Re: [Qemu-devel] [PATCH 05/14] target-i386: Enable control registers for MPX

[Richard Henderson <rth@twiddle.net>]

On 02/10/2016 12:28 AM, Paolo Bonzini wrote:
> On 09/07/2015 10:17, Richard Henderson wrote:
>> +    /* Disallow enabling only half of MPX.  */
>> +    if ((mask ^ (mask * (XSTATE_BNDCSR / XSTATE_BNDREGS))) & XSTATE_BNDCSR) {
>
> I'm refreshing patches 1-4 to add PKE support, and this caught my eye...
>
> What about just
>
> 	if (!!(mask & XSTATE_BNDCSR) != !!(mask & XSTATE_BNDREGS))
>
> ?

Hmph.  Missed optimization for your version.
It is a bit more readable though, I admit...


r~



void bar();
void foo(unsigned mask)
{
   if ((mask ^ (mask * 4)) & 8) {
     bar();
   }
}
void baz(unsigned mask)
{
   if (!!(mask & 8) != !!(mask & 2)) {
     bar();
   }
}


foo:
         leal    0(,%rdi,4), %eax
         xorl    %eax, %edi
         andl    $8, %edi
         jne     .L4
	...
baz:
         movl    %edi, %eax
         shrl    %edi
         shrl    $3, %eax
         andl    $1, %edi
         andl    $1, %eax
         cmpb    %dil, %al
         je      .L5
	...
clang_baz:
         movl    %edi, %eax
         shrl    $3, %eax
         shrl    %edi
         xorl    %eax, %edi
         testb   $1, %dil
         jne     .LBB1_2