Re: [PATCH, v2] x86-64: correct segment override prefix generation

["H.J. Lu" <hjl.tools@gmail.com>]

On Tue, Aug 7, 2012 at 3:32 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>> On 30.07.12 at 19:03, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>> Please provide a testcase to show the correct behavior.
>
> Here you go.
>
> Jan
>
> Despite them being ignored by the CPU, gas issues segment override
> prefixes for other than FS/GS in 64-bit mode. If doing so at all, it
> should clearly do this correctly. Determining the default segment,
> however, requires to take into consideration RegRex (so far, RSP, RBP,
> R12, and R13 were all treated equally here).
>
> gas/
> 2012-08-07  Jan Beulich <jbeulich@suse.com>
>         * config/tc-i386-intel.c (build_modrm_byte): Split determining
>         default segment from figuring out encoding. Honor RegRex for
>         the former.
>
> gas/testsuite/
> 2012-08-07  Jan Beulich <jbeulich@suse.com>
>
>         * gas/i386/x86-64-segovr.{s,l}: New.
>         * gas/i386/i386.exp: Run new test.
>
> --- 2012-08-07/gas/config/tc-i386.c     2012-07-31 09:45:03.000000000 +0200
> +++ 2012-08-07/gas/config/tc-i386.c     2012-08-07 12:13:39.000000000 +0200
> @@ -5729,18 +5729,14 @@ build_modrm_byte (void)
>               i.sib.base = i.base_reg->reg_num;
>               /* x86-64 ignores REX prefix bit here to avoid decoder
>                  complications.  */
> -             if ((i.base_reg->reg_num & 7) == EBP_REG_NUM)
> -               {
> +             if (!(i.base_reg->reg_flags & RegRex)
> +                 && (i.base_reg->reg_num == EBP_REG_NUM
> +                  || i.base_reg->reg_num == ESP_REG_NUM))
>                   default_seg = &ss;
> -                 if (i.disp_operands == 0)
> -                   {
> -                     fake_zero_displacement = 1;
> -                     i.types[op].bitfield.disp8 = 1;
> -                   }
> -               }
> -             else if (i.base_reg->reg_num == ESP_REG_NUM)
> +             if (i.base_reg->reg_num == 5 && i.disp_operands == 0)

Please use EBP_REG_NUM instead 5 here.

