Question

What is the relevance of Stack Overflow question/answer Why does changing 0.1f to 0 slow down performance by 10x? for Objective-C? If there is any relevance, how should this change my coding habits? Is there some way to shut off denormalized floating points on Mac OS X?

It seems like this is completely irrelevant to iOS. Is that correct?

Answer 1

As I said in response to your comment there:

it is more of a CPU than a language issue, so it probably has
relevance for Objective-C on x86. (iPhone’s ARMv7 doesn’t seem to support
denormalized floats, at least with the default runtime/build settings)

Update

I just tested. On Mac OS X on x86 the slowdown is observed, on iOS on ARMv7 it is not (default build settings).

And as to be expected, running on iOS simulator (on x86) denormalized floats appear again.

Interestingly, FLT_MIN and DBL_MIN respectively are defined to the smallest non-denormalized number (on iOS, Mac OS X, and Linux). Strange things happen using

DBL_MIN/2.0

in your code; the compiler happily sets a denormalized constant, but as soon as the (arm) CPU touches it, it is set to zero:

double test = DBL_MIN/2.0;
printf("test      == 0.0 %d\n",test==0.0);
printf("DBL_MIN/2 == 0.0 %d\n",DBL_MIN/2.0==0.0);

Outputs:

test      == 0.0 1  // computer says YES
DBL_MIN/2 == 0.0 0  // compiler says NO

So a quick runtime check if denormalization is supported can be:

#define SUPPORT_DENORMALIZATION ({volatile double t=DBL_MIN/2.0;t!=0.0;})

(“given without even the implied warranty of fitness for any purpose”)

This is what ARM has to say on flush to zero mode: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204h/Bcfheche.html

Update<<1

This is how you disable flush to zero mode on ARMv7:

int x;
asm(
    "vmrs %[result],FPSCR \r\n"
    "bic %[result],%[result],#16777216 \r\n"
    "vmsr FPSCR,%[result]"
    :[result] "=r" (x) : :
);
printf("ARM FPSCR: %08x\n",x);

with the following surprising result.

• Column 1: a float, divided by 2 for every iteration
• Column 2: the binary representation of this float
• Column 3: the time taken to sum this float 1e7 times

You can clearly see that the denormalization comes at zero cost. (For an iPad 2. On iPhone 4, it comes at a small cost of a 10% slowdown.)

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