[PATCH] D77448: [PowerPC] Canonicalize shuffles to match more single-instruction masks on LE

[Fangrui Song via Phabricator via llvm-commits]

MaskRay added a comment.

Our internal testing finds a correctness bug related to this patch (after the last known fix eafe7c14ea38946e8c1fb64d548effaee7614718 <https://reviews.llvm.org/rGeafe7c14ea38946e8c1fb64d548effaee7614718>)

The code is at https://github.com/google/dimsum/blob/master/dimsum_fuzz.cc#L402

With the following patch it can build with `-mcpu=pwr9 -O0`. When I feed the executable `dimsum_fuzz` itself to `dimsum_fuzz`, it triggers `__builtin_trap` because hmin (horizontal min) on `typedef signed char simd_char __attribute__((vector_size__(8)));` computed result is different from a naive algorithm. Our internal environment may be a bit strange and I can't immediately reproduce with https://github.com/google/dimsum/blob/master/dimsum_fuzz.cc#L402
I'll inform you when I can get a minimized reproduce or in the meantime if you can find problem that'd also be nice:)

  diff --git i/dimsum_fuzz.cc w/dimsum_fuzz.cc
  index fafd1fa..b7188fd 100644
  --- i/dimsum_fuzz.cc
  +++ w/dimsum_fuzz.cc
  @@ -532 +532,6 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   }
  +
  +int main() {
  +  const long data[] = {0x00010102464c457f, 0x0000000000000000};
  +  LLVMFuzzerTestOneInput((const uint8_t*)data, 16);
  +}
  diff --git i/simd_vsx.h w/simd_vsx.h
  index 5c59e2a..15a32fe 100644
  --- i/simd_vsx.h
  +++ w/simd_vsx.h
  @@ -23,2 +23,6 @@
   
  +inline __vector long vec_abs(__vector long a) {
  +  return (__vector long)vec_abs((__vector long long)a);
  +}
  +
   namespace dimsum {

cgdb session

  2004x typename std::enable_if<(_SimdType::size() > 1 && is_simd<_SimdType>::value &&
  2005x                          __floor_pow_of_2(_SimdType::size()) ==
  2006x                              _SimdType::size()),
  2007x                         typename _SimdType::value_type>::type
  2008x __hmin(const _SimdType& __v) {
  2009x   auto __arr = split_by<2>(__v);
  2010x   return __hmin(min(__arr[0], __arr[1]));                                                                                                       
  2011x }
  2012x
  2013x template <class _SimdType>
  2014x typename std::enable_if<(_SimdType::size() > 1 &&
  2015x                          !(is_simd<_SimdType>::value &&
  2016x                            __floor_pow_of_2(_SimdType::size()) ==
  /home/maskray/test/src/third_party/dimsum/simd.h                                                                                                      
  t third_party/dimsum/simd.h:2079
  0x0000000010083c40 in std::experimental::__hmin<std::experimental::simd<signed char, std::experimental::__simd_abi<(std::experimental::_StorageKind)2,
   16> > > (__v=...) at third_party/dimsum/simd.h:2010
  Value returned is $2 = <incomplete type>
  (gdb) s
  std::experimental::__hmin<std::experimental::simd<signed char, std::experimental::__simd_abi<(std::experimental::_StorageKind)2, 8> > > (__v=...) at t
  hird_party/dimsum/simd.h:2009
  (gdb) n
  (gdb) x/2xg &__v
  0x7fffffffed88: 0x0000000000000000      0x00010102464c457f
  (gdb) x/2xg &__arr
  0x7fffffffecf8: 0x00007fff00000000      0x00007fffffffed88
  (gdb) p sizeof(__arr)            # under the hood this is a signed char vector_size(8) type
  $3 = 8
  (gdb) 


Repository:
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  https://reviews.llvm.org/D77448/new/

https://reviews.llvm.org/D77448