Useless memory access - bt, bts, btr, btc

[mfaerevaag]

Description

Using some seemingly useless memory for simple register operation with operands 0x0c and 0xc8.

CF <- Bit(Bitcase, BitOffset)

Reference:
Ref. Intel 64 and IA-32 Architecture Software Developer's Manual Vol. 2A 3-113

Affected instructions:

0x0fa3c0    # bt
0x0fa3c8
0x0fabc0    # bts
0x0fabc8
0x0fb3c0    # btr
0x0fb3c8
0x0fbbc0    # btc
0x0fbbc8

NOTE: All combinations of prefixes are omitted.

Reproduction guide

Instruction:

00000000  0FA3C0            bt eax,eax

Input:

pyvex.IRSB("\x0F\xA3\xC0", 0x8048000, archinfo.ArchX86())

Observed output:

IRSB {
   t0:Ity_I8 t1:Ity_I8 t2:Ity_I32 t3:Ity_I32 t4:Ity_I8 t5:Ity_I32 t6:Ity_I32 t7:Ity_I32 t8:Ity_I32 t9:Ity_I32 t10:Ity_I32 t11:Ity_I32 t12:Ity_I32 t13:Ity_I8 t14:Ity_I32 t15:Ity_I32 t16:Ity_I32 t17:Ity_I32 t18:Ity_I32 t19:Ity_I32 t20:Ity_I32

   00 | ------ IMark(0x0, 3, 0) ------
   01 | t2 = GET:I32(eax)
   02 | t9 = GET:I32(esp)
   03 | t8 = Sub32(t9,0x00000080)
   04 | PUT(esp) = t8
   05 | STle(t8) = t2
   06 | t3 = And32(t2,0x0000001f)
   07 | t12 = Sar32(t3,0x03)
   08 | t11 = Add32(t8,t12)
   09 | t14 = And32(t3,0x00000007)
   10 | t13 = 32to8(t14)
   11 | t0 = LDle:I8(t11)
   12 | PUT(cc_op) = 0x00000000
   13 | PUT(cc_dep2) = 0x00000000
   14 | t17 = 8Uto32(t0)
   15 | t16 = Shr32(t17,t13)
   16 | t15 = And32(t16,0x00000001)
   17 | PUT(cc_dep1) = t15
   18 | PUT(cc_ndep) = 0x00000000
   19 | t18 = LDle:I32(t8)
   20 | PUT(eax) = t18
   21 | t19 = Add32(t8,0x00000080)
   22 | PUT(esp) = t19
   NEXT: PUT(eip) = 0x00000003; Ijk_Boring
}

Expected output:
Not using useless memory.

System Info

OS:

# uname -a
Linux ubuntu 4.10.0-28-generic #32-Ubuntu SMP Fri Jun 30 05:32:18 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
# cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=17.04
DISTRIB_CODENAME=zesty
DISTRIB_DESCRIPTION="Ubuntu 17.04"

PyVEX:

#  pip freeze | grep pyvex
pyvex==6.7.4.12