ra: Disable round-robin strategy for optimistically colorable nodes

Graphics / Mesa 3D Graphics Library / Mesa - Francisco Jerez [riseup.net] - 23 February 2015 12:55 UTC

The round-robin allocation strategy is expected to decrease the amount of false dependencies created by the register allocator and give the post-RA scheduling pass more freedom to move instructions around. On the other hand it has the disadvantage of increasing fragmentation and decreasing the number of equally-colored nearby nodes, what increases the likelihood of failure in presence of optimistically colorable nodes.

This patch disables the round-robin strategy for optimistically colorable nodes. These typically arise in situations of high register pressure or for registers with large live intervals, in both cases the task of the instruction scheduler shouldn't be constrained excessively by the dense packing of those nodes, and a spill (or on Intel hardware a fall-back to SIMD8 mode) is invariably worse than a slightly less optimal scheduling.

Shader-db results on the i965 driver:

total instructions in shared programs: 5488539 -> 5488489 (-0.00%) instructions in affected programs: 1121 -> 1071 (-4.46%) helped: 1 HURT: 0 GAINED: 49 LOST: 5

v2: Re-enable round-robin already for the lowest one of the nodes pushed optimistically onto the sack (Connor).
v3: Use UINT_MAX instead of ~0, open-code MIN2 (Jason, Connor).

f80af89 ra: Disable round-robin strategy for optimistically colorable nodes.
src/util/register_allocate.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)

Upstream: cgit.freedesktop.org

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