Auto-vectorization via `masked.load` blocks constprop

[scottmcm]

I was writing some code in Rust and ended up with the following IR, where even though everything's a constant -- it should just be ret i64 165 -- the masked loads from autovectorization on -Ctarget-cpu=x86-64-v3 kept that from happening:

define noundef i64 @test() unnamed_addr #0 {
bb3.preheader:
  %iter = alloca [64 x i8], align 8
  call void @llvm.lifetime.start.p0(i64 64, ptr nonnull %iter)
  %_3.sroa.5.0.iter.sroa_idx = getelementptr inbounds nuw i8, ptr %iter, i64 16
  store <4 x i64> <i64 23, i64 16, i64 54, i64 3>, ptr %_3.sroa.5.0.iter.sroa_idx, align 8
  %_3.sroa.9.0.iter.sroa_idx = getelementptr inbounds nuw i8, ptr %iter, i64 48
  store i64 60, ptr %_3.sroa.9.0.iter.sroa_idx, align 8
  %_3.sroa.10.0.iter.sroa_idx = getelementptr inbounds nuw i8, ptr %iter, i64 56
  store i64 9, ptr %_3.sroa.10.0.iter.sroa_idx, align 8
  %unmaskedload = load <4 x i64>, ptr %_3.sroa.5.0.iter.sroa_idx, align 8, !alias.scope !2
  %0 = getelementptr inbounds nuw i8, ptr %iter, i64 48
  %wide.masked.load.1 = call <4 x i64> @llvm.masked.load.v4i64.p0(ptr nonnull %0, i32 8, <4 x i1> <i1 true, i1 true, i1 false, i1 false>, <4 x i64> poison), !alias.scope !2
  %1 = add <4 x i64> %wide.masked.load.1, %unmaskedload
  %2 = shufflevector <4 x i64> %1, <4 x i64> %unmaskedload, <4 x i32> <i32 0, i32 1, i32 6, i32 7>
  %3 = tail call i64 @llvm.vector.reduce.add.v4i64(<4 x i64> %2)
  call void @llvm.lifetime.end.p0(i64 64, ptr nonnull %iter)
  ret i64 %3
}

It looks like trunk can't optimize that to a constant either: https://llvm.godbolt.org/z/z6MKz6cz1

(Trunk at least doesn't need the store-load of the vector constant, but it still doesn't const-prop the stores and the masked.load.)

[nikic]

What's the original IR? Ideally this should get constant folded before it gets vectorized.

[scottmcm]

Here's the full -C no-prepopulate-passes IR, @nikic : issue-101082.ll

i64 @test_eight() folds fine, probably because 8 elements is a multiple of the vector length, but i64 @test() is a 6-element array and doesn't fold.

[nikic]

What happens here is that the load of the loop counter gets load-only promoted by LICM, but at that point we just have the load in the preheader, but not the forwarded value from the initialization to 0. So LoopUnrollFull does not know that this is actually a loop with 6 iterations.

We've recently gained load-only promotion support in SROA, but we currently only use it for readonly calls. I believe we could also use it to SROA the case where we have unknown-offset loads, as long as all the stores are known-offset. That would allow the optimization to occur earlier, including direct forwarding of the initialization value.

[nikic]

It looks like that would basically work, but there's an issue with

llvm-project/llvm/lib/Transforms/Scalar/SROA.cpp

Lines 5513 to 5514 in a6853cd

	AllSameAndValid &= PartitionEnd->beginOffset() == BeginOffset &&
	PartitionEnd->endOffset() == EndOffset;

blocking the transform if there are lifetime intrinsics. Need to relax that first.