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Fix internal compiler error on "var2 = var1 + 1" patterns
This appears to be the correct fix. It was provided by Tormod Volden
(debian.tormod@gmail.com). The final commit is substantially different from
Tormod's submission mostly due to housecleaning (removing the old patches
and updating the README). Tormod's comments follow.
The original addhi_mem_1 "insn" instruction pattern /matches/ two
memory operands, just with the /constraint/ that these are the same
location. A pattern match tells the compiler "you should be able to use
this, but you might have to work on it to meet the constraints". For
typical constraints on registers the compiler can add "reloads", moving
stuff between registers or from memory, until the constraints are met
and the instruction can be used. However, in this case, no amount of
reloads can make two memory locations the same if they already weren't,
so the compiler breaks down and cries "unable to generate reloads".
It seems this issue only appears if optimization is enabled. The proof
is in gcc's reload.c and is left as an exercise to the reader.
Limiting the matching pattern to identical memory operands avoids
these situations, while allowing the common "var++" cases.
References:
The pattern/constraints difference is explained in
https://gcc.gnu.org/onlinedocs/gccint/Simple-Constraints.html#index-other-register-constraints-3335
| author | William Astle <lost@l-w.ca> |
|---|---|
| date | Tue, 29 Mar 2016 21:21:49 -0600 |
| parents | fc166b3bbae3 |
| children |
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