[Mesa-dev] [PATCH] i965: Fix output register sizes when variable ranges are interleaved

[Neil Roberts]

In 6f5abf31466aed this code was fixed to calculate the maximum size of
an attribute in a seperate pass and then allocate the registers to
that size. However this wasn’t taking into account ranges that overlap
but don’t have the same starting location. For example:

layout(location = 0, component = 0) out float a[4];
layout(location = 2, component = 1) out float b[4];

Previously, if ‘a’ was processed first then it would allocate a
register of size 4 for location 0 and it wouldn’t allocate another
register for location 2 because it would already be covered by the
range of 0. Then if something tries to write to b[2] it would try to
write past the end of the register allocated for ‘a’ and it would hit
an assert.

This patch changes it to scan for any overlapping ranges that start
within each range to calculate the maximum extent and allocate that
instead.

Fixed Piglit’s arb_enhanced_layouts/execution/component-layout/
vs-fs-array-interleave-range.shader_test
---

For what it’s worth I also made a simplified test for this for
VkRunner here:

https://github.com/Igalia/vkrunner/blob/tests/examples/overlap-outputs.shader_test

 src/intel/compiler/brw_fs_nir.cpp | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/src/intel/compiler/brw_fs_nir.cpp b/src/intel/compiler/brw_fs_nir.cpp
index 1ce89520bf1..b179018e5e8 100644
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -67,14 +67,25 @@ fs_visitor::nir_setup_outputs()
       vec4s[loc] = MAX2(vec4s[loc], var_vec4s);
    }
 
-   nir_foreach_variable(var, &nir->outputs) {
-      const int loc = var->data.driver_location;
-      if (outputs[loc].file == BAD_FILE) {
-         fs_reg reg = bld.vgrf(BRW_REGISTER_TYPE_F, 4 * vec4s[loc]);
-         for (unsigned i = 0; i < vec4s[loc]; i++) {
-            outputs[loc + i] = offset(reg, bld, 4 * i);
-         }
+   for (unsigned loc = 0; loc < ARRAY_SIZE(vec4s);) {
+      if (vec4s[loc] == 0) {
+         loc++;
+         continue;
       }
+
+      unsigned reg_size = vec4s[loc];
+
+      /* Check if there are any ranges that start within this range and extend
+       * past it. If so, include them in this allocation.
+       */
+      for (unsigned i = 1; i < reg_size; i++)
+         reg_size = MAX2(vec4s[i + loc] + i, reg_size);
+
+      fs_reg reg = bld.vgrf(BRW_REGISTER_TYPE_F, 4 * reg_size);
+      for (unsigned i = 0; i < reg_size; i++)
+         outputs[loc + i] = offset(reg, bld, 4 * i);
+
+      loc += reg_size;
    }
 }
 
-- 
2.14.3