nir: detect more induction variables

Graphics / Mesa 3D Graphics Library / Mesa - Timothy Arceri [itsqueeze.com] - 12 December 2018 23:58 EST

This allows loop analysis to detect inductions variables that are incremented in both branches of an if rather than in a main loop block. For example:

loop { block block_1: /* preds: block_0 block_7 */
vec1 32 ssa_8 = phi block_0: ssa_4, block_7: ssa_20
vec1 32 ssa_9 = phi block_0: ssa_0, block_7: ssa_4
vec1 32 ssa_10 = phi block_0: ssa_1, block_7: ssa_4
vec1 32 ssa_11 = phi block_0: ssa_2, block_7: ssa_21
vec1 32 ssa_12 = phi block_0: ssa_3, block_7: ssa_22
vec4 32 ssa_13 = vec4 ssa_12, ssa_11, ssa_10, ssa_9
vec1 32 ssa_14 = ige ssa_8, ssa_5 /* succs: block_2 block_3 */ if ssa_14 { block block_2: /* preds: block_1 */ break /* succs: block_8 */ } else { block block_3: /* preds: block_1 */ /* succs: block_4 */ } block block_4: /* preds: block_3 */
vec1 32 ssa_15 = ilt ssa_6, ssa_8 /* succs: block_5 block_6 */ if ssa_15 { block block_5: /* preds: block_4 */
vec1 32 ssa_16 = iadd ssa_8, ssa_7
vec1 32 ssa_17 = load_const (0x3f800000 /* 1.000000*/) /* succs: block_7 */ } else { block block_6: /* preds: block_4 */
vec1 32 ssa_18 = iadd ssa_8, ssa_7
vec1 32 ssa_19 = load_const (0x3f800000 /* 1.000000*/) /* succs: block_7 */ } block block_7: /* preds: block_5 block_6 */
vec1 32 ssa_20 = phi block_5: ssa_16, block_6: ssa_18
vec1 32 ssa_21 = phi block_5: ssa_17, block_6: ssa_4
vec1 32 ssa_22 = phi block_5: ssa_4, block_6: ssa_19 /* succs: block_1 */ }

Unfortunatly GCM could move the addition out of the if for us (making this patch unrequired) but we still cannot enable the GCM pass without regressions.

This unrolls a loop in Rise of The Tomb Raider.

vkpipeline-db results (VEGA):

Totals from affected shaders: SGPRS: 88 -> 96 (9.09 %)
VGPRS: 56 -> 52 (-7.14 %) Spilled SGPRs: 0 -> 0 (0.00 %) Spilled VGPRs: 0 -> 0 (0.00 %) Private memory VGPRs: 0 -> 0 (0.00 %) Scratch size: 0 -> 0 (0.00 %) dwords per thread Code Size: 2168 -> 4560 (110.33 %) bytes LDS: 0 -> 0 (0.00 %) blocks Max Waves: 4 -> 4 (0.00 %) Wait states: 0 -> 0 (0.00 %)

9e6b39e1d5 nir: detect more induction variables
src/compiler/nir/nir_loop_analyze.c | 36 ++++++++++++++++++++++++++++++++++++
1 file changed, 36 insertions(+)

Upstream: cgit.freedesktop.org


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