Created attachment 122426 [details]
c++ code to reproduce the issue

According to Intel's Instruction Set Reference A-Z
(https://www.intel.com/content/dam/www/public/us/en/documents/manuals/64-ia-32-architectures-software-developer-instruction-set-reference-manual-325383.pdf),
the instructions bt/btc/bts/btr leave the ZF unchanged.
However, in VEX/priv/guest_amd64_toIR.c:4016ff in function dis_Grp8_Imm
the ZF is cleared, unlike the implementation for bt/btc/bts/btr with non-immediate argument
in VEX/priv/guest_amd64_toIR.c:8225ff in function dis_bt_G_E which conforms to the Intel 
specification.

Also it seems that the x86 implementation clears the ZF both for immediate and non-immediate argument versions of the  bt/btc/bts/btr instructions.



See the attached code to reproduce the issue:

baumratte@linux:~/tmp/valgrind-bug> g++ -omain main.cxx 
baumratte@linux:~/tmp/valgrind-bug> ./main 
result = 0 (expecting 0)
baumratte@linux:~/tmp/valgrind-bug> valgrind ./main 
==29558== Memcheck, a memory error detector
==29558== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==29558== Using Valgrind-3.15.0 and LibVEX; rerun with -h for copyright info
==29558== Command: ./main
==29558== 
result = -1 (expecting 0)
==29558== 
==29558== HEAP SUMMARY:
==29558==     in use at exit: 0 bytes in 0 blocks
==29558==   total heap usage: 1 allocs, 1 frees, 72,704 bytes allocated
==29558== 
==29558== All heap blocks were freed -- no leaks are possible
==29558== 
==29558== For lists of detected and suppressed errors, rerun with: -s
==29558== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 0 from 0)