vince at macports.org> wrote:

> Hi folks, gurus and experts –
>
> I am not sure this is the proper list to post to, but I try nevertheless.
> I am getting a strange .o output from an assembly file parsed by clang.
>
> From:
> .text
> .text
> .globl _ATL_UGEMV
> .align 6
> _ATL_UGEMV:
>
> […]
>       addpd %xmm0, %xmm1
>       movaps %xmm1, 112-128(%r9)
>
>       sub $-128, %r9
>       sub $-128, %rdx
>       sub $8*2, %rbx
>       jnz LOOPM
>
>       cmp $0, %rcx
>       jz MCLEAN
>
>       mov %rcx, %rbx
> LOOPMCU:
>       movsd -128(%r9), %xmm1
>       movsd -128(%rdx), %xmm0
> […]
>
>       dec %rbx
>       jnz LOOPMCU
>
> MCLEAN:
>       prefetchnta 12*8+64(%r8)
>       add $12*8, %r8
>       add %r15, %rdx
>       mov %r11, %r9
>       mov %rdi, %rbx
>       sub $12, %rsi
>       jnz LOOPN
>
>
>
>       movq -8(%rsp), %rbp
>       movq -16(%rsp), %rbx
>       movq -24(%rsp), %r12
>       movq -32(%rsp), %r13
>       movq -40(%rsp), %r14
>       movq -48(%rsp), %r15
>       ret
>
> I get this after ‘clang -x assembler’ (clang version 3.3 (trunk 173279)):
>
> […]
> dmvn_sse.o[0x6f8]:  addpd  %xmm0, %xmm1
> dmvn_sse.o[0x6fc]:  movaps %xmm1, -16(%r9)
> dmvn_sse.o[0x701]:  subq   $-128, %r9
> dmvn_sse.o[0x705]:  subq   $-128, %rdx
> dmvn_sse.o[0x709]:  subq   $16, %rbx
> dmvn_sse.o[0x70d]:  jne    0xc7                      ; ATL_UGEMV + 199
> dmvn_sse.o[0x713]:  cmpq   $0, %rcx
> dmvn_sse.o[0x717]:  je     0x71d                     ; ATL_UGEMV + 1821
> dmvn_sse.o[0x71d]:  movq   %rcx, %rbx
> dmvn_sse.o[0x720]:  movsd  -128(%r9), %xmm1
> dmvn_sse.o[0x726]:  movsd  -128(%rdx), %xmm0
> […]
> dmvn_sse.o[0x7e7]:  decq   %rbx
> dmvn_sse.o[0x7ea]:  jne    0x720                     ; ATL_UGEMV + 1824
>
> The ‘jz MCLEAN’ has been replaced by a jump to the next instruction, the
> code after MCLEAN: is discarded and does not even appear in the .o file (if
> I believe lldb), so that the function abruptly ends up at the ‘jnz
> LOOPMCPU’. Needless to say, this code fails to run. Could anybody tell me
> what’s wrong?
>
> Thanks a lot!
> Vincent
>
>
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