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Re: combination of read/write and earlyclobber constraint modifier


On Tue, 1 Jul 2014, Jeff Law wrote:

On 07/01/14 13:27, Tom de Vries wrote:
Vladimir,

There are a few patterns which use both the read/write constraint
modifier (+) and the earlyclobber constraint modifier (&):
...
$ grep -c 'match_operand.*+.*&' gcc/config/*/* | grep -v :0
gcc/config/aarch64/aarch64-simd.md:1
gcc/config/arc/arc.md:1
gcc/config/arm/ldmstm.md:30
gcc/config/rs6000/spe.md:8
...

F.i., this one in gcc/config/aarch64/aarch64-simd.md:
...
(define_insn "vec_pack_trunc_<mode>"
  [(set (match_operand:<VNARROWQ2> 0 "register_operand" "+&w")
        (vec_concat:<VNARROWQ2>
          (truncate:<VNARROWQ> (match_operand:VQN 1 "register_operand"
"w"))
          (truncate:<VNARROWQ> (match_operand:VQN 2 "register_operand"
"w"))))]
...

The documentation (
https://gcc.gnu.org/onlinedocs/gccint/Modifiers.html#Modifiers ) states:
...
'‘&’ does not obviate the need to write ‘=’.
...
which seems to state that '&' implies '='.

An earlyclobber operand is defined as 'modified before the instruction
is finished using the input operands'. AFAIU that would indeed exclude
the possibility that the earlyclobber operand is an input/output operand
it self, but perhaps I misunderstand.

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.

--
Marc Glisse


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