Mesa (master): tgsi: align the definition of BFI & [UI]BFE with GLSL

[Nicolai Hähnle]

Module: Mesa
Branch: master
Commit: 6526977306ae013a3ab4946229168a30015393ec
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=6526977306ae013a3ab4946229168a30015393ec

Author: Nicolai Hähnle <nicolai.haehnle at amd.com>
Date:   Thu Oct 27 10:32:27 2016 +0200

tgsi: align the definition of BFI & [UI]BFE with GLSL

As previously written, these opcodes use the SM5 semantics which is
incompatible with GLSL when bits == 0, offset == 32.

At some point we may want to add BFI_SM5 etc. opcodes, but all users
currently either want (and expect!) the GLSL semantics or don't care.

Bitfield inserts are generated by the GLSL lower_instructions and
lower_packing_builtins passes with constant bits and offset arguments,
so any workaround code that drivers may have to emit to follow GLSL
semantics should be optimized away easily for those uses.

Reviewed-by: Marek Olšák <marek.olsak at amd.com>

---

 src/gallium/docs/source/tgsi.rst | 35 +++++++++++++++--------------------
 1 file changed, 15 insertions(+), 20 deletions(-)

diff --git a/src/gallium/docs/source/tgsi.rst b/src/gallium/docs/source/tgsi.rst
index 5068285..d2d30b4 100644
--- a/src/gallium/docs/source/tgsi.rst
+++ b/src/gallium/docs/source/tgsi.rst
@@ -1583,48 +1583,43 @@ These opcodes are used for bit-level manipulation of integers.
 
 .. opcode:: IBFE - Signed Bitfield Extract
 
-  See SM5 instruction of the same name. Extracts a set of bits from the input,
-  and sign-extends them if the high bit of the extracted window is set.
+  Like GLSL bitfieldExtract. Extracts a set of bits from the input, and
+  sign-extends them if the high bit of the extracted window is set.
 
   Pseudocode::
 
     def ibfe(value, offset, bits):
-      offset = offset & 0x1f
-      bits = bits & 0x1f
+      if offset < 0 or bits < 0 or offset + bits > 32:
+        return undefined
       if bits == 0: return 0
       # Note: >> sign-extends
-      if width + offset < 32:
-        return (value << (32 - offset - bits)) >> (32 - bits)
-      else:
-        return value >> offset
+      return (value << (32 - offset - bits)) >> (32 - bits)
 
 .. opcode:: UBFE - Unsigned Bitfield Extract
 
-  See SM5 instruction of the same name. Extracts a set of bits from the input,
-  without any sign-extension.
+  Like GLSL bitfieldExtract. Extracts a set of bits from the input, without
+  any sign-extension.
 
   Pseudocode::
 
     def ubfe(value, offset, bits):
-      offset = offset & 0x1f
-      bits = bits & 0x1f
+      if offset < 0 or bits < 0 or offset + bits > 32:
+        return undefined
       if bits == 0: return 0
       # Note: >> does not sign-extend
-      if width + offset < 32:
-        return (value << (32 - offset - bits)) >> (32 - bits)
-      else:
-        return value >> offset
+      return (value << (32 - offset - bits)) >> (32 - bits)
 
 .. opcode:: BFI - Bitfield Insert
 
-  See SM5 instruction of the same name. Replaces a bit region of 'base' with
-  the low bits of 'insert'.
+  Like GLSL bitfieldInsert. Replaces a bit region of 'base' with the low bits
+  of 'insert'.
 
   Pseudocode::
 
     def bfi(base, insert, offset, bits):
-      offset = offset & 0x1f
-      bits = bits & 0x1f
+      if offset < 0 or bits < 0 or offset + bits > 32:
+        return undefined
+      # << defined such that mask == ~0 when bits == 32, offset == 0
       mask = ((1 << bits) - 1) << offset
       return ((insert << offset) & mask) | (base & ~mask)