Re: [PATCH 17/18] target/i386/tcg: add a CCOp for SBB x,x

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/11/25 20:11, Richard Henderson wrote:
> On 12/10/25 07:16, Paolo Bonzini wrote:
>> This is more efficient both when generating code and when testing
>> flags.
> 
> I guess sbb x,x appears quite frequently in x86 setcc computation, and 
> the testing of the flags is much less important than the straight line 
> code generation?

Yes.  And to be honest, in the most common idioms generated for a modern 
processor the whole computation ends up being dead, so it doesn't really 
matter to have this vs. CC_OP_SBB or CC_OP_SUB.  For example memcmp uses 
it for "(x < y) ? -1 : 1":

                  subq     %rcx, %rax
                  sbbl     %eax, %eax
                  orl      $1, %eax

and this is also common, for "(x < y) ? VALUE : 0"

                  subq     %rcx, %rax
                  sbbq     %rax, %rax     ; could be sbbl :)
		 andl     $0x1234, %eax

In old hand-written assembly it is used more creatively, and having 
simpler generated code can matter if there are memory operations after 
the sbb.  I did this just because it's silly to compute both negsetcond 
and setcond...

Paolo