It is an extraordinarily difficult problem to transform scalar code into vector instructions. The only way to get even passable output from a vectorizing compiler is to write the code as vectors to begin with, such as with cross-platform assembly tools like Orc.
And even then you'll often end up significantly worse off than if you wrote the assembly by hand.
A run of Intel's compiler on the C versions of our DSP functions resulted in a grand total of one vectorization, which was done terribly, too.
The problem is that you used C, which doesn't have any syntax to represent meta-information about the problem you're trying to solve. When you write out C code to, say, add a list of numbers, it's hard for the compiler to optimize that. But it's very easy for the compiler when you tell it "sum this list of numbers".
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It is an extraordinarily difficult problem to transform scalar code into vector instructions. The only way to get even passable output from a vectorizing compiler is to write the code as vectors to begin with, such as with cross-platform assembly tools like Orc.
And even then you'll often end up significantly worse off than if you wrote the assembly by hand.
A run of Intel's compiler on the C versions of our DSP functions resulted in a grand total of one vectorization, which was done terribly, too.
The problem is that you used C, which doesn't have any syntax to represent meta-information about the problem you're trying to solve. When you write out C code to, say, add a list of numbers, it's hard for the compiler to optimize that. But it's very easy for the compiler when you tell it "sum this list of numbers".