This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: combination of read/write and earlyclobber constraint modifier


On 02-07-14 08:23, Marc Glisse wrote:
On Tue, 1 Jul 2014, Tom de Vries wrote:

On 01-07-14 21:58, Marc Glisse wrote:
So my question is: is the combination of '&' and '+' supported ? If so,
what is the exact semantics ? If not, should we warn or give an error ?
I don't think we can define any reasonable semantics for &+.  My
recommendation would be for this to be considered a hard error.

Uh? The doc explicitly says "An input operand can be tied to an earlyclobber
operand" and goes on to explain why that is useful. It avoids using the same
register for other input when they are identical.

Hi Marc,

That part of the doc refers to the mulsi3 insn for ARM as example:
...
;; Use `&' and then `0' to prevent the operands 0 and 1 being the same
(define_insn "*arm_mulsi3"
 [(set (match_operand:SI          0 "s_register_operand" "=&r,&r")
       (mult:SI (match_operand:SI 2 "s_register_operand" "r,r")
                (match_operand:SI 1 "s_register_operand" "%0,r")))]
 "TARGET_32BIT && !arm_arch6"
 "mul%?\\t%0, %2, %1"
 [(set_attr "type" "mul")
  (set_attr "predicable" "yes")]
)
...

Note that there's no combination of & and + here.

I think it could have used (match_dup 0) instead of operand 1, if there had been
only the first alternative. And then the constraint would have been +&.


Marc,

isn't that explicitly listed as unsupported here ( https://gcc.gnu.org/onlinedocs/gccint/RTL-Template.html#index-match_005fdup-3244 ):
...
Note that match_dup should not be used to tell the compiler that a particular register is being used for two operands (example: add that adds one register to another; the second register is both an input operand and the output operand). Use a matching constraint (see Simple Constraints) for those. match_dup is for the cases where one operand is used in two places in the template, such as an instruction that computes both a quotient and a remainder, where the opcode takes two input operands but the RTL template has to refer to each of those twice; once for the quotient pattern and once for the remainder pattern.
...
?

Thanks,
- Tom


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]