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Re: movmem pattern and missed alignment
On 10/08/2018 07:38 PM, Paul Koning wrote:
>> On Oct 8, 2018, at 1:29 PM, Andrew Haley <email@example.com> wrote:
>> On 10/08/2018 06:20 PM, Michael Matz wrote:
>>> Only if you somewhere visibly add accesses to *i and *j. Without them you
>>> only have the "accesses" via memcpy, and as Richi says, those don't imply
>>> any alignment requirements. The i and j pointers might validly be char*
>>> pointers in disguise and hence be in fact only 1-aligned. I.e. there's
>>> nothing in your small example program from which GCC can infer that those
>>> two global pointers are in fact 2-aligned.
>> So all you'd actually have to say is
>> void f1(void)
>> *i; *j;
>> __builtin_memcpy (i, j, 32);
> No, that doesn't help.
It could do.
> Not even if I make it:
> void f1(void)
> k = *i + *j;
> __builtin_memcpy (i, j, 4);
> The first line does word aligned references to *i and *j, but the memcpy stubbornly remains a byte move.
Right, so that is a missed optimization.
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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