Re: Help: How to determine big/little endian?

In describing his compiler (lcc)'s code to determine the host Endian-ness,
Chris Fraser (cwf@research.att.com) says that he uses:
...
union { char c; int i; } u;
u.i = 0; u.c = 0;
swap = (u.i == 1) != IR->little_endian;
...
I suspect this code is improved by using:
...
volatile union { char c; int i; } u;
u.i = 0; u.c = 0;
swap = (u.i == 1) != IR->little_endian;
...


So that the compiler may not decide to ``optimize'' this code into
...
union { char c; int i; } u;
u.i = 0; u.c = 0;
swap = (0 == 1) != IR->little_endian;
...
The fetch of an int from the union after storing a char is strictly
illegal. Since it is illegal, the compiler may assume the fetch must be
tied to a corresponding store, and the "u.i = 0;" strictly dominate the
fetch. So, my ``optimization'' should be a legal compiler optimization.
By making u volatile, each fetch and store must be accomplished by the
related sequence points, guaranteeing a strict order of
write u.i, write u.c, read u.i
Which is what we want.


-Scott David Daniels
daniels@cse.ogi.edu
[It occurs to me that changing the first 0 to 1 will also help a lot. -John]
--

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