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/* Definitions of target machine for GNU compiler. TMS320C[34]x
Copyright (C) 2002 Free Software Foundation, Inc.
Contributed by Michael Hayes (
and Herman Ten Brugge (
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.
GCC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with GCC; see the file COPYING. If not, write to
the Free Software Foundation, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA. */
/* C4x wants 1- and 2-word float modes, in its own peculiar format.
FIXME: Give this port a way to get rid of SFmode, DFmode, and all
the other modes it doesn't use. */
FLOAT_MODE (QF, 1, c4x_single_format);
FLOAT_MODE (HF, 2, c4x_extended_format);
RESET_FLOAT_FORMAT (SF, 0); /* not used */
RESET_FLOAT_FORMAT (DF, 0); /* not used */
/* Add any extra modes needed to represent the condition code.
On the C4x, we have a "no-overflow" mode which is used when an ADD,
SUB, NEG, or MPY insn is used to set the condition code. This is
to prevent the combiner from optimizing away a following CMP of the
result with zero when a signed conditional branch or load insn
The problem is a subtle one and deals with the manner in which the
negative condition (N) flag is used on the C4x. This flag does not
reflect the status of the actual result but of the ideal result had
no overflow occurred (when considering signed operands).
For example, 0x7fffffff + 1 => 0x80000000 Z=0 V=1 N=0 C=0. Here
the flags reflect the untruncated result, not the actual result.
While the actual result is less than zero, the N flag is not set
since the ideal result of the addition without truncation would
have been positive.
Note that the while the N flag is handled differently to most other
architectures, the use of it is self consistent and is not the
cause of the problem.
Logical operations set the N flag to the MSB of the result so if
the result is negative, N is 1. However, integer and floating
point operations set the N flag to be the MSB of the result
exclusive ored with the overflow (V) flag. Thus if an overflow
occurs and the result does not have the MSB set (i.e., the result
looks like a positive number), the N flag is set. Conversely, if
an overflow occurs and the MSB of the result is set, N is set to 0.
Thus the N flag represents the sign of the result if it could have
been stored without overflow but does not represent the apparent
sign of the result. Note that most architectures set the N flag to
be the MSB of the result.
The C4x approach to setting the N flag simplifies signed
conditional branches and loads which only have to test the state of
the N flag, whereas most architectures have to look at both the N
and V flags. The disadvantage is that there is no flag giving the
status of the sign bit of the operation. However, there are no
conditional load or branch instructions that make use of this
feature (e.g., BMI---branch minus) instruction. Note that BN and
BLT are identical in the C4x.
To handle the problem where the N flag is set differently whenever
there is an overflow we use a different CC mode, CC_NOOVmode which
says that the CC reflects the comparison of the result against zero
if no overflow occurred.
For example,
[(set (reg:CC_NOOV 21)
(compare:CC_NOOV (minus:QI (match_operand:QI 1 "src_operand" "")
(match_operand:QI 2 "src_operand" ""))
(const_int 0)))
(set (match_operand:QI 0 "ext_reg_operand" "")
(minus:QI (match_dup 1)
(match_dup 2)))]
Note that there is no problem for insns that don't return a result
like CMP, since the CC reflects the effect of operation.
An example of a potential problem is when GCC
converts (LTU (MINUS (0x80000000) (0x7fffffff) (0x80000000)))
to (LEU (MINUS (0x80000000) (0x7fffffff) (0x7fffffff)))
to (GE (MINUS (0x80000000) (0x7fffffff) (0x00000000)))
Now (MINUS (0x80000000) (0x7fffffff)) returns 0x00000001 but the
C4x sets the N flag since the result without overflow would have
been 0xffffffff when treating the operands as signed integers.
Thus (GE (MINUS (0x80000000) (0x7fffffff) (0x00000000))) sets the N
flag but (GE (0x00000001)) does not set the N flag.
The upshot is that we can not use signed branch and conditional
load instructions after an add, subtract, neg, abs or multiply.
We must emit a compare insn to check the result against 0. */