[PATCH] D51313: [LV] Fix code gen for conditionally executed uniform loads

[Ayal Zaks via Phabricator via llvm-commits]

Ayal added inline comments.


================
Comment at: lib/Transforms/Vectorize/LoopVectorize.cpp:5883
+          // gather/scatters are not considered scalar with predication.
+          !Legal->blockNeedsPredication(I.getParent())) {
         // Scalar load + broadcast
----------------
Indeed scalarizing a conditional load from a uniform address is currently broken, when it can turn into a gather. The culprit is that VPReplicateRecipe, when constructed for such instructions, uses an incorrect IsPredicate = isScalarWithPredication(). Which, as noted, is false for loads and stores of types that can be gathered/scattered. I.e., really means mustBeScalarWithPredication. (That, on top of IsUniform which is also incorrect, causing replication as noted in D50665#1209958.)

This proposed fix to refrain from scalarizing such loads, should** make sure they always turn into a gather**, rather than rely on the cost-based gather-vs-scalarize decision below.

The above holds also for conditional loads from **non-uniform** addresses, that can turn into gathers, but possibly also get incorrectly scalarized w/o branches. It's hard to test, as the scalarization cost needs to be better than the gather for this to occur. But best make sure these also always turn into a gather.

It would also/alternatively be good to fix the scalarization of such loads (by correcting IsPredicate), and enable cost-based gather-vs-scalarize decision. Presumably gather should always win, unless perhaps if the load feed scalarized users.


Repository:
  rL LLVM

https://reviews.llvm.org/D51313