[Mesa-dev] [PATCH 1/3] nir/opt_intrinsics: Fix values for gl_SubGroupG{e, t}MaskARB

[Neil Roberts]

Previously the values were calculated by just shifting ~0 by the
invocation ID. This would end up including bits that are higher than
gl_SubGroupSizeARB. The corresponding CTS test effectively requires that
these high bits be zero so it was failing. There is a Piglit test as
well but this appears to checking the wrong values so it passes.

For the two greater-than bitmasks, this patch adds an extra mask with
(~0>>(64-gl_SubGroupSizeARB)) to force these bits to zero.

Fixes: KHR-GL45.shader_ballot_tests.ShaderBallotBitmasks

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102680#c3
Signed-off-by: Neil Roberts <nroberts at igalia.com>
---
 src/compiler/nir/nir_opt_intrinsics.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/src/compiler/nir/nir_opt_intrinsics.c b/src/compiler/nir/nir_opt_intrinsics.c
index 26a0f96..d5fdc51 100644
--- a/src/compiler/nir/nir_opt_intrinsics.c
+++ b/src/compiler/nir/nir_opt_intrinsics.c
@@ -28,6 +28,26 @@
  * \file nir_opt_intrinsics.c
  */
 
+static nir_ssa_def *
+high_subgroup_mask(nir_builder *b,
+                   nir_ssa_def *count,
+                   uint64_t base_mask)
+{
+   /* group_mask could probably be calculated more efficiently but we want to
+    * be sure not to shift by 64 if the subgroup size is 64 because the GLSL
+    * shift operator is undefined in that case. In any case if we were worried
+    * about efficency this should probably be done further down because the
+    * subgroup size is likely to be known at compile time.
+    */
+   nir_ssa_def *subgroup_size = nir_load_subgroup_size(b);
+   nir_ssa_def *all_bits = nir_imm_int64(b, ~0ull);
+   nir_ssa_def *shift = nir_isub(b, nir_imm_int(b, 64), subgroup_size);
+   nir_ssa_def *group_mask = nir_ushr(b, all_bits, shift);
+   nir_ssa_def *higher_bits = nir_ishl(b, nir_imm_int64(b, base_mask), count);
+
+   return nir_iand(b, higher_bits, group_mask);
+}
+
 static bool
 opt_intrinsics_impl(nir_function_impl *impl)
 {
@@ -95,10 +115,10 @@ opt_intrinsics_impl(nir_function_impl *impl)
                replacement = nir_ishl(&b, nir_imm_int64(&b, 1ull), count);
                break;
             case nir_intrinsic_load_subgroup_ge_mask:
-               replacement = nir_ishl(&b, nir_imm_int64(&b, ~0ull), count);
+               replacement = high_subgroup_mask(&b, count, ~0ull);
                break;
             case nir_intrinsic_load_subgroup_gt_mask:
-               replacement = nir_ishl(&b, nir_imm_int64(&b, ~1ull), count);
+               replacement = high_subgroup_mask(&b, count, ~1ull);
                break;
             case nir_intrinsic_load_subgroup_le_mask:
                replacement = nir_inot(&b, nir_ishl(&b, nir_imm_int64(&b, ~1ull), count));
-- 
2.9.5