When __builtin_memcpy is replaced with libc's memcpy

Question

There is a version of C99/posix memcpy function in GCC: __builtin_memcpy.

Sometimes it can be replaced by GCC to inline version of memcpy and in other cases it is replaced by call to libc's memcpy. E.g. it was noted here:

Finally, on a compiler note, __builtin_memcpy can fall back to emitting a memcpy function call.

What is the logic in this selection? Is it logic the same in other gcc-compatible compilers, like clang/llvm, intel c++ compiler, PCC, suncc (oracle studio)?

When I should prefer of using __builtin_memcpy over plain memcpy?

It appears that it uses an inline version when the size is a compile time constant and it's less than or equal to 8192. — Z boson, Aug 14 '14 at 17:06

score 20 · Accepted Answer · edited Mar 28 '19 at 19:18

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I had been experimenting with the builtin replacement some time ago and I found out that the <string.h> functions are only replaced when the size of the source argument can be known at compile time. In which case the call to libc is replaced directly by unrolled code.

Unless you compile with -fno-builtin, -ansi, -std=c89 or something similar, it actually doesn't matter wether you use the __builtin_ prefix or not.

Although it's hard to follow, the code that deciedes whether to emit a library call or a chunk of code seems to be here.

edited Mar 28 '19 at 19:18

Marco Bonelli

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answered Sep 04 '12 at 23:05

C2H5OH

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Which gcc version and optimisation level was studied? – osgx Sep 05 '12 at 00:52
`gcc` version 4.6 using `-O3` and `-fomit-frame-pointer`. Before answering I tested with 4.7 as well. – C2H5OH Sep 05 '12 at 18:06
Sorry, my bad. The functions are replaced from the first optimization level (`-O1` or simply `-O`). – C2H5OH Sep 06 '12 at 13:33

When __builtin_memcpy is replaced with libc's memcpy

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