>                 {
> -                 default_seg = &ss;
> +                 fake_zero_displacement = 1;
> +                 i.types[op].bitfield.disp8 = 1;
>                 }
>               i.sib.scale = i.log2_scale_factor;
>               if (i.index_reg == 0)
> --- 2012-08-07/gas/testsuite/gas/i386/i386.exp  2012-07-24 14:52:59.000000000 +0200
> +++ 2012-08-07/gas/testsuite/gas/i386/i386.exp  2012-08-07 12:13:39.000000000 +0200
> @@ -311,6 +311,7 @@ if [expr ([istarget "i*86-*-*"] || [ista
>      run_dump_test "x86-64-stack-suffix"
>      run_list_test "x86-64-inval" "-al"
>      run_list_test "x86-64-segment" "-al"
> +    run_dump_test "x86-64-segovr"
>      run_list_test "x86-64-inval-seg" "-al"
>      run_dump_test "x86-64-branch"
>      run_dump_test "x86-64-relax-1"
> --- 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.d   1970-01-01 01:00:00.000000000 +0100
> +++ 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.d   2012-08-03 14:06:44.000000000 +0200
> @@ -0,0 +1,41 @@
> +#objdump: -dw
> +#name: x86-64 segment overrides
> +
> +.*: +file format .*
> +
> +Disassembly of section .text:
> +
> +0+ <segovr>:
> +[      ]*[a-f0-9]+:    8b 00[  ]+mov[  ]+\(%rax\),%eax
> +[      ]*[a-f0-9]+:    8b 01[  ]+mov[  ]+\(%rcx\),%eax
> +[      ]*[a-f0-9]+:    8b 02[  ]+mov[  ]+\(%rdx\),%eax
> +[      ]*[a-f0-9]+:    8b 03[  ]+mov[  ]+\(%rbx\),%eax
> +[      ]*[a-f0-9]+:    3e 8b 04 24[    ]+mov[  ]+%ds:\(%rsp\),%eax
> +[      ]*[a-f0-9]+:    3e 8b 45 00[    ]+mov[  ]+%ds:((0x)?0)?\(%rbp\),%eax
> +[      ]*[a-f0-9]+:    8b 06[  ]+mov[  ]+\(%rsi\),%eax
> +[      ]*[a-f0-9]+:    8b 07[  ]+mov[  ]+\(%rdi\),%eax
> +[      ]*[a-f0-9]+:    41 8b 00[       ]+mov[  ]+\(%r8\),%eax
> +[      ]*[a-f0-9]+:    41 8b 01[       ]+mov[  ]+\(%r9\),%eax
> +[      ]*[a-f0-9]+:    41 8b 02[       ]+mov[  ]+\(%r10\),%eax
> +[      ]*[a-f0-9]+:    41 8b 03[       ]+mov[  ]+\(%r11\),%eax
> +[      ]*[a-f0-9]+:    41 8b 04 24[    ]+mov[  ]+\(%r12\),%eax
> +[      ]*[a-f0-9]+:    41 8b 45 00[    ]+mov[  ]+((0x)?0)?\(%r13\),%eax
> +[      ]*[a-f0-9]+:    41 8b 06[       ]+mov[  ]+\(%r14\),%eax
> +[      ]*[a-f0-9]+:    41 8b 07[       ]+mov[  ]+\(%r15\),%eax
> +[      ]*[a-f0-9]+:    36 8b 00[       ]+mov[  ]+%ss:\(%rax\),%eax
> +[      ]*[a-f0-9]+:    36 8b 01[       ]+mov[  ]+%ss:\(%rcx\),%eax
> +[      ]*[a-f0-9]+:    36 8b 02[       ]+mov[  ]+%ss:\(%rdx\),%eax
> +[      ]*[a-f0-9]+:    36 8b 03[       ]+mov[  ]+%ss:\(%rbx\),%eax
> +[      ]*[a-f0-9]+:    8b 04 24[       ]+mov[  ]+\(%rsp\),%eax
> +[      ]*[a-f0-9]+:    8b 45 00[       ]+mov[  ]+((0x)?0)?\(%rbp\),%eax
> +[      ]*[a-f0-9]+:    36 8b 06[       ]+mov[  ]+%ss:\(%rsi\),%eax
> +[      ]*[a-f0-9]+:    36 8b 07[       ]+mov[  ]+%ss:\(%rdi\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 00[    ]+mov[  ]+%ss:\(%r8\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 01[    ]+mov[  ]+%ss:\(%r9\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 02[    ]+mov[  ]+%ss:\(%r10\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 03[    ]+mov[  ]+%ss:\(%r11\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 04 24[         ]+mov[  ]+%ss:\(%r12\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 45 00[         ]+mov[  ]+%ss:((0x)?0)?\(%r13\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 06[    ]+mov[  ]+%ss:\(%r14\),%eax
> +[      ]*[a-f0-9]+:    36 41 8b 07[    ]+mov[  ]+%ss:\(%r15\),%eax
> +#pass
> --- 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.s   1970-01-01 01:00:00.000000000 +0100
> +++ 2012-08-07/gas/testsuite/gas/i386/x86-64-segovr.s   2012-08-03 13:54:08.000000000 +0200
> @@ -0,0 +1,9 @@
> +# 64bit segment overrides
> +
> +       .text
> +segovr:
> +.irp seg, ds, ss
> + .irp reg, ax, cx, dx, bx, sp, bp, si, di, 8, 9, 10, 11, 12, 13, 14, 15
> +       mov     %\seg:(%r\reg), %eax
> + .endr
> +.endr
>
>

OK with the EBP_REG_NUM change above if Linux x86-64 kernel
compiles and runs.

Thanks.

-- 
H.J.