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Does a compiler use all x86 instructions? (2010)

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Re: Does a compiler use all x86 instructions? (2010)

#31

Post by cptskippy »

api wrote: post #7
ajenner wrote: post #2

It doesn't, because there are lots of special-purpose x86 instructions that would be more trouble than they're worth to teach a compiler about. For example, instructions for accessing particular CPU features that the C and C++ languages have no concept of (cryptographic acceleration and instructions used for OS kernel code spring to mind). Some of these the compiler might know about via intrinsic functions, but won't…

There are also AFIAK a few "deprecated" instructions that are implemented for backward compatibility but do not perform well on modern cores or have much better modern alternatives. These would be things like old MMX instructions, cruft left over from the 16-bit DOS days, etc. X86 is crufty. Of course all old architectures are crufty, and using microcode it's probably possible to keep the cruft from taking up much si…

Since P6, Intel's CPUs have used a RISC like core with a very heavy decoder that translates x86 CISC instructions to run on the internal ISA. With that in mind, do older or lesser used instructions actually perform poorly or are they just the wrong choice but actually preferred for other scenarios?

Re: Does a compiler use all x86 instructions? (2010)

#33

Post by terminalcommand »

sytelus wrote: post #25
Hydraulix989 wrote: post #13

mov is Turing complete

Source?

Here is a paper and a mov only compiler. http://www.cl.cam.ac.uk/~sd601/papers/mov.pdf, https://github.com/xoreaxeaxeax/movfuscator

PS: Found these links on hn.algolia.com, the HN threads are: https://news.ycombinator.com/item?id=6309631 https://news.ycombinator.com/item?id=9751312

Re: Does a compiler use all x86 instructions? (2010)

#34

Post by [deleted] »

api wrote: post #7
ajenner wrote: post #2

It doesn't, because there are lots of special-purpose x86 instructions that would be more trouble than they're worth to teach a compiler about. For example, instructions for accessing particular CPU features that the C and C++ languages have no concept of (cryptographic acceleration and instructions used for OS kernel code spring to mind). Some of these the compiler might know about via intrinsic functions, but won't…

There are also AFIAK a few "deprecated" instructions that are implemented for backward compatibility but do not perform well on modern cores or have much better modern alternatives. These would be things like old MMX instructions, cruft left over from the 16-bit DOS days, etc. X86 is crufty. Of course all old architectures are crufty, and using microcode it's probably possible to keep the cruft from taking up much si…

[deleted]

Re: Does a compiler use all x86 instructions? (2010)

#38

Post by wscott »

shanemhansen wrote: post #24

There are instructions that would almost never be useful. See Linus's rant on cmov http://yarchive.net/comp/linux/cmov.html The tl;dr is that it would only be useful if you are trying to optimize the size of a binary.

It is _very_ useful when hand optimizing loops in assembly. But for the compiler, the newer processors have such incredible branch predictors it is usually a bad idea for the compiler assume a branch is poorly predicted.

Now if you are using profile guided optimizations (--prof-gen/prof-use) AND you use the processor's performance counters as part of the feedback to the compiler _then_ I could see the compiler correctly using this instruction.

Re: Does a compiler use all x86 instructions? (2010)

#39

Post by Nelson69 »

shanemhansen wrote: post #24

There are instructions that would almost never be useful. See Linus's rant on cmov http://yarchive.net/comp/linux/cmov.html The tl;dr is that it would only be useful if you are trying to optimize the size of a binary.

There are also very useful instructions that don't have semantics for most programming languages and have little or no use outside of booting a system. Like LGDT and some of the tricks to switch from real mode to protected mode.

What's maybe more curious, some instructions in certain modes aren't even implemented by some assemblers. I don't think it's entirely unusual to see hand crafted opcodes in an OS boot sequence. When the assembler doesn't do it, the compiler certainly isn't going to.

Re: Does a compiler use all x86 instructions? (2010)

#40

Post by emily-c »

ajenner wrote: post #2

It doesn't, because there are lots of special-purpose x86 instructions that would be more trouble than they're worth to teach a compiler about. For example, instructions for accessing particular CPU features that the C and C++ languages have no concept of (cryptographic acceleration and instructions used for OS kernel code spring to mind). Some of these the compiler might know about via intrinsic functions, but won't…

lea's addressing mode arithmetic also doesn't wipe out any flags so it can be extremely useful for throwing in between arithmetic operations whose resulting flags you care about and the compare operation itself so as to obtain a greater degree of instruction level parallelism.
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