[llvm-dev] Instcombine and bitcast of vector. Wrong CHECKs in cast.ll, miscompile in instcombine?

[Sanjay Patel via llvm-dev]

Looks broken to me - we need to consider big/little-endian datalayout when
bitcasting to/from vectors.
We should have some documentation for this in the LangRef, but I don't see
anything currently.

The transform in question was added here:
https://reviews.llvm.org/rL103354

You can find other vector bitcast transforms that (hopefully correctly...)
account for the datalayout difference for vector elements.

Example:
https://github.com/llvm/llvm-project/blob/master/llvm/test/Transforms/InstCombine/bitcast-bigendian.ll#L10
https://github.com/llvm/llvm-project/blob/master/llvm/test/Transforms/InstCombine/bitcast.ll#L290

So we need something like this:
https://github.com/llvm/llvm-project/blob/master/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp#L487


On Thu, Nov 28, 2019 at 9:12 AM Mikael Holmén via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> In
>  llvm/test/Transforms/InstCombine/cast.ll
> there is a test like this:
>
> target datalayout = "E-p:64:64:64-p1:32:32:32-p2:64:64:64-p3:64:64:64-
> a0:0:8-f32:32:32-f64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-
> v64:64:64-v128:128:128-n8:16:32:64"
>
> [...]
>
> define <3 x i32> @test60(<4 x i32> %call4) {
> ; CHECK-LABEL: @test60(
> ; CHECK-NEXT:    [[P10:%.*]] = shufflevector <4 x i32> [[CALL4:%.*]],
> <4 x i32> undef, <3 x i32> <i32 0, i32 1, i32 2>
> ; CHECK-NEXT:    ret <3 x i32> [[P10]]
> ;
>   %p11 = bitcast <4 x i32> %call4 to i128
>   %p9 = trunc i128 %p11 to i96
>   %p10 = bitcast i96 %p9 to <3 x i32>
>   ret <3 x i32> %p10
>
> }
>
> If we assume the input vector is e.g. <1, 2, 3, 4> then I assume %p11
> would be the (hex) value 1234, %p9 would be the 234 and %p10 would then
> be the vector <2, 3, 4>.
>
> Am I right, or am I missing something here? Note that the datalayout
> says we're using big endian.
>
> But the CHECK-NEXT checks that the result is made up of the elements at
> index 0, 1 and 2 from the input vector, which would be <1, 2, 3>.
>
> So, broken testcase or am I missing something?
>
> Thanks,
> Mikael
>
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