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Re: inline asm clobbers
- From: Ian Lance Taylor <iant at google dot com>
- To: Paul_Koning at dell dot com
- Cc: David Wohlferd <dw at limegreensocks dot com>, GCC Development <gcc at gcc dot gnu dot org>
- Date: Wed, 11 Mar 2015 17:04:29 -0700
- Subject: Re: inline asm clobbers
- Authentication-results: sourceware.org; auth=none
- References: <5500C898 dot 3000601 at LimeGreenSocks dot com> <CAKOQZ8yG4fPx8Ce3JiZMtW8MwXAMFk5W3_JAJ=b2WMQ4mXDDdA at mail dot gmail dot com> <7DD652D7-C369-4E2D-AEA0-9C75ABAB8636 at dell dot com>
On Wed, Mar 11, 2015 at 4:41 PM, <Paul_Koning@dell.com> wrote:
>> On Mar 11, 2015, at 7:19 PM, Ian Lance Taylor <email@example.com> wrote:
>> On Wed, Mar 11, 2015 at 3:58 PM, David Wohlferd <firstname.lastname@example.org> wrote:
>>> Why does gcc allow you to specify clobbers using numbers:
>>> asm ("" : : "r" (var) : "0"); // i386: clobbers eax
>>> How is this better than using register names?
>>> This makes even less sense when you realize that (apparently) the indices of
>>> registers aren't fixed. Which means there is no way to know which register
>>> you have clobbered in order to use it in the template.
>>> Having just seen someone trying (unsuccessfully) to use this, it seems like
>>> there is no practical way you can.
>>> Which makes me wonder why it's there. And whether it still should be.
>> I don't know why it works. It should be consistent, though. It's
>> simply GCC's internal hard register number, which doesn't normally
>> I would agree that one should avoid it. I'd be wary of removing it
>> from GCC at this point since it might break working code.
> It certainly would. Itâs not all that common, but I have seen this done in production code. Come to think of it, this certainly makes sense in machines where some instructions act on fixed registers.
> Register names would be nice as an additional capability.
Register names are supported.