[PATCH 1/2] x86: refine special casing of insns with MSR as immediate

[H.J. Lu]

On Thu, Apr 9, 2026 at 2:47 PM Jan Beulich <jbeulich@suse.com> wrote:
>
> PR gas/34028
>
> i.op[].imms is legitimate to de-reference only when the operand actually
> is an immediate. The pointer happens to be non-NULL for most other operand
> kinds (i.e. a deref is UB, but would likely not fault), just not for
> memory operands without displacement.
>
> Leverage that the to-be-special-cased insns all only have just a single
> immediate, and leverage further that after the immediately preceding
> swapping of operands valid immediates will come first. Hence the
> conditional can be adjusted to satisfy the criteria above, and no loop is
> needed at all.
>
> With the conditional changed, leverage the property also in optimize_imm()
> itself, reducing the number of loop iterations.
>
> --- a/gas/config/tc-i386.c
> +++ b/gas/config/tc-i386.c
> @@ -7303,7 +7303,9 @@ i386_assemble (char *line)
>           && i.imm_operands && i.operands > i.imm_operands))
>        swap_2_operands (0, 1);
>
> -  if (i.imm_operands)
> +  /* All legitimate immediates are placed first now.  Others, if any, will be
> +     rejected by match_template() anyway.  */
> +  if (operand_type_check (i.types[0], imm))
>      {
>        /* For USER_MSR and MSR_IMM instructions, imm32 stands for the name of a
>          model specific register (MSR). That's an unsigned quantity, whereas all
> @@ -7313,10 +7315,7 @@ i386_assemble (char *line)
>        if (is_cpu(current_templates.start, CpuUSER_MSR)
>           || t->mnem_off == MN_rdmsr
>           || t->mnem_off == MN_wrmsrns)
> -       {
> -         for (j = 0; j < i.imm_operands; j++)
> -           i.types[j] = smallest_imm_type (i.op[j].imms->X_add_number);
> -       }
> +       i.types[0] = smallest_imm_type (i.op[0].imms->X_add_number);
>        else
>         optimize_imm ();
>      }
> @@ -8520,7 +8519,7 @@ optimize_imm (void)
>                && current_templates.start->mnem_off != MN_jmpabs))
>      guess_suffix = LONG_MNEM_SUFFIX;
>
> -  for (op = i.operands; --op >= 0;)
> +  for (op = i.imm_operands; --op >= 0;)
>      if (operand_type_check (i.types[op], imm))
>        {
>         switch (i.op[op].imms->X_op)
> --- a/gas/testsuite/gas/i386/x86-64-msr_imm-inval.l
> +++ b/gas/testsuite/gas/i386/x86-64-msr_imm-inval.l
> @@ -1,5 +1,7 @@
>  .* Assembler messages:
>  .*:5: Error: operand type mismatch for `rdmsr'
>  .*:6: Error: operand type mismatch for `rdmsr'
> -.*:7: Error: operand type mismatch for `wrmsrns'
> +.*:7: Error: operand type mismatch for `rdmsr'
>  .*:8: Error: operand type mismatch for `wrmsrns'
> +.*:9: Error: operand type mismatch for `wrmsrns'
> +.*:10: Error: operand type mismatch for `wrmsrns'
> --- a/gas/testsuite/gas/i386/x86-64-msr_imm-inval.s
> +++ b/gas/testsuite/gas/i386/x86-64-msr_imm-inval.s
> @@ -4,5 +4,7 @@
>  _start:
>         rdmsr  $5151515151515151, %r12
>         rdmsr  $-515151, %r12
> +       rdmsr  (%rax), $0
>         wrmsrns  %r12, $5151515151515151
>         wrmsrns  %r12, $-515151
> +       wrmsrns  $0, (%rax)

PR gas/34028 test crashed in 32-bit mode.   Please add a 32-bit test
to verify that it no longer happens.

> --- a/gas/testsuite/gas/i386/x86-64-user_msr-inval.l
> +++ b/gas/testsuite/gas/i386/x86-64-user_msr-inval.l
> @@ -3,7 +3,9 @@
>  .*:6: Error: operand type mismatch for `urdmsr'
>  .*:7: Error: operand type mismatch for `urdmsr'
>  .*:8: Error: operand type mismatch for `urdmsr'
> -.*:9: Error: operand type mismatch for `uwrmsr'
> +.*:9: Error: operand type mismatch for `urdmsr'
>  .*:10: Error: operand type mismatch for `uwrmsr'
>  .*:11: Error: operand type mismatch for `uwrmsr'
>  .*:12: Error: operand type mismatch for `uwrmsr'
> +.*:13: Error: operand type mismatch for `uwrmsr'
> +.*:14: Error: operand type mismatch for `uwrmsr'
> --- a/gas/testsuite/gas/i386/x86-64-user_msr-inval.s
> +++ b/gas/testsuite/gas/i386/x86-64-user_msr-inval.s
> @@ -6,7 +6,9 @@ _start:
>         urdmsr  $-32767, %r14
>         urdmsr  $-2147483648, %r14
>         urdmsr  $0x7fffffffffffffff, %r14
> +       urdmsr  (%rax), $0
>         uwrmsr  %r12, $-1
>         uwrmsr  %r12, $-32767
>         uwrmsr  %r12, $-2147483648
>         uwrmsr  %r12, $0x7fffffffffffffff
> +       uwrmsr  $0, (%rax)
>


-- 
H.J.