I'd like gcc's autovectorization (i.e. not intrinsics) to convert 0xPQ to the 64-bit value 0xPQPQPQPQPQPQPQPQ using the ssse3 opcode pshufb. However, even though I can see pshufb opcodes being output by gcc for other uses (so the compiler is definitely able to output it), I can't work out the series of C instructions needed to trigger it for this particualr case.
Any suggestions? Thanks!
I doubt that pshufb will be the most efficient solution, unless you intend to have the result in the lower part of an xmm register. If you do, provide an actual usage example.
If you write something like:
long long foo(char x)
{
long long ret;
std::memset(&ret, x, sizeof ret);
return ret;
}
Both gcc and clang essentially just multiply x by 0x0101010101010101 which is as fast as a pshufb (assuming you have that value in a register already). However, with imul you have the result already in a general purpose register (and no additional movq is required).
Godbolt compilation results: https://godbolt.org/z/dTvcsM (the -msse3 makes no difference, nor do other compilation options, as long as it is at least -O1).