Re: [Qemu-devel] x86: cvtsi2s{s,d} etc. array access

[Blue Swirl]

On Mon, May 14, 2012 at 9:05 PM, Blue Swirl <address@hidden> wrote:
> Hi,
>
> While working on the AREG0 patches, I noticed strange code in
> target-i386/translate.c.
>
> We have this table of function pointers:
> static void *sse_op_table3[4 * 3] = {
>    gen_helper_cvtsi2ss,
>    gen_helper_cvtsi2sd,
>    X86_64_ONLY(gen_helper_cvtsq2ss),
>    X86_64_ONLY(gen_helper_cvtsq2sd),
>
>    gen_helper_cvttss2si,
>    gen_helper_cvttsd2si,
>    X86_64_ONLY(gen_helper_cvttss2sq),
>    X86_64_ONLY(gen_helper_cvttsd2sq),
>
>    gen_helper_cvtss2si,
>    gen_helper_cvtsd2si,
>    X86_64_ONLY(gen_helper_cvtss2sq),
>    X86_64_ONLY(gen_helper_cvtsd2sq),
> };
>
> It's accessed like this (line 3537):
>            sse_op2 = sse_op_table3[(s->dflag == 2) * 2 + ((b >> 8) - 2)];
>
> b >> 8 can be only either 1 or 0. I don't see how this can work, won't
> the array index become negative for s->dflag != 2?
>
> The other access is as follows (line 3594):
>            sse_op2 = sse_op_table3[(s->dflag == 2) * 2 + ((b >> 8) - 2) + 4 +
>                                    (b & 1) * 4];
>
> This looks better because of + 4 but I think some array values are not
> accessible (max. 1 * 2 + (1 - 2) + 4 + 1 * 4 == 9).

I still don't understand the arithmetic, but it looks like the correct
helpers are called:

$ cat cvtsi2ss.c
int main(void)
{
    asm("cvtsi2ss %eax, %xmm0;");
    asm("cvtsi2sd %eax, %xmm0;");
#ifdef __amd64__
    asm("cvtsi2ss %rax, %xmm0;");
    asm("cvtsi2sd %rax, %xmm0;");
#endif

    asm("cvttss2si %xmm0, %eax;");
    asm("cvttsd2si %xmm0, %eax;");
#ifdef __amd64__
    asm("cvttss2si %xmm0, %rax;");
    asm("cvttsd2si %xmm0, %rax;");
#endif

    asm("cvtss2si %xmm0, %eax;");
    asm("cvtsd2si %xmm0, %eax;");
#ifdef __amd64__
    asm("cvtss2si %xmm0, %rax;");
    asm("cvtsd2si %xmm0, %rax;");
#endif

    return 0;
}

$ gcc -o cvtsi2ss cvtsi2ss.c
$ gcc -m32 -o cvtsi2ss.i386 cvtsi2ss.c

$ qemu-x86_64 -d in_asm,op_opt ./cvtsi2ss
IN: main
0x0000000000400494:  push   %rbp
0x0000000000400495:  mov    %rsp,%rbp
0x0000000000400498:  cvtsi2ss %eax,%xmm0
0x000000000040049c:  cvtsi2sd %eax,%xmm0
0x00000000004004a0:  cvtsi2ssq %rax,%xmm0
0x00000000004004a5:  cvtsi2sdq %rax,%xmm0
0x00000000004004aa:  cvttss2si %xmm0,%eax
0x00000000004004ae:  cvttsd2si %xmm0,%eax
0x00000000004004b2:  cvttss2siq %xmm0,%rax
0x00000000004004b7:  cvttsd2siq %xmm0,%rax
0x00000000004004bc:  cvtss2si %xmm0,%eax
0x00000000004004c0:  cvtsd2si %xmm0,%eax
0x00000000004004c4:  cvtss2siq %xmm0,%rax
0x00000000004004c9:  cvtsd2siq %xmm0,%rax
0x00000000004004ce:  mov    $0x0,%eax
0x00000000004004d3:  leaveq
0x00000000004004d4:  retq

