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In C, if a floating point cannot be represented exactly as an integer,
conversion from the former to the latter is implementation-defined.
Therefore, it can be flaky to test this against QBE-defined behavior.
This was discovered from (unsigned int) 4294967295.0f being an UB,
because (uint64_t) 4294967295.0f is 4294967296 > UINT_MAX
on amd64 when compiled by either gcc or clang.
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signed int can't represent all the values of unsigned int, so we
need to do the conversion to signed long, and use the lower 32 bits
as the result.
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amd64 lacks instruction for this so it has to be implemented with
float -> signed casts. The approach is borrowed from llvm.
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amd64 lacks an instruction for this so it has to be implemented with
signed -> float casts:
- Word casting is done by zero-extending the word to a long and then doing
a regular signed cast.
- Long casting is done by dividing by two with correct rounding if the
highest bit is set and casting that to float, then adding
1 to mantissa with integer addition
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