On 16/09/14 23:03, James Greenhalgh wrote: > On Tue, Sep 16, 2014 at 10:36:08PM +0100, Andrew Pinski wrote: >> On Thu, Sep 4, 2014 at 1:18 AM, James Greenhalgh >> <james.greenha...@arm.com> wrote: >>> On Thu, Sep 04, 2014 at 08:42:31AM +0100, Venkataramanan Kumar wrote: >>>> Hi maintainers, >>>> >>>> I just added "=r" and retested it. >>> >>> I had a very similar patch to this sitting in my local tree. However, >>> I am surprised you have left operand 3 as an output operand. In my tree >>> I had marked operand 3 as "&r". >>> >>> What do you think? >> >> The clobber needs to be "=&r" as you are writing to the register and >> not just reading from it. I think this is causing some issues >> including linaro bugzilla #667 >> (https://bugs.linaro.org/show_bug.cgi?id=667). > > (+CC Matthias Klose and Steve McIntyre who have also been in contact with me > regarding this bug) > > I've seen this bug locally, and had considered sending the patch you > suggested, which does indeed fix the bug. However, it feels wrong as > the operand is not a formal output of the pattern. It is clobbered - and > indeed earlyclobbered - so yes it is written to, but it isn't an output. > This makes the fix look like a band-aid around the real problem. > > The bug looks similar to pr52573 - regrename fails to spot that it should > not rename to a register used in an earlyclobber operand of any type, rather > than just an output+earlyclobber operand as it does now. > > I've played about with a fix that sits in regrename, and forces it to think > of all earlyclobber operands as starting and ending chains but this didn't > bootstrap clean - we end up with what I believe are false reports of stack > smashing in libstdc++. > > I was planning to look again at my approach tomorrow, I would like to > convince myself that this isn't a deficiency in regrename before I would > support just marking this operand "=&r". > > If you have any other suggestions, or if "=&r" is actually correct and > I am misreading the documentation please let me know. > > Thanks, > James >
"=&r" is correct for an early-clobbered scratch. R.
