[PATCH] D93229: [VectorCombine] allow peeking through GEPs when creating a vector load

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

In D93229#2452695 <https://reviews.llvm.org/D93229#2452695>, @lebedev.ri wrote:

> I'm having trouble coming up with an example because there appears to be a preexisting soundness problems, example: (CC @nlopes @aqjune)
>
>   define <8 x i16> @t(i8* align 128 dereferenceable(128) %base) {
>     %ptr = getelementptr inbounds i8, i8* %base, i64 1
>     %p = bitcast i8* %ptr to <8 x i16>*
>   
>     %gep = getelementptr inbounds <8 x i16>, <8 x i16>* %p, i64 0, i64 1
>     %s = load i16, i16* %gep, align 1
>     %r = insertelement <8 x i16> undef, i16 %s, i64 0
>     ret <8 x i16> %r
>   }
>
>   /builddirs/llvm-project/build-Clang11-unknown$ /builddirs/llvm-project/build-Clang11-unknown/bin/opt -load /repositories/alive2/build-Clang-release/tv/tv.so -tv -vector-combine -mtriple=x86_64-- -mattr=avx2 -tv -o /dev/null --tv-smt-to=60000 /tmp/D93229.ll 
>   
>   ----------------------------------------
>   define <8 x i16> @t(* dereferenceable(128) align(128) %base) {
>   %0:
>     %ptr = gep inbounds * dereferenceable(128) align(128) %base, 1 x i64 1
>     %p = bitcast * %ptr to *
>     %gep = gep inbounds * %p, 16 x i64 0, 2 x i64 1
>     %s = load i16, * %gep, align 1
>     %r = insertelement <8 x i16> undef, i16 %s, i64 0
>     ret <8 x i16> %r
>   }
>   =>
>   define <8 x i16> @t(* dereferenceable(128) align(128) %base) {
>   %0:
>     %ptr = gep inbounds * dereferenceable(128) align(128) %base, 1 x i64 1
>     %p = bitcast * %ptr to *
>     %gep = gep inbounds * %p, 16 x i64 0, 2 x i64 1
>     %1 = bitcast * %gep to *
>     %r = load <8 x i16>, * %1, align 1
>     ret <8 x i16> %r
>   }
>   Transformation doesn't verify!
>   ERROR: Target is more poisonous than source
>   
>   Example:
>   * dereferenceable(128) align(128) %base = pointer(non-local, block_id=1, offset=1664)
>   
>   Source:
>   * %ptr = pointer(non-local, block_id=1, offset=1665)
>   * %p = pointer(non-local, block_id=1, offset=1665)
>   * %gep = pointer(non-local, block_id=1, offset=1667)
>   i16 %s = poison
>   <8 x i16> %r = < poison, any, any, any, any, any, any, any >
>   
>   SOURCE MEMORY STATE
>   ===================
>   NON-LOCAL BLOCKS:
>   Block 0 >       size: 0 align: 1        alloc type: 0
>   Block 1 >       size: 2048      align: 128      alloc type: 0
>   
>   Target:
>   * %ptr = pointer(non-local, block_id=1, offset=1665)
>   * %p = pointer(non-local, block_id=1, offset=1665)
>   * %gep = pointer(non-local, block_id=1, offset=1667)
>   * %1 = pointer(non-local, block_id=1, offset=1667)
>   <8 x i16> %r = < poison, poison, poison, poison, poison, poison, poison, poison >
>   Source value: < poison, any, any, any, any, any, any, any >
>   Target value: < poison, poison, poison, poison, poison, poison, poison, poison >
>   
>   Alive2: Transform doesn't verify!

IIUC, this is a question of allowing poison (from the unused loaded memory elements) to propagate?
So we have to freeze or explicitly make those elements undef again?
https://alive2.llvm.org/ce/z/LKqBVW


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