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Differential D112142 Diff 380964 llvm/test/Transforms/InstCombine/vscale_load_bitcast.ll

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llvm/test/Transforms/InstCombine/vscale_load_bitcast.ll

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; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt < %s -instcombine -S -verify | FileCheck %s
define <8 x i8> @constprop_load_bitcast() {
; CHECK-LABEL: @constprop_load_bitcast(
; CHECK-NEXT: ret <8 x i8> zeroinitializer
david-armUnsubmitted
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I guess this fold is happening because effectively i1 is being promoted to i8? Otherwise the minimum size of <vscale x 16 x i1> would be just 2 bytes, i.e. less than than <8 x i8>.

david-arm: I guess this fold is happening because effectively i1 is being promoted to i8? Otherwise the…
paulwalker-armUnsubmitted
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This is likely due to the issue above but at the same time the test is relying on undefined behaviour. You either want to use a smaller fixed length vector or perhaps add a vscale_range so the load does overlap the store.

Can you also add a negative test for the case where the load & store must remain. I guess if you want to tick all the boxes you could also add a test where the store remains but the load is removed.

paulwalker-arm: This is likely due to the issue above but at the same time the test is relying on undefined…
peterwaller-armAuthorUnsubmitted
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Right. I've added a negative test and fixed up the size query, thanks both for your attention to detail!

I considered your comment about vscale_range but:

  1. I don't see a straightforward route to get at it without propagating the function attributes through the constant folding chain. Maybe that's the right thing to do? Or is there a way to take Constant* and a Type*, and ask "what vscale_range is in effect?". I'm guessing not since these aren't in principle bound to functions. I don't want to fiddle with the function signature without a positive steer from someone that it's the right thing to do.
  1. What is the best way to make isKnown* queries in the presence of a vscale_range attribute?
peterwaller-arm: Right. I've added a negative test and fixed up the size query, thanks both for your attention…
;
%stack_var = alloca <vscale x 16 x i1>, align 16
store <vscale x 16 x i1> zeroinitializer, <vscale x 16 x i1>* %stack_var, align 16
%cast_to_fixed = bitcast <vscale x 16 x i1>* %stack_var to <8 x i8>*
%a = load <8 x i8>, <8 x i8>* %cast_to_fixed, align 16
ret <8 x i8> %a
}