44 lines
2.0 KiB
Diff
44 lines
2.0 KiB
Diff
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commit 256cf43c5eadb28edb45436aca6fda8ee55eb10e
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Author: Mark Wielaard <mark@klomp.org>
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Date: Thu Feb 21 17:21:53 2019 +0100
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memcheck powerpc subfe x, x, x initializes x to 0 or -1 based on CA
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GCC might use subfe x, x, x to initialize x to 0 or -1, based on
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whether the carry flag is set. This happens in some cases when g++
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compiles resetting a unique_ptr. The "trick" used by the compiler is
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that it can AND a pointer with the register x (now 0x0 or 0xffffffff)
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to set something to NULL or to the given pointer.
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subfe is implemented as rD = (log not)rA + rB + XER[CA]
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if we instead implement it as rD = rB - rA - (XER[CA] ^ 1)
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then memcheck can see that rB and Ra cancel each other out if they
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are the same.
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https://bugs.kde.org/show_bug.cgi?id=404054
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diff --git a/VEX/priv/guest_ppc_toIR.c b/VEX/priv/guest_ppc_toIR.c
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index e207642..00ae6df 100644
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--- a/VEX/priv/guest_ppc_toIR.c
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+++ b/VEX/priv/guest_ppc_toIR.c
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@@ -5361,11 +5361,15 @@ static Bool dis_int_arith ( UInt theInstr )
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flag_OE ? "o" : "", flag_rC ? ".":"",
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rD_addr, rA_addr, rB_addr);
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// rD = (log not)rA + rB + XER[CA]
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+ // ==>
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+ // rD = rB - rA - (XER[CA] ^ 1)
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assign( old_xer_ca, mkWidenFrom32(ty, getXER_CA_32(), False) );
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- assign( rD, binop( mkSzOp(ty, Iop_Add8),
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- unop( mkSzOp(ty, Iop_Not8), mkexpr(rA)),
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- binop( mkSzOp(ty, Iop_Add8),
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- mkexpr(rB), mkexpr(old_xer_ca))) );
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+ assign( rD, binop( mkSzOp(ty, Iop_Sub8),
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+ binop( mkSzOp(ty, Iop_Sub8),
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+ mkexpr(rB), mkexpr(rA)),
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+ binop(mkSzOp(ty, Iop_Xor8),
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+ mkexpr(old_xer_ca),
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+ mkSzImm(ty, 1))) );
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set_XER_CA_CA32( ty, PPCG_FLAG_OP_SUBFE,
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mkexpr(rD), mkexpr(rA), mkexpr(rB),
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mkexpr(old_xer_ca) );
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