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combination of read/write and earlyclobber constraint modifier
- From: Tom de Vries <Tom_deVries at mentor dot com>
- To: Vladimir Makarov <vmakarov at redhat dot com>
- Cc: <gcc at gcc dot gnu dot org>
- Date: Tue, 1 Jul 2014 21:27:18 +0200
- Subject: combination of read/write and earlyclobber constraint modifier
- Authentication-results: sourceware.org; auth=none
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
F.i., this one in gcc/config/aarch64/aarch64-simd.md:
[(set (match_operand:<VNARROWQ2> 0 "register_operand" "+&w")
(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 ?