GNU Info
(gcc-295.info)Condition Code
Condition Code Status
=====================
This describes the condition code status.
The file `conditions.h' defines a variable `cc_status' to describe
how the condition code was computed (in case the interpretation of the
condition code depends on the instruction that it was set by). This
variable contains the RTL expressions on which the condition code is
currently based, and several standard flags.
Sometimes additional machine-specific flags must be defined in the
machine description header file. It can also add additional
machine-specific information by defining `CC_STATUS_MDEP'.
`CC_STATUS_MDEP'
C code for a data type which is used for declaring the `mdep'
component of `cc_status'. It defaults to `int'.
This macro is not used on machines that do not use `cc0'.
`CC_STATUS_MDEP_INIT'
A C expression to initialize the `mdep' field to "empty". The
default definition does nothing, since most machines don't use the
field anyway. If you want to use the field, you should probably
define this macro to initialize it.
This macro is not used on machines that do not use `cc0'.
`NOTICE_UPDATE_CC (EXP, INSN)'
A C compound statement to set the components of `cc_status'
appropriately for an insn INSN whose body is EXP. It is this
macro's responsibility to recognize insns that set the condition
code as a byproduct of other activity as well as those that
explicitly set `(cc0)'.
This macro is not used on machines that do not use `cc0'.
If there are insns that do not set the condition code but do alter
other machine registers, this macro must check to see whether they
invalidate the expressions that the condition code is recorded as
reflecting. For example, on the 68000, insns that store in address
registers do not set the condition code, which means that usually
`NOTICE_UPDATE_CC' can leave `cc_status' unaltered for such insns.
But suppose that the previous insn set the condition code based
on location `a4@(102)' and the current insn stores a new value in
`a4'. Although the condition code is not changed by this, it will
no longer be true that it reflects the contents of `a4@(102)'.
Therefore, `NOTICE_UPDATE_CC' must alter `cc_status' in this case
to say that nothing is known about the condition code value.
The definition of `NOTICE_UPDATE_CC' must be prepared to deal with
the results of peephole optimization: insns whose patterns are
`parallel' RTXs containing various `reg', `mem' or constants which
are just the operands. The RTL structure of these insns is not
sufficient to indicate what the insns actually do. What
`NOTICE_UPDATE_CC' should do when it sees one is just to run
`CC_STATUS_INIT'.
A possible definition of `NOTICE_UPDATE_CC' is to call a function
that looks at an attribute (Note: Insn Attributes.) named, for
example, `cc'. This avoids having detailed information about
patterns in two places, the `md' file and in `NOTICE_UPDATE_CC'.
`EXTRA_CC_MODES'
A list of names to be used for additional modes for condition code
values in registers (Note: Jump Patterns.). These names are
added to `enum machine_mode' and all have class `MODE_CC'. By
convention, they should start with `CC' and end with `mode'.
You should only define this macro if your machine does not use
`cc0' and only if additional modes are required.
`EXTRA_CC_NAMES'
A list of C strings giving the names for the modes listed in
`EXTRA_CC_MODES'. For example, the Sparc defines this macro and
`EXTRA_CC_MODES' as
#define EXTRA_CC_MODES CC_NOOVmode, CCFPmode, CCFPEmode
#define EXTRA_CC_NAMES "CC_NOOV", "CCFP", "CCFPE"
This macro is not required if `EXTRA_CC_MODES' is not defined.
`SELECT_CC_MODE (OP, X, Y)'
Returns a mode from class `MODE_CC' to be used when comparison
operation code OP is applied to rtx X and Y. For example, on the
Sparc, `SELECT_CC_MODE' is defined as (see Note: Jump Patterns.
for a description of the reason for this definition)
#define SELECT_CC_MODE(OP,X,Y) \
(GET_MODE_CLASS (GET_MODE (X)) == MODE_FLOAT \
? ((OP == EQ || OP == NE) ? CCFPmode : CCFPEmode) \
: ((GET_CODE (X) == PLUS || GET_CODE (X) == MINUS \
|| GET_CODE (X) == NEG) \
? CC_NOOVmode : CCmode))
You need not define this macro if `EXTRA_CC_MODES' is not defined.
`CANONICALIZE_COMPARISON (CODE, OP0, OP1)'
One some machines not all possible comparisons are defined, but
you can convert an invalid comparison into a valid one. For
example, the Alpha does not have a `GT' comparison, but you can
use an `LT' comparison instead and swap the order of the operands.
On such machines, define this macro to be a C statement to do any
required conversions. CODE is the initial comparison code and OP0
and OP1 are the left and right operands of the comparison,
respectively. You should modify CODE, OP0, and OP1 as required.
GNU CC will not assume that the comparison resulting from this
macro is valid but will see if the resulting insn matches a
pattern in the `md' file.
You need not define this macro if it would never change the
comparison code or operands.
`REVERSIBLE_CC_MODE (MODE)'
A C expression whose value is one if it is always safe to reverse a
comparison whose mode is MODE. If `SELECT_CC_MODE' can ever
return MODE for a floating-point inequality comparison, then
`REVERSIBLE_CC_MODE (MODE)' must be zero.
You need not define this macro if it would always returns zero or
if the floating-point format is anything other than
`IEEE_FLOAT_FORMAT'. For example, here is the definition used on
the Sparc, where floating-point inequality comparisons are always
given `CCFPEmode':
#define REVERSIBLE_CC_MODE(MODE) ((MODE) != CCFPEmode)
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