[PATCH][RFC]AArch64 SVE: Fix multiple comparison masks on inverted operands

Richard Sandiford richard.sandiford@arm.com
Mon Jun 14 14:50:09 GMT 2021


Tamar Christina <tamar.christina@arm.com> writes:
> Hi All,
>
> This RFC is trying to address the following inefficiency when vectorizing
> conditional statements with SVE.
>
> Consider the case
>
> void f10(double * restrict z, double * restrict w, double * restrict x,
> 	 double * restrict y, int n)
> {
>     for (int i = 0; i < n; i++) {
>         z[i] = (w[i] > 0) ? x[i] + w[i] : y[i] - w[i];
>     }
> }
>
>
> For which we currently generate at -O3:
>
> f10:
>         cmp     w4, 0
>         ble     .L1
>         mov     x5, 0
>         whilelo p1.d, wzr, w4
>         ptrue   p3.b, all
> .L3:
>         ld1d    z1.d, p1/z, [x1, x5, lsl 3]
>         fcmgt   p2.d, p1/z, z1.d, #0.0
>         fcmgt   p0.d, p3/z, z1.d, #0.0
>         ld1d    z2.d, p2/z, [x2, x5, lsl 3]
>         bic     p0.b, p3/z, p1.b, p0.b
>         ld1d    z0.d, p0/z, [x3, x5, lsl 3]
>         fsub    z0.d, p0/m, z0.d, z1.d
>         movprfx z0.d, p2/m, z1.d
>         fadd    z0.d, p2/m, z0.d, z2.d
>         st1d    z0.d, p1, [x0, x5, lsl 3]
>         incd    x5
>         whilelo p1.d, w5, w4
>         b.any   .L3
> .L1:
>         ret
>
> Notice that the condition for the else branch duplicates the same predicate as
> the then branch and then uses BIC to negate the results.
>
> The reason for this is that during instruction generation in the vectorizer we
> emit
>
>   mask__41.11_66 = vect__4.10_64 > vect_cst__65;
>   vec_mask_and_69 = mask__41.11_66 & loop_mask_63;
>   vec_mask_and_71 = mask__41.11_66 & loop_mask_63;
>   mask__43.16_73 = ~mask__41.11_66;
>   vec_mask_and_76 = mask__43.16_73 & loop_mask_63;
>   vec_mask_and_78 = mask__43.16_73 & loop_mask_63;
>
> which ultimately gets optimized to
>
>   mask__41.11_66 = vect__4.10_64 > { 0.0, ... };
>   vec_mask_and_69 = loop_mask_63 & mask__41.11_66;
>   mask__43.16_73 = ~mask__41.11_66;
>   vec_mask_and_76 = loop_mask_63 & mask__43.16_73;
>
> Notice how the negate is on the operation and not the predicate resulting from
> the operation.  When this is expanded this turns into RTL where the negate is on
> the compare directly.  This means the RTL is different from the one without the
> negate and so CSE is unable to recognize that they are essentially same operation.
>
> To fix this my patch changes it so you negate the mask rather than the operation
>
>   mask__41.13_55 = vect__4.12_53 > { 0.0, ... };
>   vec_mask_and_58 = loop_mask_52 & mask__41.13_55;
>   vec_mask_op_67 = ~vec_mask_and_58;
>   vec_mask_and_65 = loop_mask_52 & vec_mask_op_67;

But to me this looks like a pessimisation in gimple terms.  We've increased
the length of the critical path: vec_mask_and_65 now needs a chain of
4 operations instead of 3.

We also need to be careful not to pessimise the case in which the
comparison is an integer one.  At the moment we'll generate opposed
conditions, which is the intended behaviour:

.L3:
        ld1d    z1.d, p0/z, [x1, x5, lsl 3]
        cmpgt   p2.d, p0/z, z1.d, #0
        movprfx z2, z1
        scvtf   z2.d, p3/m, z1.d
        cmple   p1.d, p0/z, z1.d, #0
        ld1d    z0.d, p2/z, [x2, x5, lsl 3]
        ld1d    z1.d, p1/z, [x3, x5, lsl 3]
        fadd    z0.d, p2/m, z0.d, z2.d
        movprfx z0.d, p1/m, z1.d
        fsub    z0.d, p1/m, z0.d, z2.d
        st1d    z0.d, p0, [x0, x5, lsl 3]
        add     x5, x5, x6
        whilelo p0.d, w5, w4
        b.any   .L3

Could we handle the fcmp case using a 3->2 define_split instead: convert

   (set res (and (not (fcmp X Y)) Z)) ->
     (set res (fcmp X Y))
     (set res (and (not res) Z))

?

Thanks,
Richard


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