Re: [PATCH]AArch64 fix regexp for live_1.c sve test

[Richard Sandiford <richard.sandiford@arm.com>]

Tamar Christina <tamar.christina@arm.com> writes:
> Hi All,
>
> The resulting predicate register of a whilelo is not
> restricted to the lower half of the predicate register file.
>
> As such these tests started failing after recent changes
> because the whilelo outside the loop is getting assigned p15.

It's the whilelo in the loop for me.  We go from:

.L3:
        ld1b    z31.b, p7/z, [x4, x3]
        movprfx z30, z31
        mul     z30.b, p5/m, z30.b, z29.b
        st1b    z30.b, p7, [x4, x3]
        mov     p6.b, p7.b
        add     x3, x3, x0
        whilelo p7.b, w3, w1
        b.any   .L3

to:

.L3:
        ld1b    z31.b, p7/z, [x3, x2]
        movprfx z29, z31
        mul     z29.b, p6/m, z29.b, z30.b
        st1b    z29.b, p7, [x3, x2]
        add     x2, x2, x0
        whilelo p15.b, w2, w1
        b.any   .L4
        [...]
        .p2align 2,,3
.L4:
        mov     p7.b, p15.b
        b       .L3

This adds an extra (admittedly unconditional) branch to every non-final
vector iteration, which seems unfortunate.  I don't think we'd see
p8-p15 otherwise, since the result of the whilelo is used as a
governing predicate by the next iteration of the loop.

This happens because the scalar loop is given an 89% chance of iterating.
Previously we gave the vector loop an 83.33% chance of iterating, whereas
after 061f74c06735e1fa35b910ae we give it a 12% chance.  0.89^16 == 15.50%,
so the new probabilities definitely preserve the original probabilities
more closely.  But for purely heuristic probabilities like these, I'm
not sure we should lean so heavily into the idea that the vector
latch is unlikely.

Honza, Richi, any thoughts?  Just wanted to double-check that this
was operating as expected before making the tests accept the (arguably)
less efficient code.  It looks like the commit was more aimed at fixing
the profile counts for the epilogues, rather than the main loop.

Thanks,
Richard

> This widens the regexp.
>
> Tested on aarch64-none-linux-gnu and passes again.
>
> Ok for master?
>
> Thanks,
> Tamar
>
> gcc/testsuite/ChangeLog:
>
> 	* gcc.target/aarch64/sve/live_1.c: Update assembly.
>
> --- inline copy of patch -- 
> diff --git a/gcc/testsuite/gcc.target/aarch64/sve/live_1.c b/gcc/testsuite/gcc.target/aarch64/sve/live_1.c
> index 80ee176d1807bf628ad47551d69ff5d84deda79e..2db6c3c209a9514646e92628f3d2dd58d466539c 100644
> --- a/gcc/testsuite/gcc.target/aarch64/sve/live_1.c
> +++ b/gcc/testsuite/gcc.target/aarch64/sve/live_1.c
> @@ -27,10 +27,10 @@
>  
>  TEST_ALL (EXTRACT_LAST)
>  
> -/* { dg-final { scan-assembler-times {\twhilelo\tp[0-7].b, } 2 } } */
> -/* { dg-final { scan-assembler-times {\twhilelo\tp[0-7].h, } 4 } } */
> -/* { dg-final { scan-assembler-times {\twhilelo\tp[0-7].s, } 4 } } */
> -/* { dg-final { scan-assembler-times {\twhilelo\tp[0-7].d, } 4 } } */
> +/* { dg-final { scan-assembler-times {\twhilelo\tp[0-9]+.b, } 2 } } */
> +/* { dg-final { scan-assembler-times {\twhilelo\tp[0-9]+.h, } 4 } } */
> +/* { dg-final { scan-assembler-times {\twhilelo\tp[0-9]+.s, } 4 } } */
> +/* { dg-final { scan-assembler-times {\twhilelo\tp[0-9]+.d, } 4 } } */
>  
>  /* { dg-final { scan-assembler-times {\tlastb\tb[0-9]+, p[0-7], z[0-9]+\.b\n} 1 } } */
>  /* { dg-final { scan-assembler-times {\tlastb\th[0-9]+, p[0-7], z[0-9]+\.h\n} 2 } } */