OP after liveness analysis:
 mov_i64 tmp0,rbp
 mov_i64 tmp2,rsp
 movi_i64 tmp12,$0xfffffffffffffff8
 add_i64 tmp2,tmp2,tmp12
 qemu_st64 tmp0,tmp2,$0xffffffffffffffff
 mov_i64 rsp,tmp2
 mov_i64 tmp0,rsp
 mov_i64 rbp,tmp0
 mov_i64 tmp0,rax
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 mov_i32 tmp6,tmp0
 movi_i64 tmp12,$cvtsi2ss
 call tmp12,$0x0,$0,tmp10,tmp6
 mov_i64 tmp0,rax
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 mov_i32 tmp6,tmp0
 movi_i64 tmp12,$cvtsi2sd
 call tmp12,$0x0,$0,tmp10,tmp6
 mov_i64 tmp0,rax
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvtsq2ss
 call tmp12,$0x0,$0,tmp10,tmp0
 mov_i64 tmp0,rax
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvtsq2sd
 call tmp12,$0x0,$0,tmp10,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvttss2si
 call tmp12,$0x0,$1,tmp6,tmp10
 ext32u_i64 tmp0,tmp6
 ext32u_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvttsd2si
 call tmp12,$0x0,$1,tmp6,tmp10
 ext32u_i64 tmp0,tmp6
 ext32u_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvttss2sq
 call tmp12,$0x0,$1,tmp0,tmp10
 mov_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvttsd2sq
 call tmp12,$0x0,$1,tmp0,tmp10
 mov_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvtss2si
 call tmp12,$0x0,$1,tmp6,tmp10
 ext32u_i64 tmp0,tmp6
 ext32u_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvtsd2si
 call tmp12,$0x0,$1,tmp6,tmp10
 ext32u_i64 tmp0,tmp6
 ext32u_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvtss2sq
 call tmp12,$0x0,$1,tmp0,tmp10
 mov_i64 rax,tmp0
 movi_i64 tmp12,$0x2a8
 add_i64 tmp10,env,tmp12
 movi_i64 tmp12,$cvtsd2sq

$ qemu-i386 -d in_asm,op_opt ./cvtsi2ss.i386
$ grep -B3 -A29 cvtsi2ss /tmp/qemu.log
IN: main
0x08048394:  push   %ebp
0x08048395:  mov    %esp,%ebp
0x08048397:  cvtsi2ss %eax,%xmm0
0x0804839b:  cvtsi2sd %eax,%xmm0
0x0804839f:  cvttss2si %xmm0,%eax
0x080483a3:  cvttsd2si %xmm0,%eax
0x080483a7:  cvtss2si %xmm0,%eax
0x080483ab:  cvtsd2si %xmm0,%eax
0x080483af:  mov    $0x0,%eax
0x080483b4:  pop    %ebp
0x080483b5:  ret

OP after liveness analysis:
 mov_i32 tmp0,ebp
 mov_i32 tmp2,esp
 movi_i32 tmp12,$0xfffffffc
 add_i32 tmp2,tmp2,tmp12
 qemu_st32 tmp0,tmp2,$0xffffffffffffffff
 mov_i32 esp,tmp2
 mov_i32 tmp0,esp
 mov_i32 ebp,tmp0
 mov_i32 tmp0,eax
 movi_i64 tmp13,$0x1d8
 add_i64 tmp10,env,tmp13
 mov_i32 tmp6,tmp0
 movi_i64 tmp13,$cvtsi2ss
 call tmp13,$0x0,$0,tmp10,tmp6
 mov_i32 tmp0,eax
 movi_i64 tmp13,$0x1d8
 add_i64 tmp10,env,tmp13
 mov_i32 tmp6,tmp0
 movi_i64 tmp13,$cvtsi2sd
 call tmp13,$0x0,$0,tmp10,tmp6
 movi_i64 tmp13,$0x1d8
 add_i64 tmp10,env,tmp13
 movi_i64 tmp13,$cvttss2si
 call tmp13,$0x0,$1,tmp6,tmp10
 nopn $0x2,$0x2
 mov_i32 eax,tmp6
 movi_i64 tmp13,$0x1d8
 add_i64 tmp10,env,tmp13
 movi_i64 tmp13,$cvttsd2si
 call tmp13,$0x0,$1,tmp6,tmp10
 nopn $0x2,$0x2
 mov_i32 eax,tmp6
 movi_i64 tmp13,$0x1d8
 add_i64 tmp10,env,tmp13
 movi_i64 tmp13,$cvtss2si
 call tmp13,$0x0,$1,tmp6,tmp10
 nopn $0x2,$0x2
 mov_i32 eax,tmp6
 movi_i64 tmp13,$0x1d8
 add_i64 tmp10,env,tmp13
 movi_i64 tmp13,$cvtsd2si
 call tmp13,$0x0,$1,tmp6,tmp10