Three arithmetic situations had no defined behaviour and the two backends
disagreed about all three: a divide or remainder by zero, which was a raw
SIGFPE with no message and no location; (/ min -1), whose quotient is one past
the top of the type; and a float to integer cast whose value does not fit,
which LLVM called undefined and would fold to anything.
They now signal ArithError with `error`, exactly as a bad index signals
BoundsError, and die with a sentence naming the file, the line and the operands
only if nothing answered. The guards ride the same --checks flag as the bounds
check and are elided with it.
No restart is established at the failing operation. The sketch this started
from asked for use-value, and the implementation ruled it out: a restart frame
is allocated by the restart-case that offers it, on its own stack, so the
runtime cannot hold one on a program's behalf and use-value here would mean an
alloca and a restart frame at every division in every checked build. That is
the cost already refused for indexing, buying a silently different answer.
The x86 backend is unchanged and is the next commit.
Joseph Ferano
a431cddd3b