Ahah, Apple does have different math
nntp.perl.org
nntp.perl.org
This isn't a bug. Both code paths produce results of the highest representable precision of the hardware in question. It's just that there are multiple hardware units capable of giving you the answer, and Apple's toolchain picks a different one than whatever Tom is using elsewhere.
And as has been pointed out in FooBarWidget's comment -- any code that relies on bit-precise results from floating point computation is almost certainly concealing precision bugs anyway. That's not the right way to approach floating point architecture.
x87 does floating point math with 80 bits (in registers). But the variables are stored in memory locations with 64 bits. So the results are rounded.
The problem comes from when to do the rounding. And that can vary depending on if a register is needed for something else, optimizations, the order of code, etc.
To put it another way, it isn't Apple doing something wrong. If your code depends on rounding errors in FP representations, your code is buggy.
That's also why good numerical algorithms are hard to design. You need to worry about losing precision. As a simple example, if you calculate (a-b)c and a is almost equal to b, you're much better off doing ac-b*c. These tricks are important.
For example, on x86, many C / C++ compilers interpret long double as a 64-bit float while others interpret it as 80-bit extended precision.
And extended precision TBYTE (in Intel syntax) does actually take up 10 bytes. There's a speed loss for not aligning, but no correctness loss. In a possibly related note, however, Apple has bizarre and unnecessarily strict alignment policies for Mac OS on x86:
That's a stretch. Just because it's called "GCC" doesn't mean it behaves exactly the same on every platform, even if the CPU architecture is the same.
Apple makes extensive modifications, has their own ABI (see below), etc.
In a possibly related note, however, Apple has bizarre and unnecessarily strict alignment policies for Mac OS on x86:
It's not "bizarre and unnecessarily strict" if you want to be able to rely on SSE2+. Apple had the advantage of being able to define their ABI without regard to most legacy concerns, and so they did.
The reason that it's strictly enforced everywhere is that since Apple's compilers use SSE2+ they must be able to assume that, at function entry, the stack is properly (16 byte) aligned.
I understand your pain -- I've had to update a JIT implementation to deal with this, along with quite a bit of assembly that assumed 4 byte alignment, but Apple's reasoning makes sense.
See also: http://stackoverflow.com/questions/612443/why-does-the-mac-a...
The stack has --known alignment-- on entry, which removes the need to compute alignment at runtime. Any other approach requires more instructions overall.
It's not so hard for the callee side of the ABI to make sure the stack is aligned if it's going to use SSE2 and friends; it's rather more onerous to require every call site to make the alignments for the benefit of the callee.
I disagree that it's onerous. It seems silly to increase the runtime costs in exchange for a minutely simplified compiler port. It's not as if non-4-byte aligned ABIs are unusual.
And it's not a "minutely simplified compiler port". That statement is startlingly naive. Do you have any idea how much hand-coded inline assembly, both in the runtime library and in customer code, needs to be carefully reviewed and modified to port from a platform without this requirement to one with it? Particularly since almost every other platform targeting the same architecture doesn't have the requirement?
SSE2 is used everywhere. That's unlikely.
And it's not a "minutely simplified compiler port". That statement is startlingly naive. Do you have any idea how much hand-coded inline assembly, both in the runtime library and in customer code, needs to be carefully reviewed and modified to port from a platform without this requirement to one with it? Particularly since almost every other platform targeting the same architecture doesn't have the requirement?
Do you have any idea what the advantages are of being able to use SSE2+ everywhere? I find your position to be startling naive, especially given the fact that the vast majority of the existing Mac OS X developer base did not have any hand-coded inline assembly targeted at x86-32.
Other than game developers, how many legacy x86-32 developers is Apple genuinely interested in courting? Even for game developers (or JIT authors, or otherwise) with an overabundance of x86 4-byte-alignment-assuming assembly, fixing stack alignment is an annoying issue, not an impossible one.
The reasoning makes sense and I'd have done the same. I fixed our code and moved on.
However, if you put the technical reasons aside, and only focus on the business reasons for making a choice here, it's clear to me that the best way to go is to fall in line with the existing precedents for the platform. That way maximizes your business upside. There is no business reason for wanting 16-byte alignment, only business reasons for not wanting it.
The technical case would need to outweigh the business case in order for it to win. But I don't see the technical case as being that strong. Floating point code is rare. Outside of scalable vector-oriented UI libraries and domain-specific number crunching, it's hardly ever used. Many architectures survived for decades with only optional support for floating point, in a coprocessor. Many embedded architectures still use emulated FP, if it's needed at all.
And I really meant it about shockingly naive back there. It tells me everything I need to know about what you know about commercial compilers: that you think of them in the academic sense of being the bit that turns text into code. There's more to it than that in the real world.
Wait a minute, does that mean that floating point types don't have a stable ABI? If I have a library with function foo(float bar) compiled with compiler A, then it's not safe to compile an app - that uses that library - with compiler B?
For example, the following C statement:
printf("%d\n", sizeof(long double));
Cygwin gcc: 12
MSVC: 8
Embarcadero bcc32: 10
Just `float`, though, is rather more solid.
Trying compiler constants first...
f is 0.12345, rounded to 0.1235, expanded to 0.123450003564357760000000000000
d is 0.12345, rounded to 0.1235, expanded to 0.123450000000000000000000000000
ld is 0.12345, rounded to 0.1235, expanded to 0.123450000000000000000000000000
Now trying derived values...
f is 0.12345, rounded to 0.1235, expanded to 0.123450003564357760000000000000
d is 0.12345, rounded to 0.1235, expanded to 0.123450000000000000000000000000
ld is 0.12345, rounded to 0.1235, expanded to 0.123450000000000000000000000000