tgsi: add BALLOT/READ_* opcodes

v2 (Nicolai):
- BALLOT isn't per-channel
- expand the documentation (also for VOTE_*)

v3:
- only BALLOT returns a 64-bit lanemask (Boyan)
- relax the requirement on READ_INVOC: the invocation number to read
  from must be uniform within a sub-group. This matches the
  GL_ARB_shader_ballot spect (and the v_readlane instruction of AMD
  GCN)

v4:
- hopefully really fix the doc of VOTE_* returns (Ilia)

Signed-off-by: Ilia Mirkin <[email protected]>
Signed-off-by: Nicolai Hähnle <[email protected]>
Reviewed-by: Marek Olšák <[email protected]> (v2)

1 files changed, 57 insertions, 11 deletions

diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
index 05b06ce6f1e..9362d430f07 100644
--- a/src/gallium/docs/source/tgsi.rst
+++ b/src/gallium/docs/source/tgsi.rst
@@ -2859,22 +2859,68 @@ only be used with 32-bit integer image formats.
   resource[offset] = (dst_x > src_x ? dst_x : src_x)
 
 
-.. _voteopcodes:
+.. _interlaneopcodes:
+
+Inter-lane opcodes
+^^^^^^^^^^^^^^^^^^
+
+These opcodes reduce the given value across the shader invocations
+running in the current SIMD group. Every thread in the subgroup will receive
+the same result. The BALLOT operations accept a single-channel argument that
+is treated as a boolean and produce a 64-bit value.
+
+.. opcode:: VOTE_ANY - Value is set in any of the active invocations
+
+  Syntax: ``VOTE_ANY dst, value``
+
+  Example: ``VOTE_ANY TEMP[0].x, TEMP[1].x``
+
+
+.. opcode:: VOTE_ALL - Value is set in all of the active invocations
+
+  Syntax: ``VOTE_ALL dst, value``
+
+  Example: ``VOTE_ALL TEMP[0].x, TEMP[1].x``
+
+
+.. opcode:: VOTE_EQ - Value is the same in all of the active invocations
+
+  Syntax: ``VOTE_EQ dst, value``
+
+  Example: ``VOTE_EQ TEMP[0].x, TEMP[1].x``
+
+
+.. opcode:: BALLOT - Lanemask of whether the value is set in each active
+            invocation
+
+  Syntax: ``BALLOT dst, value``
+
+  Example: ``BALLOT TEMP[0].xy, TEMP[1].x``
+
+  When the argument is a constant true, this produces a bitmask of active
+  invocations. In fragment shaders, this can include helper invocations
+  (invocations whose outputs and writes to memory are discarded, but which
+  are used to compute derivatives).
+
+
+.. opcode:: READ_FIRST - Broadcast the value from the first active
+            invocation to all active lanes
+
+  Syntax: ``READ_FIRST dst, value``
+
+  Example: ``READ_FIRST TEMP[0], TEMP[1]``
 
-Vote opcodes
-^^^^^^^^^^^^
 
-These opcodes compare the given value across the shader invocations
-running in the current SIMD group. The details of exactly which
-invocations get compared are implementation-defined, and it would be a
-correct implementation to only ever consider the current thread's
-value. (i.e. SIMD group of 1). The argument is treated as a boolean.
+.. opcode:: READ_INVOC - Retrieve the value from the given invocation
+            (need not be uniform)
 
-.. opcode:: VOTE_ANY - Value is set in any of the current invocations
+  Syntax: ``READ_INVOC dst, value, invocation``
 
-.. opcode:: VOTE_ALL - Value is set in all of the current invocations
+  Example: ``READ_INVOC TEMP[0].xy, TEMP[1].xy, TEMP[2].x``
 
-.. opcode:: VOTE_EQ - Value is the same in all of the current invocations
+  invocation.x controls the invocation number to read from for all channels.
+  The invocation number must be the same across all active invocations in a
+  sub-group; otherwise, the results are undefined.
 
 
 Explanation of symbols used