variable-shiftA shift by a distance read at run time

Reported by --explain after a verdict.

mettle explain variable-shift

The kernels carry a shift only when the distance is written in the source. A distance the loop reads has no kernel, in either direction:

b[i] = a[i] >> k;      // `k` a parameter or a variable
b[i] = a[i] << k;

Both stay scalar. A constant distance vectorizes:

b[i] = a[i] >> 8;

There is no source rewrite that reaches a kernel while the distance stays a run-time value, so this is a gap in the compiler rather than a problem with the loop. Splitting into one loop per distance works when the distances are few and known.

Related

• call-in-bodyThe loop body calls a function
• extern-call-in-bodyThe loop body calls an external function
• indirect-callThe loop body calls through a function pointer
• alloc-in-bodyThe loop body allocates
• inline-asmThe loop body contains inline assembly
• control-flowThe loop body branches
• early-exitThe loop can leave before its trip count
• int16-elements16-bit integer elements have no kernel