kmclaughlin added a comment.
Thanks all for the suggestions on this patch :)
I had a look at the LowerSwitch pass as suggested by @junparser, and I did find
that running it before vectorisation transforms the switch and allows the same
loops to be vectorised. However, I did find that if the loop is not vectorised
then the switch is not created again later by SimplifyCFG (possibly because the
pass is also arbitrarily splitting cases into ranges and creating multiple
branches to the default block?). Tests such as
Transforms/PhaseOrdering/X86/simplifycfg-late.ll then fail, which attempts to
convert a switch statement into a lookup table.
For example, running the @switch_no_vectorize test (from remove-switches.ll)
with -lowerswitch results in:
for.body: ; preds = %L3, %entry
%i = phi i64 [ %inc, %L3 ], [ 0, %entry ]
%sum.033 = phi float [ %conv20, %L3 ], [ 2.000000e+00, %entry ]
%arrayidx = getelementptr inbounds i32, i32* %a, i64 %i
%0 = load i32, i32* %arrayidx, align 4
br label %NodeBlock
NodeBlock: ; preds = %for.body
%Pivot = icmp slt i32 %0, 3
br i1 %Pivot, label %LeafBlock, label %LeafBlock1
LeafBlock1: ; preds = %NodeBlock
%SwitchLeaf2 = icmp eq i32 %0, 3
br i1 %SwitchLeaf2, label %L3, label %NewDefault
LeafBlock: ; preds = %NodeBlock
%SwitchLeaf = icmp eq i32 %0, 2
br i1 %SwitchLeaf, label %L2, label %NewDefault
NewDefault: ; preds = %LeafBlock1,
%LeafBlock
br label %L1
I also found that any weights assigned to the switch statement are ignored when
creating the new branches in LowerSwitch.
I'm not sure what the best approach to this is - I could try to change
LowerSwitch to create branches which SimplifyCFG will be able to recognise and
replace with a switch, or try to change SimplifyCFG to recognise this pattern
of compares & branches. Alternatively, the changes in this patch could be used
as the basis for a new pass which runs before the vectoriser. I wondered if
anyone has any thoughts or preferences on which would be the best option here?
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D108138/new/
https://reviews.llvm.org/D108138
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