commit 256cf43c5eadb28edb45436aca6fda8ee55eb10e
Author: Mark Wielaard <mark@klomp.org>
Date:   Thu Feb 21 17:21:53 2019 +0100

    memcheck powerpc subfe x, x, x initializes x to 0 or -1 based on CA
    
    GCC might use subfe x, x, x to initialize x to 0 or -1, based on
    whether the carry flag is set. This happens in some cases when g++
    compiles resetting a unique_ptr. The "trick" used by the compiler is
    that it can AND a pointer with the register x (now 0x0 or 0xffffffff)
    to set something to NULL or to the given pointer.
    
    subfe is implemented as rD = (log not)rA + rB + XER[CA]
    if we instead implement it as rD = rB - rA - (XER[CA] ^ 1)
    then memcheck can see that rB and Ra cancel each other out if they
    are the same.
    
    https://bugs.kde.org/show_bug.cgi?id=404054

diff --git a/VEX/priv/guest_ppc_toIR.c b/VEX/priv/guest_ppc_toIR.c
index e207642..00ae6df 100644
--- a/VEX/priv/guest_ppc_toIR.c
+++ b/VEX/priv/guest_ppc_toIR.c
@@ -5361,11 +5361,15 @@ static Bool dis_int_arith ( UInt theInstr )
              flag_OE ? "o" : "", flag_rC ? ".":"",
              rD_addr, rA_addr, rB_addr);
          // rD = (log not)rA + rB + XER[CA]
+         //    ==>
+         // rD = rB - rA - (XER[CA] ^ 1)
          assign( old_xer_ca, mkWidenFrom32(ty, getXER_CA_32(), False) );
-         assign( rD, binop( mkSzOp(ty, Iop_Add8),
-                            unop( mkSzOp(ty, Iop_Not8), mkexpr(rA)),
-                            binop( mkSzOp(ty, Iop_Add8),
-                                   mkexpr(rB), mkexpr(old_xer_ca))) );
+         assign( rD, binop( mkSzOp(ty, Iop_Sub8),
+                            binop( mkSzOp(ty, Iop_Sub8),
+                                   mkexpr(rB), mkexpr(rA)),
+                            binop(mkSzOp(ty, Iop_Xor8),
+                                  mkexpr(old_xer_ca),
+                                  mkSzImm(ty, 1))) );
          set_XER_CA_CA32( ty, PPCG_FLAG_OP_SUBFE,
                           mkexpr(rD), mkexpr(rA), mkexpr(rB),
                           mkexpr(old_xer_ca) );