Question

The 'select' instruction is used to choose one value based on a condition, without branching.

I want to know the differences between branching and select instructions (preferably for both x86 architectures and PTX). As far as I know, select is more optimal compared to branching instructions, but I don’t have a clear picture.

Answer 1

Branching is a general-purpose mechanism used to redirect control flow. It is used to implement most forms of the if statement (when specific optimizations don’t apply).

Selection is a specialized instruction available on some instruction sets which can implement some forms of the conditional expression

z = (cond) ? x : y;

or

if(cond) z = x;

provided that x and y are plain values (if they were expressions, they would both have to be computed before the select, which might incur performance penalties or incorrect side-effect evaluation). Such an instruction is necessarily more limited than branching, but has the distinct advantage that the instruction pointer doesn’t change. As a result, the processor does not need to flush its pipeline on a branch misprediction (since there is no branch). Because of this, a select instruction (where available) is faster.

On some superscalar architectures, e.g. CUDA, branches are very expensive performance-wise because the parallel units must remain perfectly synchronized. On CUDA, for example, every execution unit in a block must take the same execution path; if one thread branches, then every unit steps through both branches (but will execute no-operations on the branch not taken). A select instruction, however, doesn’t incur this kind of penalty.

Note that most compilers will, with suitable options, generate ‘select’-style instructions like cmov if given a simple-enough if statement. Also, in some cases, it is possible to use bitwise manipulation or logical operations to combine a boolean conditional with expression values without performing a branch.