Hi all,

In this testcase the codegen for VLA SVE is worse than it could be due to unrolling:

fully_peel_me:
         mov     x1, 5
         ptrue   p1.d, all
         whilelo p0.d, xzr, x1
         ld1d    z0.d, p0/z, [x0]
         fadd    z0.d, z0.d, z0.d
         st1d    z0.d, p0, [x0]
         cntd    x2
         addvl   x3, x0, #1
         whilelo p0.d, x2, x1
         beq     .L1
         ld1d    z0.d, p0/z, [x0, #1, mul vl]
         fadd    z0.d, z0.d, z0.d
         st1d    z0.d, p0, [x3]
         cntw    x2
         incb    x0, all, mul #2
         whilelo p0.d, x2, x1
         beq     .L1
         ld1d    z0.d, p0/z, [x0]
         fadd    z0.d, z0.d, z0.d
         st1d    z0.d, p0, [x0]
.L1:
         ret

In this case, due to the vector-length-agnostic nature of SVE the compiler doesn't know the loop iteration count.
For such loops we don't want to unroll if we don't end up eliminating branches as this just bloats code size
and hurts icache performance.

This patch introduces a new unroll-known-loop-iterations-only param that disables cunroll when the loop iteration
count is unknown (SCEV_NOT_KNOWN). This case occurs much more often for SVE VLA code, but it does help some
Advanced SIMD cases as well where loops with an unknown iteration count are not unrolled when it doesn't eliminate
the branches.

So for the above testcase we generate now:
fully_peel_me:
         mov     x2, 5
         mov     x3, x2
         mov     x1, 0
         whilelo p0.d, xzr, x2
         ptrue   p1.d, all
.L2:
         ld1d    z0.d, p0/z, [x0, x1, lsl 3]
         fadd    z0.d, z0.d, z0.d
         st1d    z0.d, p0, [x0, x1, lsl 3]
         incd    x1
         whilelo p0.d, x1, x3
         bne     .L2
         ret

Not perfect still, but it's preferable to the original code.
The new param is enabled by default on aarch64 but disabled for other targets, leaving their behaviour unchanged
(until other target people experiment with it and set it, if appropriate).

Bootstrapped and tested on aarch64-none-linux-gnu.
Benchmarked on SPEC2017 on a Cortex-A57 and there are no differences in performance.

Ok for trunk?
Thanks,
Kyrill


2018-11-09  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* params.def (PARAM_UNROLL_KNOWN_LOOP_ITERATIONS_ONLY): Define.
	* tree-ssa-loop-ivcanon.c (try_unroll_loop_completely): Use above to
	disable unrolling on unknown iteration count.
	* config/aarch64/aarch64.c (aarch64_override_options_internal): Set
	PARAM_UNROLL_KNOWN_LOOP_ITERATIONS_ONLY to 1.
	* doc/invoke.texi (--param unroll-known-loop-iterations-only):
	Document.

2018-11-09  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

	* gcc.target/aarch64/sve/unroll-1.c: New test.