|q: gcc backend condition codes firstname.lastname@example.org (Erez Doron) (2003-06-20)|
|Re: q: gcc backend condition codes email@example.com (Chris Dollin) (2003-06-22)|
|Re: q: gcc backend condition codes firstname.lastname@example.org (2003-06-25)|
|Re: q: gcc backend condition codes email@example.com (Michael Meissner) (2003-07-02)|
|From:||Michael Meissner <firstname.lastname@example.org>|
|Date:||2 Jul 2003 00:42:15 -0400|
|Posted-Date:||02 Jul 2003 00:42:15 EDT|
Erez Doron <email@example.com> writes:
> I'm trying to port gcc to a new processor.
> the assembeler does not have a flags register.
> instead, every instruction can be preceded with a condition.
> if r1>r2 mov r2,r1
> is a maximum function.
> how can i acomplish that ? (examples is good)
There are two levels to answering your question. The first is how to
add the basic support for things like if-then-else, etc. to the
compiler, and the second is how to enable condition execution without
In terms of adding the basic if-then-else support, it depends on how
the condition is set. You have machines like the ARM which have a
single set of condition codes, that a previous comparison sets. On
machines like the ARM, you typically treat the condition codes as one
register using CCmode, etc. to set it.
(set (reg:CC <cc>)
(compare:CC (reg:SI <reg1>)
(if_then_else (ne:CC (reg:CC <cc>)
On the other hand you have machines like the IA64 which have a set of 1 bit
predicate registers, which is evaluated for every instruction (with typically
one predicate register that is hardwired to always return1 1). The IA-64 has
64 predicates, and the compare instructions set 2 predicates, one if the
compare succeeded, and one if it failed. By convention, the IA-64 GCC port
uses the even register for the true value and the odd register for the false,
and pretends there are only 1/2 as many predicates.
(set (reg:BI <pred>)
(gt:BI (reg:SI <reg1>)
(if_then_else (ne:BI (reg:BI <pred>)
The FRV is a third way of doing things, in that it has a subset of the
instructions that are conditionally executed, but still has multiple condition
To enable conditional execution of instructions (other than jumps), you use the
(ne:BI (reg:BI <pred>)
(set (reg:SI <reg1>)
"move %3,%2 if %1")
To avoid doubling or tripling the number of patterns that cond_exec would
introduce, you want to use match-operator. For example from the FRV:
(match_operator 0 "ccr_eqne_operator"
[(match_operand 1 "cr_operand" "C")
(set (match_operand:SI 2 "integer_register_operand" "=d")
(match_operator:SI 3 "condexec_si_unary_operator"
[(match_operand:SI 4 "integer_register_operand" "d")])))]
switch (GET_CODE (operands))
case NOT: return \"cnot %4, %2, %1, %e0\";
case NEG: return \"csub %., %4, %2, %1, %e0\";
default: abort ();
[(set_attr "length" "4")
(set_attr "type" "int")])
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