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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)

#2

Post by ajenner »

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 generate for pure C/C++ code.

Regarding the large number of LEA instructions in x86 code - this is actually a very useful instruction for doing several mathematical operations very compactly. You can multiply the value in a register by 1, 2, 4 or 8, add the value in another register (which can be the same one, yielding multiplication by 3, 5 or 9), add a constant value and place the result in a third register.

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

#3

Post by Majster »

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…

Could you provide some links or examples? I'd love to learn more about this.

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

#7

Post by api »

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 silicon.

There are also instructions not universally available. A typical compiler will not emit these unless told to do so or unless they are coded in intrinsics or inline ASM.

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

#9

Post by nneonneo »

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 is also used frequently to actually load addresses. When passing the address of a stack buffer, for example, the compiler will usually generate code like "lea eax, [ebp-0x80]; push eax". Or, when loading the address of a struct member, you might have "lea ecx, [eax+8]".
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