5 ms·

Worth noting this is for the x87 fsin instruction. For x86_64 typically a library function is called (which in turn uses SSE2 math instead of x87) rather than g

by jabl 8y ago

Worth noting this is for the x87 fsin instruction. For x86_64 typically a library function is called (which in turn uses SSE2 math instead of x87) rather than generating inline code.

ajross 8y ago
More "pervasively" than merely typically. The x87 registers still exist in a technical sense, but except for compatibility in legacy binaries those instructions are never used.
- burfog 8y ago
  They are used by gcc on x86_64 Ubuntu when dealing with long double. I just tried it, with a trivial function that adds arguments and returns the resulting sum. I think this is true on any x86_64 platform for which sizeof(long double) exceeds 8.
  - ajross 8y ago
    Sure, because "long double" is fundamentally an x86-specific compiler extension[1] designed to exploit the 80 bit FPU precision in the 8087 architecture. Nothing for an SSE (or any other 64 bit IEEE FPU) will be using long double, except for compatibility. [1] Yeah, I think there's a software emulation layer to give you 128 bits on other platforms or somesuch. Again, not important nor relevant to a discussion abou FSIN.
Joky 8y ago
I’m puzzled by what you mean here: you refer to an x87 instruction, which makes me think it is assembly/binary level and then you’re saying that a library function is called. Where is the magic? Is there an x86 feature where you can instruct the CPU to jump to an address when a specific instruction is encountered? Or are you maybe using “library” for microcode in the CPU itself? Edit: or are you referring to fsin at the C language level (in which case the x87 is hard to relate)
- AlotOfReading 8y ago
  There is an extension to the original x86 instruction set called x87 which has an instruction called fsin. Unfortunately, it's very slow and somewhat inaccurate. On newer processors it's actually faster to call a function using SSE2 the normal way instead of emitting a single fsin instruction.
  - microcolonel 8y ago
    To somewhat clarify, x87 is the instruction set of the Intel 8087 and its successors, which were floating point coprocessors designed to work with x86 family processors. The coprocessor was eventually integrated into x86 chips. The design was, in retrospect, rather poor, and so SSE2 was designed with (mostly) replacing x87 in mind (along with narrow SIMD operations), and since then x87 instructions have largely been second-class.
    - shawnz 8y ago
      Why wasn't the x87 instruction updated to use the implementation of the SSE2 instruction?
      - microcolonel 8y ago
        In order for that to work well, I have a hunch that they would need to do some very dirty tricks with register renaming, and add a 80-bit ("long double") mode to the SSE hardware, and I suspect that's why they are not as fast as eachother (whether that's as a result of fully separate implementation, or very conservative microcode [possibly involving fencing and swapping registers, emulation of arithmetic quirks of x87]) in practice. Though, disclaimer: I am not even really an amateur at this level of analysis, so take what I've said with its very own salt lick.
      - DiabloD3 8y ago
        Because then it would no longer comply with the x87 specification.

by jabl 8y ago

Worth noting this is for the x87 fsin instruction. For x86_64 typically a library function is called (which in turn uses SSE2 math instead of x87) rather than generating inline code.

ajross 8y ago

More "pervasively" than merely typically. The x87 registers still exist in a technical sense, but except for compatibility in legacy binaries those instructions are never used.

burfog 8y ago
They are used by gcc on x86_64 Ubuntu when dealing with long double. I just tried it, with a trivial function that adds arguments and returns the resulting sum. I think this is true on any x86_64 platform for which sizeof(long double) exceeds 8.
- ajross 8y ago
  Sure, because "long double" is fundamentally an x86-specific compiler extension[1] designed to exploit the 80 bit FPU precision in the 8087 architecture. Nothing for an SSE (or any other 64 bit IEEE FPU) will be using long double, except for compatibility. [1] Yeah, I think there's a software emulation layer to give you 128 bits on other platforms or somesuch. Again, not important nor relevant to a discussion abou FSIN.

Joky 8y ago

I’m puzzled by what you mean here: you refer to an x87 instruction, which makes me think it is assembly/binary level and then you’re saying that a library function is called. Where is the magic? Is there an x86 feature where you can instruct the CPU to jump to an address when a specific instruction is encountered? Or are you maybe using “library” for microcode in the CPU itself? Edit: or are you referring to fsin at the C language level (in which case the x87 is hard to relate)

AlotOfReading 8y ago
There is an extension to the original x86 instruction set called x87 which has an instruction called fsin. Unfortunately, it's very slow and somewhat inaccurate. On newer processors it's actually faster to call a function using SSE2 the normal way instead of emitting a single fsin instruction.
- microcolonel 8y ago
  To somewhat clarify, x87 is the instruction set of the Intel 8087 and its successors, which were floating point coprocessors designed to work with x86 family processors. The coprocessor was eventually integrated into x86 chips. The design was, in retrospect, rather poor, and so SSE2 was designed with (mostly) replacing x87 in mind (along with narrow SIMD operations), and since then x87 instructions have largely been second-class.
  - shawnz 8y ago
    Why wasn't the x87 instruction updated to use the implementation of the SSE2 instruction?
    - microcolonel 8y ago
      In order for that to work well, I have a hunch that they would need to do some very dirty tricks with register renaming, and add a 80-bit ("long double") mode to the SSE hardware, and I suspect that's why they are not as fast as eachother (whether that's as a result of fully separate implementation, or very conservative microcode [possibly involving fencing and swapping registers, emulation of arithmetic quirks of x87]) in practice. Though, disclaimer: I am not even really an amateur at this level of analysis, so take what I've said with its very own salt lick.
    - DiabloD3 8y ago
      Because then it would no longer comply with the x87 specification.