> On 22 Nov 2017, at 09:14, Richard Biener <richard.guent...@gmail.com> wrote:
>
> On Tue, Nov 21, 2017 at 5:43 PM, Kilian Verhetsel
> <kilian.verhet...@uclouvain.be> wrote:
>>
>>> This is PR81179 I think, please mention that in the changelog.
>>
>> Correct, my bad for missing that.
>>
>>> This unconditionally pessimizes code even if there is no valid index
>>> zero, right?
>>
>> Almost, since for a loop such as:
>>
>> #define OFFSET 1
>> unsigned int find(const unsigned int *a, unsigned int v) {
>> unsigned int result = 120;
>> for (unsigned int i = OFFSET; i < 32+OFFSET; i++) {
>> if (a[i-OFFSET] == v) result = i;
>> }
>> return result;
>> }
>>
>> the index i will match the contents of the index vector used here ---
>> but this does indeed pessimize the code generated for, say, OFFSET
>> = 2. It is probably more sensible to use the existing code path in those
>> situations.
>>
>>> The issue with the COND_REDUCITION index vector is overflow IIRC.
>>
>> Does that mean such overflows can already manifest themselves for
>> regular COND_REDUCTION? I had assumed sufficient checks were already in
>> place because of the presence of the is_nonwrapping_integer_induction
>> test.
>
> But only if we need the index vector? The patch looked like you're changing
> how other modes are handled (in my look I didn't make myself familiar with
> the various modes again...). Anyway, Alan hopefully remembers what he
> coded so I'll defer to him here.
>
> If Alan is happy with the patch consider it approved.
>
Richard’s right with his question.
The optimisation needs to fail if the number of interactions of the loop + 1
doesn’t
fit into the data type used for the result.
I took the test pr65947-14.c
First I set N to 0xffffffff-1. This compiled and vectorised. That’s ok.
Now if I set N to 0xffffffff it still vectorises, but this should fail.
Compare to pr65947-14.c where we set last = a[I]; inside the if.
When set N to 0xffffffff-1, it compiled and vectorised. That’s ok.
When set N to 0xffffffff it fails to vectorise with the message
"loop size is greater than data size”.
Looks like you might just need to add the one check.
Also see pr65947-9.c which uses the slightly more useful char indexes.
Alan.
