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Re: [PATCH 06/15] rs6000: Use rldiwi in constant construction
- From: Richard Henderson <rth at redhat dot com>
- To: Segher Boessenkool <segher at kernel dot crashing dot org>
- Cc: gcc-patches at gcc dot gnu dot org, David Edelsohn <dje dot gcc at gmail dot com>
- Date: Wed, 12 Aug 2015 08:55:51 -0700
- Subject: Re: [PATCH 06/15] rs6000: Use rldiwi in constant construction
- Authentication-results: sourceware.org; auth=none
- References: <1439341904-9345-1-git-send-email-rth at redhat dot com> <1439341904-9345-7-git-send-email-rth at redhat dot com> <20150812140251 dot GH4711 at gate dot crashing dot org>
On 08/12/2015 07:02 AM, Segher Boessenkool wrote:
> On Tue, Aug 11, 2015 at 06:11:35PM -0700, Richard Henderson wrote:
>> @@ -8173,6 +8173,13 @@ genimm_ppc::exam_search (HOST_WIDE_INT c, int budget)
>> if (exam_mask (-1, c, sub_budget))
>> return true;
>>
>> + /* If the two halves are equal, use an insert. */
>> + if (c >> 32 == test && exam_sub (test, sub_budget))
>> + {
>> + opN (VEC_DUPLICATE, 0xffffffffu); /* RLDIMI */
>> + return true;
>> + }
>
> Does this work for c with the high bit set? I think you need
> to cast it to unsigned HOST_WIDE_INT first?
Indeed, a sign-extension works better. It means the base constant will use
LIS+ORIS without trying to create an unsigned version.
If you're talking about ubsan sort of restrictions on shifting signed
constants... I choose to totally ignore that. Certainly no where else in gcc
has been audited for that, beginning with hwint.h itself.
r~