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Re: combine_simplify_rtx (doesn't) commute XOR and ASHIFTRT ???
- From: kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)
- To: law at redhat dot com
- Cc: alan dot lawrence at arm dot com, gcc at gcc dot gnu dot org
- Date: Tue, 24 Jun 2014 17:26:33 EDT
- Subject: Re: combine_simplify_rtx (doesn't) commute XOR and ASHIFTRT ???
- Authentication-results: sourceware.org; auth=none
- References: <53A98028 dot 2000604 at arm dot com> <53A9B28E dot 80803 at redhat dot com>
> > and wondering if anyone can explain to me what's wrong with this
> > transformation. Having worked through all four cases of A and C1
> > positive and negative, it seems to me that the extra bits 'fed in' to
> > the most-significant end of the result are the same either way (i.e. the
> > XOR of the sign bits of A and C1).
>
> I haven't heard from Kenner in a while, but you could always try to
> contact him directly and see if he can recall the issue. It was a long
> time ago though...
I haven't gone anywhere ...
But indeed ten years is a long time and I don't have any recollection of
this at all. The above analysis seems right, but this isn't the sort of
thing I'd have done if there weren't some sort of bug. Perhaps it's only
relevant if result_mode != shift_mode?