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) );