Type punning confusion
- From: Finn Wilcox <finnw@xxxxxxxxxxx>
- To: LuaJIT mailing list <luajit@xxxxxxxxxxxxx>
- Date: Wed, 06 Nov 2013 21:40:51 +0000
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Is this a legitimate use of type punning via a union?
The assert fails shortly after entering the compiled loop (next time
the i<32 branch is taken.) Is this related to aliasing rules? Are
there caveats when using a union for type punning? Or is it that I
should not be comparing a cdata value type to itself? (That seems to
work most of the time.)
It works with -joff, or if I create 2 unions and ffi.copy() from one
to the other.
- --
local ffi = require 'ffi'
assert(ffi.abi 'le')
local s = ffi.new [[
union {
int64_t q;
struct { int32_t lo, hi; };
}
]]
local function oneBit(i)
i = bit.band(i, 63)
s.q = 0
if i < 32 then
s.lo = bit.rol(1, i)
else
s.hi = bit.rol(1, i-32)
end
return s.q
end
for i = 1, 999 do
assert(oneBit(i) == oneBit(i))
end
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