Being Sneaky in C

Re:Being Sneaky in C by pcwalton on Friday June 12, 2015 @08:57PM (#9709432)

LLVM can and will do it. It will assume it knows what a function named "memcpy" (for example) does and optimizes accordingly. (Look at TargetLibraryInfo.cpp and grep for LibFunc::memset in, for example, SimplifyLibCalls.cpp.)

(That said, I think TheLoneWolfling is being too strong with his/her claims. You can get modern compilers to avoid dangerous optimizations; it's just not for the faint of heart.)

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Re:Being Sneaky in C by TheLoneWolfling on Friday June 12, 2015 @10:02PM (#9709581)

I never said you couldn't get a particular compiler to. Or indeed, all current compilers.

I am saying that it's impossible to do so and remain in the realm of portable C / C++.

There is a distinction.

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Re:Being Sneaky in C by TheLoneWolfling on Friday June 12, 2015 @10:13PM (#9709599)

Also: isn't that a bug? Is there something in a C / C++ standard that states that a function named "memcpy" (for example) is necessarily the normal function?

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Re:Being Sneaky in C by pcwalton on Saturday June 13, 2015 @01:30AM (#9710006)

Compilers have been doing this for a long time. The optimizations that this enables are essential for performance. They shouldn't stop; if the spec prohibits it, the spec should change (and if it doesn't, the compilers should ignore the spec).

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Re:Being Sneaky in C by TheLoneWolfling on Saturday June 13, 2015 @01:44AM (#9710044)

Good to know about the first and last part.

And as for the second part... Meh. I don't see any optimizations that hard-coding calling something named "memcpy" (or whatever) does that cannot be enabled by looking at the actual code that gets linked. Albeit with more difficulty.

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