draw: Add lower_opcodes NIR pass

Gallivm runs shaders that are originally compiled with another backend's
compiler options, which may have optimizations that introduce opcodes
that gallivm does not support. Add a pass to lower these.

Assisted-by: Claude Opus 4.6
Signed-off-by: Benjamin Cheng <benjamin.cheng@amd.com>
Reviewed-by: Georg Lehmann <dadschoorse@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/41302>

src/gallium/auxiliary/draw/draw_nir.h

@@ -0,0 +1,17 @@
+/**************************************************************************
+ *
+ * Copyright 2026 Advanced Micro Devices, Inc.
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ **************************************************************************/
+
+#ifndef DRAW_NIR_H
+#define DRAW_NIR_H
+
+#include "compiler/nir/nir.h"
+
+bool
+draw_nir_lower_opcodes(struct nir_shader *shader);
+
+#endif

src/gallium/auxiliary/draw/draw_nir_lower_opcodes.py

@@ -0,0 +1,48 @@
+#
+# Copyright 2026 Advanced Micro Devices, Inc.
+#
+# SPDX-License-Identifier: MIT
+
+import argparse
+import sys
+
+a, b, c = 'a', 'b', 'c'
+
+# This contains lowerings for opcodes that driver-specific optimizations
+# may introduce but gallivm does not support.
+lower = [
+   (('fmulz', a, b),    ('bcsel', ('ior', ('feq', a, 0.0), ('feq', b, 0.0)), 0.0, ('fmul', a, b))),
+   (('ffmaz', a, b, c), ('bcsel', ('ior', ('feq', a, 0.0), ('feq', b, 0.0)), c,   ('ffma_weak', a, b, c))),
+
+   # draw shaders are GLSL shaders, which means we can just use ffma_weak
+   (('ffma', a, b, c),  ('ffma_weak', a, b, c)),
+
+   (('bitfield_select', a, b, c), ('ixor', c, ('iand', a, ('ixor', b, c)))),
+   (('ubfe', a, b, c), ('ubitfield_extract', a, ('iand', b, 0x1f), ('iand', c, 0x1f))),
+   (('ibfe', a, b, c), ('ibitfield_extract', a, ('iand', b, 0x1f), ('iand', c, 0x1f))),
+   (('bfm', a, b), ('ishl', ('isub', ('ishl', 1, ('iand', a, 0x1f)), 1), ('iand', b, 0x1f))),
+   # bfi(a, b, c) -> bitfield_select(a, ishl(b, find_lsb(a)), c)
+   (('bfi', a, b, c), ('ixor', c, ('iand', a, ('ixor', ('ishl', b, ('find_lsb', a)), c)))),
+
+   (('ufind_msb_rev', a), ('bcsel', ('ige', ('ufind_msb', a), 0), ('isub', 31, ('ufind_msb', a)), -1)),
+   (('ifind_msb_rev', a), ('bcsel', ('ige', ('ifind_msb', a), 0), ('isub', 31, ('ifind_msb', a)), -1)),
+
+   # uclz(a) -> umin(32, ufind_msb_rev(a))
+   (('uclz', a), ('umin', 32, ('bcsel', ('ige', ('ufind_msb', a), 0), ('isub', 31, ('ufind_msb', a)), -1))),
+
+   (('shfr', a, b, c),
+    ('bcsel', ('ieq', ('iand', c, 0x1f), 0), b,
+                      ('ior', ('ishl', a, ('iadd', 32, ('ineg', ('iand', c, 0x1f)))),
+                              ('ushr', b, ('iand', c, 0x1f))))),
+]
+
+parser = argparse.ArgumentParser()
+parser.add_argument('-p', '--import-path', required=True)
+args = parser.parse_args()
+sys.path.insert(0, args.import_path)
+
+import nir_algebraic
+
+p = nir_algebraic.AlgebraicPass("draw_nir_lower_opcodes", lower)
+print('#include "draw/draw_nir.h"')
+print(p.render())

src/gallium/auxiliary/meson.build

@@ -271,6 +271,17 @@ if with_gfx_compute
     'util/u_simple_shaders.c',
     'util/u_tests.c',
   )
+
+  files_libgallium += custom_target(
+    'draw_nir_lower_opcodes.c',
+    input : 'draw/draw_nir_lower_opcodes.py',
+    output : 'draw_nir_lower_opcodes.c',
+    command : [
+      prog_python, '@INPUT@', '-p', dir_compiler_nir,
+    ],
+    capture : true,
+    depend_files : nir_algebraic_depends,
+  )
 endif
 
 if dep_libdrm.found()

src/mesa/state_tracker/st_program.c

@@ -52,6 +52,7 @@
 #include "pipe/p_defines.h"
 #include "pipe/p_shader_tokens.h"
 #include "draw/draw_context.h"
+#include "draw/draw_nir.h"
 
 #include "util/u_dump.h"
 #include "util/u_memory.h"