Re: What registers are protected by "cc" in a clobberlist when using inline assembly?

[Jeffrey Walton <noloader at gmail dot com>]

On Tue, Feb 4, 2014 at 4:55 PM, Ian Lance Taylor <iant@google.com> wrote:
> On Tue, Feb 4, 2014 at 1:16 PM, Jeffrey Walton <noloader@gmail.com> wrote:
>>
>> I'm having trouble locating information on the register(s) protected
>> by adding "cc" to a clobber list.
>>
>> Can anyone confirm (1) FLAGS/EFLAGS on x86/c64, and (2) CPSR on ARM?
>>
>> The reason I ask is I came across some code that sets the Carry Flag
>> (CF) on success, but "cc" was not specified in a clobber list:
>>
>>     char rc;
>>     unsigned int val;
>>
>>     __asm__ volatile(
>>         "rdrand %0 ; setc %1"
>>         : "=r" (val), "=qm" (rc)
>>     );
>>
>>     // 1 = success, 0 = underflow
>>     if(rc) {
>>         // use val
>>         ...
>>     }
>>
>> So I'm trying to understand why "cc" was not specified.
>
> I'm not sure that explicitly clobbering "cc" has any effect on x86.
> For x86 GCC assumes that all asm statements clobber the flags
> register.  See ix86_md_asm_clobbers in gcc/config/i386/i386.c.
>
> For ARM "cc" represents the internal representation of the condition
> code flags.  It doesn't mean a specific register, but clobbering it
> will mean that GCC drops any information it is carrying about
> condition flags.  Internally this is represented as a pseudo-register,
> CC_REGNUM.
Thanks Ian.

That's basically what others have told me without the reference to
ix86_md_asm_clobber.

Perhaps that would be a good topic to add to
http://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html.

Jeff