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Re: Byteswapping floating point types
On Jul 14, 2009, at 4:44 PM, Andrew Troschinetz wrote:
Testing with GCC 3.5.x to GCC 4.x has shown that this issue doesn't
surface unless you attempt some mathematical operation on the float
containing a byteswaped value. In other words:
That was me talking about results on my x86_64 machine, I went to my
i686 machine instead of my x86_64 machine and tried this code: http://codepad.org/OZhEz0az
If I use GCC 2.95, or GCC 3.4.6 to compile the code and run it, the
problem presents itself. But if I use GCC 4.1.2 the code compiles and
runs just fine.
Unfortunately we're in a position where we need to support at least
GCC 3.4.6 on i686 hardware, so this problem affects us. My co-worker
has, predictably, expressed dismay that returning a float as a byte-
sequence, accommodating unsigned int type, or a user defined swapped
type is unnecessarily complicated. And that if the test fails it is
because of some kind of misconfiguration or compiler bug. He actually
suggested a comment in the header file that says "this may not work on
floating point types" as a good resolution to this issue. D:
On Jul 15, 2009, at 12:20 PM, Andrew Haley wrote:
Well, it looks like our lovely (?) hardware is indeed silently
converting a signaling NaN to a quiet NaN, as I guessed. Thanks for
the test case; it's always nice to know the real reason for things.
Doesn't this new information suggest that the problem is in the
compiler, and not the hardware? And might there a way perhaps to force
GCC to generate code that will not cause this problem to arise? Maybe
something like --no-nan-quietification?
I tried -fsignaling-nans but that didn't do the trick.
--
Andrew Troschinetz
Applied Research Laboratories