Is it necessary to save the FPU state here?

Question

I wrote a simple cooperative multi-threading library. Currently I always save and restore the fpu state with fxsave / fxrstor when switching to a new context. But is this necessary in the cdecl calling convention?

As a simple example:

float thread_using_fpu(float x)
{
    float y = x / 2; // do some fpu operation
    yield();         // context switch, possibly altering fpu state.
    y = y / 2;       // another fpu operation
    return y;
}

May the compiler make any assumptions about the FPU state after the call to yield()?

No, the usual convention mandates FPU state be empty upon entry and exit (unless used for return value obviously). — Jester, Mar 12 '16 at 22:17
Thanks, do you happen to have a source for this? I couldn't find much about this myself. Not having to save and restore a 512-byte buffer every time would really help improve performance, and I want to be 100% sure this won't cause any issues. — user5434231, Mar 12 '16 at 23:15

Rob L · Answer 1 · 2016-03-12T23:12:57.957

3

No. You don't have to do any saving of the state. If one thread is in the middle of a floating point calculation where there is, for example, a denormalized flag set, and that thread is interrupted, then when it resumes the O/S or kernel will set the flags, just like it will restore other registers. Likewise, you don't have to worry about it in a yield().

Edit: If you are doing your own context switching, it is possible you would need to save the precision and rounding control flags if you need to set them to non-default values. Otherwise, again you're fine.

edited Mar 12 '16 at 23:12

answered Mar 12 '16 at 22:32

Rob L

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Just to clarify, I'm doing my own context switching, not using OS-managed threads. – user5434231 Mar 12 '16 at 23:10

Jester · Accepted Answer · 2016-03-12T23:59:08.420

As per the The SYSTEM V APPLICATION BINARY INTERFACE Intel386TM Architecture Processor Supplement, page 3-12:

%st(0): If the function does not return a floating-point value, then this register must be empty. This register must be empty before entry to a function.

%st(1) through %st(7): Floating-point scratch registers have no specified role in the standard calling sequence. These registers must be empty before entry and upon exit from a function.

Thus, you do not need to context switch them.

Another, newer version says this:

The CPU shall be in x87 mode upon entry to a function. Therefore, every function that uses the MMX registers is required to issue an emms or femms instruction after using MMX registers, before returning or calling another function. [...] The control bits of the MXCSR register are callee-saved (preserved across calls), while the status bits are caller-saved (not preserved). The x87 status word register is caller-saved, whereas the x87 control word is callee-saved. [...] All x87 registers are caller-saved, so callees that make use of the MMX registers may use the faster femms instruction.

So, you may need to save the control word.

Is it necessary to save the FPU state here?

2 Answers2