[LLVMdev] Loop unrolling opportunity in SPEC's libquantum with profile info

[Sean Silva]

On Thu, Jan 16, 2014 at 1:02 PM, Diego Novillo <dnovillo at google.com> wrote:

> On Thu, Jan 16, 2014 at 9:26 AM, Nadav Rotem <nrotem at apple.com> wrote:
> > Hi Diego,
> >
> > It looks like the problem is with the code in the vectorizer that tries
> to estimate the most profitable vectorization factor:
> >
> >> LV: Found an estimated cost of 6 for VF 2 For instruction:   %3 = load
> >> i64* %state, align 8, !dbg !58, !tbaa !61
> >
> >
> > It looks like a cost model problem.  The vectorizer thinks that loading
> %3 (above) is non consecutive and would require scatter/gather.  Is that
> correct? I wonder that SCEV is reporting. Is there an index overflow
> problem that is preventing us from loading consecutive elements?
>
> Yes, I forgot to mention that. The access is non-consecutive:
>
>       for(i=0; i<reg->size; i++)
>          {
>              /* Flip the target bit of each basis state */
>              reg->node[i].state ^= ((MAX_UNSIGNED) 1 << target);
> }
>
> The code is writing to the 'state' field. The data structures look like
> this:
>
> typedef struct quantum_matrix_struct quantum_matrix;
>
> struct quantum_reg_node_struct
> {
>   COMPLEX_FLOAT amplitude; /* alpha_j */
>   MAX_UNSIGNED state;      /* j */
> };
>
> typedef struct quantum_reg_node_struct quantum_reg_node;
>
> /* The quantum register */
>
> struct quantum_reg_struct
> {
>   int width;    /* number of qubits in the qureg */
>   int size;     /* number of non-zero vectors */
>   int hashw;    /* width of the hash array */
>   quantum_reg_node *node;
>   int *hash;
> };
>
> If you do the trick of writing to a separate array, then the loop can
> be vectorized.
>

Or use scatter-gather.

-- Sean Silva


>
>
> Diego.
>
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