Richard Biener <richard.guent...@gmail.com> writes: > On Fri, Jun 23, 2017 at 2:05 PM, Richard Sandiford > <richard.sandif...@linaro.org> wrote: >> Richard Biener <richard.guent...@gmail.com> writes: >>> On Thu, Jun 22, 2017 at 1:30 PM, Richard Sandiford >>> <richard.sandif...@linaro.org> wrote: >>>> The test case triggered this assert in vect_update_misalignment_for_peel: >>>> >>>> gcc_assert (DR_MISALIGNMENT (dr) / dr_size == >>>> DR_MISALIGNMENT (dr_peel) / dr_peel_size); >>>> >>>> We knew that the two DRs had the same misalignment at runtime, but when >>>> considered in isolation, one data reference guaranteed a higher >>>> compile-time >>>> base alignment than the other. >>>> >>>> In the test case this looks like a missed opportunity. Both references >>>> are unconditional, so it should be possible to use the highest of the >>>> available base alignment guarantees when analyzing each reference. >>>> The patch does this. >>>> >>>> However, as the comment in the patch says, the base alignment guarantees >>>> provided by a conditional reference only apply if the reference occurs >>>> at least once. In this case it would be legitimate for two references >>>> to have the same runtime misalignment and for one reference to provide a >>>> stronger compile-time guarantee than the other about what the misalignment >>>> actually is. The patch therefore relaxes the assert to handle that case. >>> >>> Hmm, but you don't actually check whether a reference occurs only > conditional, >>> do you? You just seem to say that for masked loads/stores the reference >>> is conditional (I believe that's not true). But for a loop like >>> >>> for (;;) >>> if (a[i]) >>> sum += b[j]; >>> >>> you still assume b[j] executes unconditionally? >> >> Maybe the documentation isn't clear enough, but DR_IS_CONDITIONAL >> was supposed to mean "even if the containing statement executes >> and runs to completion, the reference might not actually occur". >> The example above isn't conditional in that sense because the >> reference to b[j] does occur if the store is reached and completes. >> >> Masked loads and stores are conditional in that sense though. >> The reference only occurs if the mask is nonzero; the memory >> isn't touched otherwise. The functions are used to if-convert >> things like: >> >> for (...) >> a[i] = b[i] ? c[i] : d[i]; >> >> where there's no guarantee that it's safe to access c[i] when !b[i] >> (or d[i] when b[i]). No reference occurs for an all-false mask. > > But as you touch generic data-ref code here you should apply more > sensible semantics to DR_IS_CONDITIONAL than just marking > masked loads/stores but not DRs occuring inside BBs only executed > conditionally ...
I don't see why that's more sensible though. If a statement is only conditionally executed in a loop, it's up to the consumer to decide what to do about that. The conditions under which the statement is reached are a control-flow issue and tree-data-ref.c doesn't have any special information about it. Masked loads and stores are special because the DR_REFs created by tree-data-ref.c are artificial: they didn't exist as MEM_REFs in the original DR_STMT. And AIUI they didn't exist as MEM_REFs precisely because they're not guaranteed to happen, even if the load or store statement itself is executed. So in this case the DR_IS_CONDITIONAL is reflecting something that tree-data-ref.c itself has done. How about calling it DR_IS_CONDITIONAL_IN_STMT to avoid the general-sounding name? >>> The vectorizer of course only sees unconditionally executed stmts. >>> >>> So - I'd simply not add this DR_IS_CONDITIONAL. Did you run into >>> any real-world (testsuite) issues without this? >> >> Dropping DR_IS_CONDITIONAL would cause us to make invalid alignment >> assumptions in silly corner cases. I could add a scan test for it, >> for targets with masked loads and stores. It wouldn't trigger >> an execution failure though because we assume that targets with >> masked loads and stores allow unaligned accesses: >> >> /* For now assume all conditional loads/stores support unaligned >> access without any special code. */ >> if (is_gimple_call (stmt) >> && gimple_call_internal_p (stmt) >> && (gimple_call_internal_fn (stmt) == IFN_MASK_LOAD >> || gimple_call_internal_fn (stmt) == IFN_MASK_STORE)) >> return dr_unaligned_supported; >> >> So the worst that would happen is that we'd supposedly peel for >> alignment, but actually misalign everything instead, and so make >> things slower rather than quicker. >> >>> Note that the assert is to prevent bogus information. Iff we aligned >>> DR with base alignment 8 and misalign 3 then if another same-align >>> DR has base alignment 16 we can't simply zero its DR_MISALIGNMENT >>> as it still can be 8 after aligning DR. >>> >>> So I think it's wrong to put DRs with differing base-alignment into >>> the same-align-refs chain, those should get their DR_MISALIGNMENT >>> updated independenlty after peeling. >> >> DR_MISALIGNMENT is relative to the vector alignment rather than >> the base alignment though. So: > > We seem to use it that way, yes (looking at set_ptr_info_alignment > uses). So why not fix the assert then by capping the alignment/misalignment > we compute at this value as well? (and document this in the header > around DR_MISALIGNMENT) > > Ideally we'd do alignment analysis independent of the vector size > though (for those stupid targets with multiple vector sizes to consider...). > >> a) when looking for references *A1 and *A2 with the same alignment, >> we simply have to prove that A1 % vecalign == A2 % vecalign. >> This doesn't require any knowledge about the base alignment. >> If we break the addresses down as: >> >> A1 = BASE1 + REST1, REST1 = INIT1 + OFFSET1 + X * STEP1 >> A2 = BASE2 + REST2, REST2 = INIT2 + OFFSET2 + X * STEP2 >> >> and can prove that BASE1 == BASE2, the alignment of that base >> isn't important. We simply need to prove that REST1 % vecalign >> == REST2 % vecalign for all X. >> >> b) In the assert, we've peeled the loop so that DR_PEEL is guaranteed >> to be vector-aligned. If DR_PEEL is A1 in the example above, we have >> A1 % vecalign == 0, so A2 % vecalign must be 0 too. This again doesn't >> rely on the base alignment being known. >> >> What a high base alignment for DR_PEEL gives us is the ability to know >> at compile how many iterations need to be peeled to make DR_PEEL aligned. >> But the points above apply regardless of whether we know that value at >> compile time or not. >> >> In examples like the test case, we would have known at compile time that >> VF-1 iterations would need to be peeled if we'd picked the store as the >> DR_PEEL, but would have treated the number of peels as variable if we'd >> picked the load. The value calculated at runtime would still have been >> VF-1, it's just that the code wouldn't have been as efficient. >> >> One of the benefits of pooling the alignments for unconditional references >> is that it no longer matters which DR we pick: the number of peels will >> be a compile-time constant both ways. >> >> Thanks, >> Richard >> >>> I'd rather not mix fixing this with the improvement to eventuall use a >>> larger align for the other DR if possible. > > ^^^ > > So can you fix the ICE with capping base alignment / DR_MISALIGNMENT? I don't think the problem is the lack of a cap. In the test case we see that: 1. B is known at compile time to be X * vecsize + Y when considered in isolation, because the base alignment derived from its DR_REF >= vecsize. So DR_MISALIGNMENT (B) == Y. 2. A's misalignment wrt vecsize is not known at compile time when considered in isolation, because no useful base alignment can be derived from its DR_REF. (The DR_REF is to a plain int rather than to a structure with a high alignment.) So DR_MISALIGNMENT (A) == -1. 3. A and B when considered as a pair trivially have the same misalignment wrt vecsize, for the reasons above. Each of these results is individually correct. The problem is that the assert is conflating two things: it's saying that if we know two datarefs have the same misaligment, we must either be able to calculate a compile-time misalignment for both datarefs in isolation, or we must fail to calculate a compile-time misalignment for both datarefs in isolation. That isn't true: it's valid to have situations in which the compile-time misalignment is known for one dataref in isolation but not for the other. Thanks, Richard
