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Re: combination of read/write and earlyclobber constraint modifier
- From: Jeff Law <law at redhat dot com>
- To: gcc at gcc dot gnu dot org
- Cc: Tom de Vries <Tom_deVries at mentor dot com>, Vladimir Makarov <vmakarov at redhat dot com>
- Date: Wed, 02 Jul 2014 09:49:53 -0600
- Subject: Re: combination of read/write and earlyclobber constraint modifier
- Authentication-results: sourceware.org; auth=none
- References: <53B30B96 dot 2000603 at mentor dot com> <53B31041 dot 8060608 at redhat dot com> <alpine dot DEB dot 2 dot 10 dot 1407012155000 dot 2640 at laptop-mg dot saclay dot inria dot fr>
On 07/01/14 13:58, Marc Glisse wrote:
Nothing like making a statement on the internet to bring out folks more
"in the know" to refute what was said :-)
I don't think we can define any reasonable semantics for &+. My
recommendation would be for this to be considered a hard error.
Uh? The doc explicitly says "An input operand can be tied to an
earlyclobber operand" and goes on to explain why that is useful. It
avoids using the same register for other input when they are identical.
Reviewing that paragraph, it does sound vaguely familiar. So the
backend developer explicitly ties the input to an earlyclobbered output
and the other inputs are marked as conflicting.
I guess I can see how that is helpful, though I wonder how often it is
helpful in practice.
Regardless, I stand corrected :-)